Pag.1

HELICOPTER MODIFICATIONS AND UPGRADES

Pag.2

FOREWORD The operational life of helicopters and aircrafts is usually over 20 years; some models exceed this period by far (CH-47 Chinook, DC-3, etc.). During those years, many changes occur in legislation, operation requirements and, for military helicopters/aircrafts, mission scenarios and threats. Many excellent books have been written covering most disciplines involving helicopter development (aerodynamics, structures, performance, hydraulic systems, fuel systems, etc). Most of them focussed, as mentioned above, on the development phase, that is, they provide valuable information to design a new helicopter, and although that information is perfectly valid to achieve any modification, I haven’t found a book specifically orientated to helicopter modifications and upgrades. This book is intended to be a guide for those small and mid size companies that carry out most of the modifications and upgrades that any helicopter will undergo during its operational life. These companies are not the helicopter manufacturer; therefore most of the helicopter engineering data is not available to them, often none at all. In most of the cases, only maintenance manuals and the helicopter itself are available. I hope this book can solve similar problems, at least a few of them, to those my co-workers (and friends) and I have had to face for many years when coping with many helicopters’ modifications. Madrid, August 2006. Adolfo Sanchez Manso

Pag.3

TABLE OF CONTENTS 1. OBJECT .............................................................................................................................4 2. DEFINITION .....................................................................................................................5 3. RELATIONSHIP WITH CUSTOMER .............................................................................6

Documentation: ......................................................................................................................6 4. CERTIFICATION............................................................................................................13 5. MATERIAL PROCUREMENT AND LOGISTICS........................................................19 6. MODIFICATION DESIGN .............................................................................................20

6.1 SAFETY ASSESSMENT ..............................................................................................20 6.2 STRUCTURAL DESIGN ..............................................................................................22

6.2.1 Geometrical definition.............................................................................................22 6.2.3 Actual stress distribution .........................................................................................23 6.2.4 Material selection and documentation.....................................................................28 6.2.5 Weight and balance .................................................................................................31

6.3 ELECTRICAL DESIGN ................................................................................................32 Pin to pin diagrams...............................................................................................................32 Harness routing.....................................................................................................................33 Electrical load analysis .........................................................................................................34 6.4 AERODYNAMIC AND PERFORMANCE DESIGN ..................................................35 6.5 SYSTEMS DESIGN ......................................................................................................39

6.5.1 Fluid systems (hydraulic, air conditioning, antiicing, fuel, etc.) .............................39 6.5.2 Mechanical systems.................................................................................................40

6.6 NOISE ............................................................................................................................40 6.7 ADDITIONAL DOCUMENTS FOR CERTIFICATION .............................................41 6.8 MANUAL SUPPLEMENTS..........................................................................................43

7. PROTOTYPE INSTALLATION.....................................................................................48 7.1 SERVICE BULLETIN...................................................................................................49 7.2 WORK ORDER .............................................................................................................50

8. PROTOTYPE TESTS ......................................................................................................52 8.1 FUNCTIONAL AND GROUND TESTS ......................................................................52 8.2 EMI/EMC TESTS ..........................................................................................................58 Equipment level tests............................................................................................................58 Interoperability test...............................................................................................................58 Allowable radiation level .....................................................................................................65 8.3 FLIGHT TESTS .............................................................................................................65

9. ENTRY INTO SERVICE.................................................................................................72 10. SUMMARY .................................................................................................................73

Pag.4

1. OBJECT

The purpose of this book is to provide information to allow the reader to successfully accomplish any modification or upgrade of a helicopter (civil or military). This book makes the assumption that a small or mid size company has been selected to carry out a modification. This implies that an employee has been designated as project manager and is responsible for the whole project with the following conditions:

a) Helicopter engineering data is not available b) Maintenance manuals are available c) The helicopter is available, for limited periods of time, because operativity must be

kept until the modification starts d) The company doesn’t have experts in all the required disciplines e) The company is a minor consumer of raw materials (aluminium sheets, bolts, rivets,

electrical connectors, etc.) compared to any helicopter manufacturer f) Direct contact with the customer is the responsibility of the project manager g) Direct contact with the certification authority is also the project manager’s

responsibility The following chapters are organised trying to follow the logical sequence of a modification project.

Pag.5

2. DEFINITION

It is important to define what a modification or upgrade is, in order to understand the implications regarding certification (see chapter 4 for more details). Modification/Upgrade: Any change of helicopter configuration carried out with non-approved data by any airworthiness certification authority. This data has to be approved by the pertinent certification authority prior to the release of the helicopter into normal service. It is the responsibility of the company carrying out the modification to get the certificate of the aforementioned modification from the applicable airworthiness authority. The process to be followed to approve a modification, as far as certification is concerned, is described in chapter 4. On the other hand, when an approved (by any airworthiness authority) Service Bulletin (the document describing how to carry out a particular modification) is implemented, the certification process is greatly reduced, as mentioned in chapter 4.

Pag.6

3. RELATIONSHIP WITH CUSTOMER

It is extremely important to establish a relationship with the customer based on confidence, to prevent future disappointments, complaints and other problems. The customer has to be informed (in advance) of any limitation imposed on the performance of the systems installed. These limitations may occur because of the helicopter’s geometry, electrical power generation, structural stress state or many other reasons. Keep in mind that every customer expects the systems or equipment to operate as stated in their specifications. Unfortunately, most of the time these specifications are based on laboratory tests, performed in ideal conditions. Ideal conditions simply do not exist in a helicopter (eg, most, if not all, of the Forward Looking Infrared devices currently in use in helicopters are able to provide 360º azimuth view and between +90º to –120º in elevation. When installed in a helicopter it is very difficult to attain those angles, simply because the fuselage exists). This example is quite evident and easy to understand by the customer, but even in these cases, inform the customer as soon as possible to prevent future trouble. There are a large variety of customers and the project manager has to deal with all of them. Some of them provide very detailed specifications of what exactly they want, others simply say they want this equipment to be installed without any further information. Some customers define (model and part numbers) the systems and equipment they require to be installed, others just mention the system they need (a satellite phone, a conversion to medical evacuation, etc) and expect the modification company to select the systems for them. Because of this variety, this book will assume the worst case, with no specification provided and systems or equipment to be selected by the company. Obviously, if the customer has a written specification and has selected the equipment, these tasks do not have to be carried out by the modification company. Several quality assurance standards (EN 9100, AQAP 110, etc) define the documentation to be written and the design reviews to be held with the customer in any project. Documentation:

a) Installation specification b) Design management plan c) Configuration control management plan d) Quality assurance plan

Installation specification: It is advisable to issue a specification to define the installation requirements and send it to the customer for their approval. This document makes clear what the installation goals are and the final design can be checked against them. In other words, the final design can be accepted or rejected based on facts and not on opinions. Depending on the modification complexity, the installation specification may be as brief as one page or as long as required. But the minimum information to be included is the following:

Pag.7

a) Model and part numbers of the systems or equipment to be installed b) Environmental and electromagnetic requirements to be withstood by the

systems/equipment. These requirements are those of the helicopter. If no information is available about these requirements, use those mentioned in the military standards MIL-STD-810 (environment) and MIL-STD-461 (electromagnetic) for military helicopters, and the DO-160 for civil helicopters

c) Expected operational performance of systems and equipment d) Ergonomic requirements. If there is not data available, use MIL-STD-1472 e) Certification standards to be applied. FAR 27 or 29 (USA), EASA Part 27 or 29

(Europe), etc for civil helicopters. Specific military standards, depending on the country, for military helicopters

f) Maximum weight increase allowed to the modification g) Cooling requirements for electronic equipment h) Maximum acceptable temperature and humidity for air conditioning modifications i) Maximum acceptable CO levels inside the cabin (when number of passengers has

been increased) j) Non standard corrosion prevention methods, if any k) Any other relevant requirement (specific configuration for avionics modifications,

stiffness requirements for major structural modifications, expected fatigue life for structural modifications, etc)

It can seem cumbersome and time consuming to issue a specification for many minor modifications, but even in such cases it is worth doing . Remember that the modification has to be accepted by the customer and we are all generals after the battle. It is much easier to get acceptance of a modification after filling in a checklist, where the operational parameters (with tolerances) are described, than discussing several opinions. Design management plan: This document mentions how the modification design and implementation will be carried out, what resources are assigned to the project, who is responsible for every discipline involved in the modification (structure, electrical systems, tests, prototype installation, etc) and which means will be utilised ( eg 3D design tools, finite element analysis codes, etc). It should also mention who has to sign, and in what order, the documents being issued (ie, a mechanical drawing requires several signatures, such as drawn by, checked by, stress check by, approved by, etc). The design reviews are established (how many, dates and locations (customer facilities or modification company premises)) and record formats and procedures. Configuration control management plan: This document describes how the documentation to be issued will be numbered and controlled. It also states how the changes, concessions and deviations will be treated (drawings, documents, equipments, etc), who is responsible for doing it and with what means the configuration control is achieved.

Pag.8

Quality assurance plan: This document is exactly what it says. It summarises the methods to be used to ensure the modification meets the company’s quality standards. This document is issued by the company’s quality assurance department, which is independent of the engineering and production departments. Design reviews: Three design reviews are normally set up for major modifications, however, for minor ones this can be too much and even meaningless, because it may take less time to finish the design than to prepare the three reviews. But at leas, one design review has to be held for minor modifications. For major modifications, the design reviews are the following (this is the minimum number of reviews, more reviews can be defined if deemed necessary).

a) PDR (preliminary design review) b) CDR (critical design review) c) FDR (final design review)

PDR: The preliminary design review is held when the design concepts are completely defined, although minor changes could take place. During this meeting the following modification aspects are analysed:

a) Systems/equipment selection b) Equipment locations on helicopter c) Structural modification design d) Structural stress analysis (static, dynamic and fatigue) e) Material selection (structural and mechanical) f) Corrosion prevention program g) Weight estimation h) Ergonomics i) Cooling requirements j) Electrical consumption k) Electrical modification design (pin to pin diagrams and harness routing) l) Material selection (electrical components such as connectors, switches, etc) m) Aerodynamic impact n) Helicopter performance impact (if any) o) System/equipment actual expected performance p) Noise requirements (if applicable) q) Preliminary functional and ground test procedure r) Preliminary EMI/EMC test procedure

Pag.9

s) Preliminary flight test procedure t) Preliminary flight manual supplement u) Preliminary documents for continuous airworthiness (maintenance manual

supplement, wiring manual supplement, illustrated part catalogue supplement) v) Project schedule w) Any other relevant aspect

After the PDR is finished, a document has to be issued (normally a check list) contemplating all the analysed aspects. It has to make clear if any particular point is approved or rejected and what remarks (if any) have been stated. This document has to be signed off by the customer representative and the project manager. If the PDR has been approved by the customer, it has to be explicitly written that permission has been granted to begin the next modification phase (detail design). CDR: The critical design review is held when the detail design is finished. During this meeting the following modification aspects are analysed:

x) Definitive equipment locations on helicopter y) Definitive Structural modification design z) Definitive Structural stress analysis (static, dynamic and fatigue) aa) Material selection (structural and mechanical) bb) Corrosion prevention program cc) Weight impact dd) Ergonomics ee) Cooling requirements ff) Electrical consumption gg) Electrical modification design (pin to pin diagrams and harness routing) hh) Material selection (electrical components such as connectors, switches, etc.) ii) Aerodynamic impact jj) Helicopter performance impact (if any) kk) System/equipment actual expected performance ll) Noise requirements (if applicable) mm) Definitive functional and ground test procedure nn) Definitive EMI/EMC test procedure oo) Definitive flight test procedure pp) Preliminary flight manual supplement qq) Preliminary documents for continuous airworthiness (maintenance manual

supplement, wiring manual supplement, illustrated part catalogue supplement) rr) Project schedule ss) Any other relevant aspect

After the CDR is finished, a document has to be issued (normally a check list), which lists all the analysed aspects. It has to make clear if any particular point is approved or rejected and what remarks (if any) have been stated. This document has to be signed off by the customer representative and the project manager.

Pag.10

If the CDR has been approved by the customer, it has to be explicitly written that permission has been granted to begin the next modification phase (parts’ manufacturing, prototype installation and tests). FDR: The final design review is held to wrap up the project. A final document is issued and signed off by customer and project manager, declaring the modification has been accepted by the former. The following documentation is given to the customer:

a) Flight manual supplement b) Maintenance manual supplement c) Wiring manual supplement d) Illustrated Part Catalogue supplement e) Copy of the approved modification airworthiness certificate (not mandatory) f) Filled in helicopter log book g) For military helicopters in NATO countries: NSN’s (NATO Stock numbers of the

applicable parts) And the following items are also delivered:

a) The modified helicopter b) Spare parts (if applicable) c) Special tools (if applicable) d) Ground support equipment (if applicable)

It is noteworthy to say that performance of systems and equipment has to be demonstrated to the customer. This can take place prior to or during the final design review. Some differences exist between system performance shown to the certification authority and system performance to be demonstrated to the customer. In chapter 4 “Certification” this is explained in greater detail. As far as system or equipment selection is concerned, assuming the customer has not already selected the system or equipment, keep them informed and involved in the selection, , as much as possible, and take into account the following suggestions. Disregarding economical issues (it is assumed that an agreement about equipment prices has been reached somehow), the following has to be considered when dealing with equipment suppliers (manufacturers or vendors).

a) Lead-time: this is the first information to be required from the supplier, preferably in writing. It is evident that without this information, a realistic modification schedule cannot be completed. It is sensible to provide the customer with a non-excessively optimistic schedule in order to prevent future problems. If some particular equipment has a very long lead time, one year for instance, inform the customer from the beginning that this is the time they have to wait, and do not trust the supplier’s goodwill promises saying that it may be delivered sooner. It will not be or, even worse, the equipment will have some flaws that, in the long term, cause more delays.

Pag.11

b) Specification: Ask for a comprehensive specification of the product. This will let you know what you should expect from the system.

c) Declaration of Design and Performance (DDP): Demand a DDP from the supplier. Several items of equipment do not fulfil the specification requirements, and the DDP states the deviations and actual equipment performance.

d) Environmental and EMI/EMC test reports: Pay special attention to this issue. Make sure that the equipment to be installed in the helicopter is able to withstand its operational environment (vibration, temperature variations, EMI, etc.). Therefore, if possible, ask the supplier for a copy of these test reports. As the very minimum, ask the supplier for the equipment fragility curve (that shows the vibration level the equipment is able to withstand without shock absorbers)

e) Failure analysis report: If the system to be installed may have any failure mode that could be dangerous or catastrophic for the helicopter, demand a Failure Mode and Effect Analysis report (FMEA) from the supplier

f) Interface Control Document (ICD): this is the document needed to achieve the installation on the helicopter. This document describes the mechanical (dimensions, attachment points, weight, centre of gravity, etc), electrical (signals, connectors, pins, etc), cooling (forced ventilation required, minimum distances to enclosure walls, etc.) and any other interface. This document has also to be provided by the supplier

g) Acceptance Test Procedure (ATP): The supplier has to deliver the procedures to test the equipment when received at the modification company’s facilities. This test checks whether the equipment is in the same condition as when it was sent from the supplier factory or, for any reason, has been damaged during transportation.

h) Data sheets: The supplier has to send the Data sheets (those recording the data obtained in the supplier factory when the equipment is tested prior to being shipped to the modification company), otherwise it would be impossible to determine if the equipment has suffered any damage during transportation or it was sent with some problem by the supplier

i) Certificate of conformity: The equipment, when shipped, has to be provided with a certificate of conformity stating the product has been manufactured according to the approved design data.

j) Maintenance manuals: If contractually demanded by the customer. These manuals can be for O (operational), I (intermediate) and D (Depot) maintenance levels.

k) For electronic equipment, ask the supplier for all the equipment needed to operate the system. All equipment means all the components to be installed in the helicopter and all the auxiliary items (for ground use) required to operate and test the system. Some companies say nothing about these auxiliary items, and when you realise it is necessary, the wasted time is hard to regain and the price is usually quite high.

It is more expensive to order any system or equipment with the stated documentation than without it. However, those documents ease the certification process (which could become very expensive if the certification authority requires a demonstration of compliance of any equipment with all or some environmental requirements, firstly because tests are expensive and, secondly, because the modification company has no control over the equipment in case a failure occurred during these tests). On the other hand, if there are functional problems with the supplied equipment, the documentation (ATP and data sheets) helps to detect where the problem is and who is responsible for it.

Pag.12

The previous paragraph leads to the warranty issue. It is customary to get a one year warranty for most equipment and systems. If the customer requires a warranty extension, the equipment manufacturer or vendor has to be informed and the modification company will be charged for this extension. The following surcharges can be expected:

a) Software: from 7% to 12% of equipment net price per year of warranty extension b) Electronic equipment hardware: 5% to 8% of equipment net price per year of warranty

extension c) Other equipment/systems: 4% to 9% (depending on equipment complexity) of

equipment net price per year of warranty extension The liability of the modification carried out belongs to the modification company; this has to demand the same level of responsibility from its suppliers. Therefore, the contracts to be issued have to contain the following information, as a minimum:

a) Number of equipment/systems to be supplied b) Delivery schedule c) Type of supplied equipment: only new equipments are acceptable, no “exchange”

items or second hand ones are tolerated d) Unit price e) Auxiliary equipment (if needed) price and delivery schedule f) Documentation to be delivered (Specification, DDP, test reports, ATP, Data sheets,

etc) g) Warranty and warranty extensions if applicable (price of these extensions) h) Equipment/systems delivery location (at manufacturer’s facilities, at modification

company premises, at modification company closest customer, etc) i) Equipment/system transportation method (by air, sea, etc) j) Down payment (if previously agreed) k) Payment schedule (usually linked to the delivery schedule) l) Indemnity (in case some delivery delay occurs or the equipment works below the

specification operation levels). Usually the indemnities are the same as those imposed by the customer on the modification company. Contract cancellation details are included in this paragraph

m) Maintenance plan: it has to be clearly mentioned if the intermediate and depot level maintenance will be carried out exclusively by the equipment manufacturer or some maintenance capabilities have to be developed by the modification company or the helicopter operator (customer). Obviously, this is particularly important after the warranty period has expired

n) Future maintenance policy. It will be defined how the items needing maintenance will be managed. Basically there are two ways of doing this. In the first case, the item is sent to the maintenance facility and when properly revised or repaired it is returned to the modification company (or the customer) and installed in the helicopter. The other alternative (available only for certain equipment) is called “exchange”; it means that an item is sent to the maintenance facility and you receive a revised or repaired one belonging to another operator. Both methods have pros and cons; a careful study is required before deciding which one to select. In fact, in most cases the customer will be the one to decide which method to choose according to their current and future needs

Pag.13

4. CERTIFICATION The certification authority that validates the modification will verify the following and only this: The modification does not adversely affect the safe operation of the helicopter. There are two very important certification paragraphs in FAR and EASA standards; 27/29.1301 and 27/29.1309. They are enunciated in an apparently simple and straightforward form, but to show compliance with them is not that simple. The philosophy of paragraph 1301 is: Every system/piece of equipment installed in a helicopter has to work for what it is designed (and in some occasions limitations have to be implemented because of restrictions imposed by the helicopter on the system or, vice versa, the system on the helicopter). The philosophy of paragraph 1309 is: Every system/piece of equipment installed in a helicopter has to work in a safe manner (in all the conceivable conditions compatible with the helicopter operation). It implies that the authority will check if each and every one of the systems/pieces of equipment installed affect the helicopter, but the authority will not express any opinion about the quality of the new systems/equipment. An example will clarify this point. Assume that a machine gun has been installed in a helicopter. The certification authority will check the following:

a) The machine gun is able to withstand and operate safely in the helicopter environment b) The ammunition is safe to be shot on board the helicopter c) No bullet will impact, in any conceivable flight condition when the machine gun is

shooting, with any part of the helicopter (blades, sponson, etc) d) Gases from the machine gun do not adversely affect the helicopter e) Vibration caused by the machine gun does not adversely affect the equipment nearby f) There is sufficient time to bail out of the helicopter in case of an accident, when the

machine gun is installed g) Some other issues

But the certification authority will not verify if the maximum range of the machine gun meets that of the specification, if the machine gun is accurate enough to hit some targets under specific shooting conditions, etc. These characteristics have to be demonstrated to the customer, not to the certification authority. That is why, as mentioned in chapter 3, a FDR is sometimes necessary. Safety operation has to be demonstrated to the certification authority, and performance according to the specification has to be demonstrated to the customer. The certification process starts with an official letter sent by the company to the authority. It mentions that a modification (ie, a GPS model XXXX) is going to be carried out on a

Pag.14

helicopter model XXXX and asks for a meeting to be held between the company and the authority to make a presentation of the project and start discussions on the certification process. The certification documentation to be issued for a generic modification is:

a) Certification plan b) Modification description c) Safety assessment d) Master drawing list e) Helicopter configuration report f) Equipment qualification report g) Mechanical drawings h) Static structural analysis i) Dynamic structural analysis j) Fatigue analysis k) Electrical drawings l) Electrical load analysis m) Weight and balance n) Aerodynamic analysis o) Helicopter performance analysis p) Fire protection analysis q) Hydraulic system functional analysis r) Thermal analysis s) Pneumatic system functional analysis t) Anti icing system functional analysis u) Fire extinguishing system functional analysis v) Flight controls kinematical analysis w) Landing gear kinematical analysis x) Fuel system functional analysis y) Noise analysis z) Air conditioning functional analysis aa) Heating system functional analysis bb) Illumination system functional analysis cc) Main rotor kinematical analysis

dd) Tail rotor kinematical analysis ee) Blade folding system functional analysis ff) Transmission functional analysis gg) Safety analysis (if needed depending on safety assessment results) hh) FMEA (if needed depending on hazard analysis results) ii) Functional and ground test procedure jj) Functional and ground test results report kk) EMI/EMC test procedure ll) EMI/EMC test results report mm) Flight test procedure mn) Flight test results report mo) Modification checklist mp) Service bulletin mq) Flight manual supplement

Pag.15

mr) Maintenance manual supplement ms) Wiring manual supplement mt) If a modification is applied on an engine, equivalent documentation as above has to be edited plus 1) engine functional analysis 2) windmill effect analysis 3) rotating part containment analysis

The certification plan is the document that sets up the certification bases and how compliance with those certification requirements will be demonstrated (compliance methods). The certification bases depend on whether the helicopter is civil or military. For civil helicopters, the certification bases are: For helicopters designed in the USA: FAR 27 or FAR 29 (it depends on the weight of the helicopter. FAR 27 is applicable for helicopters with a take off weight equal or less than 6000 lbs and FAR29 for those with a take off weight exceeding 6000 lbs). For helicopters designed in Europe: EASA Part 27 or Part 29 (it also depends on the weight of the helicopter). FAR 27 and EASA Part 27 are equivalent, but there are some differences between FAR 29 and EASA Part 29. For military helicopters, the certification bases can be selected by the applicant (the modification company). According to the type of modification, it may even be valid to use civil standards (FAR/EASA 29 for instance) and discuss with the authority special paragraphs for the specific military characteristics not covered by the civil standards. In the USA applicable military standards for helicopters are recommended, in Canada DEF-STAN (Volume 2: rotary wing) are also suggested. In other countries, however, military standards for helicopters are not so readily available or they simply do not exist. A suggested table of contents for a certification plan is the following:

1) Scope 2) Applicability 3) Reference documentation 4) Modification description 5) Configurations 6) Design definition 7) Documentation control 8) Certification bases 9) Certification requirements 10) Compliance methods 11) List of documents 12) Limitations

Scope:

Pag.16

This paragraph defines the purpose of the document: to obtain a certification by the authority for the modification (whichever it is) to be implemented on the helicopter model XXXX, manufactured by XXXX. Applicability: This paragraph indicates which helicopters will be affected by the modifications. Include serial number and tail number, if possible. Reference documentation: List the document used to issue the certification plan (flight manual, Type Certificate, maintenance manuals, certification standards, etc) Modification description: Summarise the modification (what it consists of , where the equipment is located, how it works, which are the interfaces with the helicopter, etc) Configurations: The modification will be implemented in a particular helicopter configuration. This configuration has to be clearly defined. Refer to the helicopter configuration report. Bear in mind that, at this stage, all possible variations of the standard helicopter configuration compatible with the modification have to be defined in order to obtain a certification covering all possible situations. An example will be used to clarify this point. Assume a VHF radio has to be installed in a helicopter. This helicopter has a configuration (whatever it is). This configuration will be called “standard”. Some optionals exists for this helicopter model eg a rescue hoist, skis and stretchers, although they are not installed in the aircraft. All these optionals are compatible with the VHF radio. The applicable configuration is the standard + rescue hoist + skis + stretchers. Of course, this compatibility has to be demonstrated to the certification authority. If the stated optionals are not included in the helicopter configuration report, the use of the VHF radio will not be possible, from a legal standpoint, when some or all of the optionals are installed in the helicopter. Design definition: The modification design is completed by the drawings and documents in the Master drawing list. Documentation control: This paragraph defines how the certification documentation to be sent to the authority will be controlled by the modification company. Certification bases:

Pag.17

This paragraph states what the certification bases are (ie, FAR29), what the applicable standards’ amendments are, and the reasons to apply these bases. Certification requirements: Each paragraph of the applicable certification standard has to be analysed and determined if it is applicable (or not) to the modification. A table, similar to the one shown below, has to be created listing all the applicable paragraphs.

SECTION TITLE (Amdt) METHOD OF COMPLIANCE (MOC)

General 29.25 (a) (b) (c) Weight limits (Amdt 29-43) Analysis

29.27 Centre of gravity limits (Amdt 29-3) Analysis

29.29 (a) (b) Empty weight and corresponding center of gravity (Amdt 29-15)

Analysis

Strength Requirements

29.303 Factor of Safety. (Original Edition) Stress analysis

29.305 (a) (b) Strength and Deformation. (Original Edition) Stress analysis

29.307 (a) (b) Proof of structure (Amdt 29-26) Stress analysis

29.309 Design Limitations. (Original Edition) Stress analysis

29.561(a) (b) (c) (d)

Emergency landing Conditions (Amdt 29-38) Stress analysis

Compliance methods: These are the methods used to demonstrate that each certification paragraph is accomplished by the modification. Several methods of compliance can be utilised, such as:

a) Compliance statement b) Design reviews c) Analysis (structural, electrical, aerodynamic, etc) d) Qualification of equipment, components and materials e) Aircraft inspection f) Ground and functional tests g) EMI/EMC tests h) Flight tests

Pag.18

i) Laboratory tests j) Any other acceptable by the authority

A brief explanation of these methods has to be included in the certification plan. List of documents: All the certification documents have to be listed with title, number and issue. Limitations: Expected limitations in the flight envelope or any other operational limitation has to be mentioned. If no limitation is envisaged or it is not possible to determine it at this stage, then it will be mentioned that any limitations detected during the certification process will be included in the flight manual supplement and/or maintenance manual supplement (if applicable). The rest of the certification documents will be further detailed in the following chapters. Note for civil modifications made in the USA and applied to European helicopters: Before the creation of EASA, it was customary in some European countries that the pertinent Civil Aviation Authority validated directly any modification made in the USA, and approved by the FAA, applicable to a civil helicopter. In other words, any modification approved by FAA could be implemented in some European helicopters without further investigation. Since EASA exists, no modification approved by FAA is directly applicable in any European country; all these modifications have to be validated by EASA before they could be applied. Note for minor civil modifications in Europe: A minor modification is that complying with the requirements set up in EASA Part 21 Check list (major and minor modifications). If the minor modification is carried out by a DOA (Design Organization Approval) company, the certification authority delegates the modification approval to the company (similar to the DER’s and the FAA,This means that certification documentation does not need to be sent to the authority for approval, but the work to be completed is exactly the same as for a major modification. It has to be substantiated in the same way. Minor modification does not mean that the justification documents do not have to be completed because the modification is quite simple.

Pag.19

5. MATERIAL PROCUREMENT AND LOGISTICS

Modification companies are usually small consumers of raw materials; therefore, they are always at the bottom of the vendor’s priorities. If this company is located in a country where all or most of these aeronautical materials (aluminium, titanium, rivets, bolts, electrical connectors, hydraulic connectors, electrical wires, etc) have to be imported, the situation is even worse, and it will have a negative impact on the project schedule. In order to minimize delays, the following actions can be implemented:

a) Use the same standards as those of the helicopter, design with the same bolts, rivets, nuts, etc. Some materials spared for maintenance can be used for the modification until the purchase order is received

b) Define standard part numbers instead of vendor’s part numbers on the parts lists. Define MS, NAS, LN, etc part numbers and avoid part numbers of suppliers, so more vendors can provide the same part

c) When rivets and bolts have been selected, order those bolts and rivets and also some of the same part number but with one longer dash number and others with the same part number but with one shorter dash number

d) Select the electrical connectors as soon as possible and place an order immediately (lead-time is variable depending on the connector. It can range from one week to more than twelve . Four to six weeks is an average lead-time). Obviously, it is absolutely necessary to know what the equipment connectors are before any purchase order can be placed. This information is obtained from the ICD (see chapter 3)

e) When ordering a system or piece of equipment, state explicitly that the following items are required:

a. System/equipment part number XXXX b. System/equipment specification c. Declaration of design and performance (DDP) d. Interface control document (ICD) e. Environmental and EMI/EMC test reports f. Acceptance test procedure and data sheets g. FMEA (if necessary) h. System/equipment maintenance manuals (if required)

f) Order metal sheets as soon as possible and at least 10% in excess of what is expected

to be consumed g) If special paint is needed (eg radomes), select and order it as soon as possible h) Bear in mind that some static strength tests (ie, for metallic materials’ yield and

ultimate tensile stress tests) have to be carried out on some specimen items to ensure the material complies with the strength requirements. These tests will be performed by the quality assurance department; according to the material reception procedure set up by the company. Tests of composites are more complex and require more time than their metallic counterparts

Pag.20

6. MODIFICATION DESIGN

In principle, any modification can be attempted, but with regard to the assumptions of chapter 1 it is very wise not to be involved in modifications concerning the following items, or at least, subcontract this part of the job to the helicopter/engine manufacturer as soon as the project starts.

a) Rotor (both main and tail rotor, including hydraulic system) and blades b) Transmissions and gear boxes (particularly the main gear box) c) Autopilot (those modifications involving autopilot control laws and autopilot

interconnection with other navigation or attitude equipment) d) Automatic augmentation stabilization systems e) Engine (engine itself, external devices (EAPS) affecting the engine can be

installed without special problems) Another important issue is that of the helicopters endowed with second-generation avionics; examples are the Eurocopter EC-135 (civil) and Eurocopter Tiger (military). When a new system or equipment is installed in such helicopters, the software has to be adapted in order to “inform” the helicopter that a new component has been added and its behaviour has to be modified accordingly. The helicopter manufacturer is the only one possessing that capability (because only they have the software), therefore ask for their support at the beginning of the project. Any other modification can be achieved without the support of the helicopter manufacturer. The following subchapters are listed sequentially but they can be addressed in parallel except the safety assessment; that should be the first analysis completed, even before the modification design is started. It is worth a reminder of what the purposes of any modification are, although they seem quite obvious.

a) Helicopter safety (this is the main consideration to be taken into account) b) Proper operation of the new equipment in order to accomplish its intended use c) Maintenance simplicity (the helicopter will be maintained for many year after the

modification is completed, so try to ease the modification maintenance tasks and not complicate the previously established maintenance actions)

d) Ergonomics (design modifications in such a way so as not to increase the work load of the crew more than necessary; and prevent tiredness through a lack of comfort (eg improper illumination, reduced space to operate, unsuitable vision angles, etc)

6.1 SAFETY ASSESSMENT Prior to discussing safety assessment, it is necessary to mention another safety issue that is of great concern for the certification authorities. This is the modification fire protection. What has been devised to prevent a fire due to the modification has to be documented in the “Fire protection analysis report”. For most modifications, this report deals with the materials selected (mechanical and electrical) for the modification and the new installed equipment. It has to be demonstrated that

Pag.21

the materials selected are fire resistant according to FAR or EASA 27/29-853/855/861 and 863; the same applies to the new equipment. Compliance with those paragraphs is demonstrated by suitable selection of materials and fire tests successfully passed by the equipment. In some extreme cases it may require that a fire extinguishing system be installed. As far as a safety assessment is concerned, it has to be carried out to comply with FAR or EASA 27/29.1309. This analysis will reveal what failures may occur, due to the modification, and how dangerous they are. The following documents can be consulted to clarify the scope of paragraph 27/29.1309:

a) FAA AC 29.1309 b) FAA AC 25.1309-1A c) FAA AC 23-1309-1C d) JAA AMJ 25-1509-1 (NPA 25F-281 Final versión)

The functional hazard analysis is the first to be done, even before the modification design is started. The conclusions of this analysis will drive the design process in order to prevent any potential major, hazardous or catastrophic failure. Note: the documentation referred above classifies the failures as minor, major, hazardous or catastrophic. The following is a brief summary of that classification. Consult the aforementioned documents for further details Minor failures are those that do not affect the aircraft’s essential flight characteristics Major failures are those affecting the helicopter’s flight characteristics and causing increased work for the crew Hazardous failures are those that may cause serious injuries to the passengers but the crew is able to perform its tasks Catastrophic failures are those that may cause the destruction of the helicopter and the death of its occupants. The functional hazard analysis has to state the capabilities of the new installed equipment. It has to investigate if any failure (capability loss, wrong indications, electrical failure, mechanical failure, electromagnetic interferences, etc) of the new equipment can adversely affect the essential helicopter functions. All the failures are classified according to minor, major, hazardous or catastrophic. If any potential failure of the new equipment can cause only minor failures, no further analysis is required. Notwithstanding this, the modification design has to consider the means to prevent as many minor failures as possible. On the contrary, if major, hazardous or catastrophic failures may occur, further investigation is needed. This is recorded in the following documents:

a) Safety analysis b) FMEA (failure modes and effect analysis)

Pag.22

One of the best references to learn how to perform these analyses is the ARP procedures (926, 5580) of the SAE (Society of Automotive Engineers) of the USA. The safety analysis determines the probability that a particular failure occurs and what has to be done to increase this probability (increase redundancy, establish inspections, specific designs, etc) and in the event this increase is not feasible, how the design has to be redefined to diminish the consequences of that particular failure. Several techniques are used to perform this task. One of them, fault tree analysis, uses probability to assure a particular system complies with the safety requirements. Note: Minor failures are those with an occurrence probability from 10-3 to 10-5. Major failures from 10-5 to 10-7. Hazardous failures from 10-7 to 10-9 and Catastrophic failures less than 10-9. The FMEA shows most of the failures that could happen in a system of special importance and what their consequences are, both in the system and in the helicopter. The concept of this analysis is simple: but imagine all the potential failures an autopilot could have and their consequences. In other words, to perform a FMEA analysis many tests and much time are required. This analysis has to identify the failures modes, has to set up quantitative failure rates, has to identify the failure effects and, finally, it has to show how the failure is detected. 6.2 STRUCTURAL DESIGN From a structural standpoint, the designer faces two problems regarding the lack of available engineering data, namely:

a) Geometrical helicopter definition b) Actual stress distribution in the helicopter structure

How to overcome these two problems will be subsequently shown.

6.2.1 Geometrical definition When a piece of equipment has to be installed in a helicopter and no reliable drawings exist, depending on the money allocated for the modification and the means available, the helicopter structure can be reproduced by two different methods. It is noteworthy to mention that only very complex areas, those with double curvature surfaces, ie, helicopter nose, need to resort to these methods; other areas with single curvature surfaces can be reproduced with a few measures taken on the helicopter. Method a: You need a laser tracker, or similar equipment, to measure a grid of points on the helicopter selected area. First of all, using tape or any other means, draw a point grid on the actual helicopter skin, using as many points as necessary. This sentence means: use many many points, and number them in a logical sequence (this is very important), the more complex the surface is, the more points are needed to get a reliable reproduction. Using the laser tracker, get the 3-D coordinates of all these points. These coordinates are related to the laser tracker origin. Record the stated measurements.

Pag.23

Transfer the laser tracker file to a 3-D CAD program, like CATIA V5, V4 or any other equivalent. Using the measured points, create a 3-D surface. It will be an absolutely reliable reproduction of the helicopter skin. Consult the original IPC or structural repair manual to know the skin thickness at this area, apply this thickness to the 3-D surface. In those documents you will also find the thickness of frames and stringers, and their materials. So measure on the helicopter the position of those elements and reproduce them on the 3-D surface, in the appropriate location. The reliable definition of the actual helicopter has been accomplished. Using this method, the whole helicopter structure can be reproduced, although the cost will be ridiculously expensive. Method b: The goal and procedure are similar to those of method “a”; the only difference is the available means. If no laser tracker or similar equipment is available, then make some metallic templates of the selected area in two perpendicular set of planes respect to the surface (that is, templates parallel to the existing frames and in their perpendicular direction (that of the stringers)). These templates will be made of thin aluminium or any other material easy to work. The metallic plate will be adapted to the helicopter geometry in successive steps; as you can imagine, this is a very time consuming operation, and not as accurate as the laser tracker measurement, but reliable enough for any modification. Differences between the actual surface and that created from the templates are usually of the order of one mm. It is wise to create the template at the same location as the frames and stringers bounding the selected area. The more complex the surface is, the more templates are needed. Next step is to obtain the coordinates of some points of these templates (bear in mind that these template are 2-D elements); a gauge can be used for this task, and then transfer these coordinates to a 3-D CAD program. From this step, the work to do is exactly the same as that of method “a”. The only difference will be precision. This method is less accurate, but expect errors between 0.5 and 1.5 mm when comparing the actual helicopter surface with the one created by this method.

6.2.3 Actual stress distribution This is another important problem to face. It is relatively easy to determine how the new equipment increases the helicopter’s weight, because the modification company has got the data of these pieces of equipment (weight, centre of gravity location, external envelope, etc): but how stressed is the actual helicopter structure in the area where the new equipment will be placed? The obvious answer to this problem is to measure those stresses on the actual helicopter. However, this is so expensive that is out of the question for many modifications.

Pag.24

If a very sensitive area will be affected by the modification, and very sensitive area means the structure supporting the main gearbox or the tail rotor, then those areas will be flight-tested. As many strain gauges as needed will be located in the structural elements in order to get the real stresses of the structure under all representative flight conditions. It is an expensive procedure because more flights than expected will have to be done, the reason is the equipment recording the strain gauges data; unfortunately they do not have an infinite number of channels, in fact they do not have many, and in order to get a detailed knowledge of how the supporting structure is loaded, many strain gauges are needed and several flight conditions have to be evaluated. However, if those areas will be affected by the modification, it is wise to proceed in the aforementioned manner. For all other modifications, this is simply unacceptable from an economical standpoint and in fact, not absolutely necessary. But if you have enough economical resources do not hesitate to flight test the helicopter, as it is the only absolutely reliable method to know the structural stress distribution; if not, then the following assumption can be used. Assume the helicopter structure is loaded to its 100% capability. It means that metallic components are assumed to be loaded to their yield allowable stress (the component is not loaded beyond that because otherwise it will have yielded, and obviously that is not the case. Besides, there are compressive stresses in the helicopter and those buckling and crippling ones are usually lower than the yield allowable stress). Composite elements are assumed to be loaded to their allowable ultimate tension stress. This assumption implies two considerations:

a) Any modification structural design tends to look bulky, and in fact, it is. Because the actual stress distribution is unknown, a conservative approach has to be undertaken.

b) Engineering judgement and experience is needed, because if this assumption were applied to its last consequences, it would imply that if any tiny hole is drilled, the whole helicopter structure should be reinforced, and obviously, it is not needed.

How to practically apply this assumption will be subsequently shown. 1) Select the area where new equipment will be installed and make a Finite Element Model of this area and its surroundings. This area has to be as large as possible, regarding the loads the new equipment introduces in the helicopter. For a small antenna, reproducing the skin, a couple of stringers and the two frames bounding the antenna location are sufficient to get conservative results. For a chaff & flare dispenser located in the helicopter tail boom, it is required to reproduce one or two complete bays of tail boom. The purpose of this area is to show the stress distribution, so if you have selected too small an area, extend it. Unfortunately, it is not possible to check if the area selected is adequate until step 3) of this procedure has been completed. 2) At the centre of gravity of the selected area, enter a generalised load. It means a unit load in the X, Y and Z-axis and a unit moment around X, Y and Z-axis. Ignore how the helicopter structure is actually loaded, that’s why a generalized load is used.

Pag.25

Connect the centre of gravity by infinite stiffness bars to the modelled structural elements and run the calculations. A stress distribution will be attained. Then, the maximum stress has to be worked out. It is accomplished by taking into account the materials of all the modelled elements and the stress distribution. Extrapolate the results until the first element (considering its material allowables) reach its allowable yield stress (for metallic components) or its allowable ultimate tensile stress (for composites). The stress of all other elements has to be adjusted in this same ratio (that of the most loaded element, that is, yield stress/stress obtained in the FEM). Some compressive stresses will be shown in several elements; assume they do not cause either buckling or crippling. At this point we have two important achievements:

a) A conservative stress level in all the modelled structural elements. b) A stress distribution in the selected area. This distribution come from a generalised

load, so all the components of any conceivable load have been taken into account 3) The next step is to design the new equipment installation and, regarding the stress level and distribution obtained previously, reinforce locally the helicopter structure. The helicopter structure always has to be reinforced because the assumption is that the original structure is loaded to its maximum. When the design is finished, make a finite element model of the new installation and the same helicopter structure area as before (the same of step 2), however this will be now locally reinforced. Introduce into the FEM the new loads coming from the new equipment and run the calculation. Again a stress distribution will be attained. Now the following checks have to be done:

a) Verify that the elements of the new installation have positive margins of safety b) Verify that the reinforced elements of the helicopter structure have positive margins of

safety (bear in mind that the new obtained stresses have to be added to those adjusted in step 2)

c) Verify no buckling or crippling occurs in any element. If so, reinforce that element d) Verify the new stress distribution in the reinforced structure is similar to that of step 2.

The reason for that is to prevent the creation of fatigue problems. If the original stress distribution did not cause any fatigue problem or, at least, it was known when and where a crack will appear, and the new reinforced structure does not show a higher stress level and the stress distribution is similar to the original, it is reasonable to expect similar fatigue behaviour of the new structure (when discussing fatigue analysis report, it will be explained why some variances from this expectation occur, but so far, the reasoning is good enough)

At some distance from the location of the new equipment, the stress distribution has to be similar to that of the original structure. This is the reason why some extra area has to be modelled. If the selected area is too small, model some additional elements so you can verify the stress level at certain distance in both structures (original and modified one) are similar.

Pag.26

The following figures show how to introduce the unit load, the stress distribution before the structure has been modified, and the stress distribution after the modification (reinforcement) of an actual design respectively.

Figure 6.2.3-1. Unit load transmission from centre of gravity to structure Courtesy of Eurocopter Spain

Pag.27

Figure 6.2.3-2. Original structure stress distribution

Courtesy of Eurocopter Spain

Pag.28

Figure 6.2.3-3. Reinforced structure stress distribution Courtesy of Eurocopter Spain

Two of the main problems to be solved in any modification (the third one is fatigue, to be discussed later) can now be addressed. The structural design has to be substantiated with installation and detailed part drawings, static analysis, dynamic analysis and fatigue reports.

6.2.4 Material selection and documentation When selecting the material to carry out a modification, metallic materials are preferred to composites for the following reasons:

a) They are far cheaper, and considering that in many cases the number of helicopters to be modified is limited, the price of composites is a good reason not to select them

b) Allowable stress data is abundant for metallic materials and accepted by the certification authorities (ie data contained in MIL-HDBK-5); however, tests are required for composites to present acceptable allowable stress data to the authority

c) Metallic materials are easier to work and repair compared to their composite counterparts

Pag.29

d) Metallic components can be built with simple tooling or, even without any. On the contrary, composites need special tools to manufacture the parts. These tools are expensive and they require time to be built

e) Regarding the geometrical definition of the helicopter, depending on the method used, some differences between the actual helicopter surface and that obtained to manufacture the new required parts can exist. It is much easier to adapt a metallic component if required than a composite one

Therefore, if possible, select metallic materials. There are some exceptions if some helicopter characteristics must be kept (ie radar absorption requirements, etc). Otherwise, the following basic guidelines may be helpful:

a) Use aluminium 2024 where possible for formed components (because of its good fatigue characteristics); if not possible, 7075 is one alternative

b) For machined parts, utilise 7075-T6 or 7075-T7351 (better fatigue behaviour). 7050-T6 is an alternative

c) If some specific part is very loaded, and steel is required, use CRES (corrosion resistant steels), particularly if the helicopter is a naval one

Many other materials may be chosen, but those mentioned are readily available. Others will have a longer lead-time. The analyses required to validate the structural design are:

a) Static analysis report: This document should contain, at least, a brief description of the structural design, the materials and rivets, bolts, anchor nuts, etc allowable list, the load cases to be analysed, a description of the FEM and the checks carried out (if finite element analysis has been used), the load case analysis results, a summary result table (including stress level of the analysed parts, load case and margin of safety) and conclusions.

b) Dynamic analysis report: For most modifications it is enough to check the new designed parts will not have a natural frequency close to that of the helicopter. The helicopter frequencies to be considered are the main rotor angular velocity (A), the main rotor angular velocity times the number of blades (B), and the first, second and third harmonic of the latter, that is, (2XB, 3XB and 4XB). Obtain the natural frequencies of the new modified structure and compare these natural frequencies with ±10% of A, B, 2XB, 3XB and 4XB. The reason to use a ±10% range is to compensate for the simplifications the finite element model could have and some rotor angular velocity variations that may occur. There are some modifications where dynamic loads are the primary ones: that is the case when a machine gun is installed. In those cases, the higher stresses are caused by the dynamic load, and it has to be checked that these loads do not have an adverse effect on the structure and they do not coincide with the frequencies of A, B, etc

c) Thermal analysis report: If the structure in the engine surroundings has been modified, a thermal analysis is required to verify that no additional induced stresses have been introduced in the structure, particularly those due to restrained thermal expansions

d) Fatigue analysis report: A fatigue analysis has to be achieved to assess the “modification life”, and therefore set up inspection intervals, if necessary. The

Pag.30

problem is how to do it if the original helicopter structure actual stress level is not known.

There are two different situations when a modification is carried out from a fatigue viewpoint, namely:

a) The helicopter operator will fly the same type of missions as before the modification has been implemented

b) The helicopter operator will fly different missions because that was the modification goal. This is the reason why it is not enough to have a similar stress distribution in the modified structure compared with the original (see last sentence of paragraph 6.2.3-3d) to ensure no fatigue problems will arise, because the sequence of flights (fatigue spectrum) influences the structure fatigue response

In both cases, the fatigue life goal has to be established and it depends on the helicopter’s age and the missions to be flown. As a general rule, consider a typical helicopter life of 30 years, hence, find out how many average hours the operator flies per year, how old the helicopter is, how many different missions the helicopter flies each year and how many cycles (one cycle is one takeoff and one landing) the helicopter will accumulate per year. When the total number of cycles (up to the helicopter retirement) has been defined, apply a scatter factor of five to your structure, and therefore, your fatigue life goal will be: Fatigue life goal = Total number of cycles x 5 The reasons to select such scatter factor are the following:

a) Fatigue is a very variable phenomenon b) The fatigue spectrum (the missions and their sequences) is an estimation, and its

accuracy is also variable (and debatable on many occasions) c) No actual fatigue tests have been achieved

In other words, it is best to be conservative. The fatigue analysis is performed as follows:

a) Obtain the locations with higher stresses (thickness changes, holes, machined radii, etc) (this information can be found in the static stress analysis) and define the stress concentration factors

b) Apply the fatigue loads for each load case (mission); (they are usually different to the static loads) and attain the stresses at those locations

c) Apply the Miner’s rule to get the maximum number of cycles your structure can withstand (it means that at this number of cycles a crack will appear at this location)

d) An alternative method is to use the fatigue formulas shown in MIL-HDBK-5 for several materials. In this case, the concentration factor of the location under study has to match with that of the MIL-HDBK-5 and the stress ratio (R = minimum stress/maximum stress) will be determined

When the fatigue analysis is finished, the fatigue life goal has to be compared with the number of cycles the structure can withstand. That is, a kind of margin of safety can be defined

Pag.31

MS = (Fatigue life goal/cycle number) -1 If this value is greater than cero (0) the designed structure is valid from a fatigue standpoint, otherwise, it has to be redesigned. As a “rule of thumb” estimate that a 20% load increase reduces the fatigue life by approximately 50% . There is a point missed in the above procedure: the actual stresses in the areas of the helicopter structure affected by the modification are unknown. Then, how can a fatigue life be estimated? The response is to use a very conservative approach. Consider the affected structure is loaded to its maximum when static load is applied (as defined for static analysis). Divide the fatigue loads by the static load and apply this ratio to the structure stresses. This assumption is conservative, too much if you wish, but it is hard to refine further the reasoning, particularly if new missions will be flown. In many occasions, this assumption leads to the setting up of quite tight inspection intervals. If after a few inspections no damage has been detected, they can be relaxed after careful engineering judgement.

6.2.5 Weight and balance The modification impact as far as weight and balance are concerned has to be determined and substantiated in the “weight and balance report”. The weight of the modified helicopter will not exceed the approved maximum takeoff weight (MTOW). The centre of gravity positions will remain within the approved limits, both longitudinal and lateral, after the helicopter has been modified. Make a table including the weight and movement of every piece of new equipment, the supporting structure (weight of this structure can be estimated between 15 to 30% of supported equipment weight) and the electrical harness. Make another table, which includes the structure, equipment and electrical harness removed due to the modification, if any. Subtract the added weight from the removed and follow suit for the moment. The result will be the modification weight and balance impact. Using the previous result, perform weight and balance calculations for any possible flying conditions (different number of passengers, different fuel quantity, rescue hoist removed and installed, stretchers, etc), at least, for those shown in the original flight manual (weight and balance chapter), and make sure the helicopter weight does not exceed the MTOW and the centre of gravity positions remain within the approved limits set up in the flight manual. Should any centre of gravity position lie outside the approved limits, some limitations should be specified in the flight manual supplement or, if possible, some ballast may be added to keep the centre of gravity within limits.

Pag.32

In case the helicopter has undergone a heavy modification, set up a weighing test in the functional and ground test procedure in order to attain a precise centre of gravity position and an accurate helicopter weight. 6.3 ELECTRICAL DESIGN When a new equipment or system is installed in a helicopter, the first step from an electrical design standpoint is to assign it to one of the electrical bus bars depending on its characteristics (ie it has to be connected to the emergency bus bar if its operation is vital for the helicopter safety) and check that the selected bus bar is able to power the new equipment. If so, the electrical design is then completed and documented. The documentation to be issued is:

a) Electrical drawings b) Electrical load analysis

Two different types of electrical drawings have to be created, namely:

a) Pin to pin diagrams b) Harness routing

Harness manufacturing drawings have been omitted and the reason will be explained later. Pin to pin diagrams These are the drawings showing all the interconnections among all the elements comprising the installation and the helicopter interfaces (bonding connections, bus bar connections (at breaker panels), etc). The designer will follow the recommendations of the system manufacturer to interconnect the equipment and will use the company electrical design manual, if any, to design the harnesses and the interfaces with the helicopter. If there is no an electrical design manual issued by the company, then, the following standards can be utilised as guidelines: MIL-W-5088 and MIL-B-5087. The former shows how to select wires and carry out an electrical design, and the latter is involved with electrical bonding and harness routing standard practices. All the wires have to be numbered with an alphanumeric code. There are several standards, so the codification varies depending on the standard used. In any case, the codification will inform about the system type (communications, navigations, power, etc), the wire gauge, the electrical phases, if applicable, etc A part list mentioning all the necessary material to build those harnesses depicted in the pin to pin diagrams has to be added to each particular diagram, and also some drawing notes concerning harness manufacturing and installation should be added to the pin to pin diagrams. It would not be necessary if harness manufacturing drawing were issued; unfortunately, this is not possible in most modifications. The reason is that in order to create a reliable electrical harness manufacturing drawing, the harness length and routing and electrical connector keyway positions are needed, and this information will be on electrical drawings not available most of the time, or, for helicopters

Pag.33

designed from 1990 to the present, in 3-D files where the whole helicopter is depicted (fuselage, equipments, electrical harness routing, hydraulic routing, pneumatic system routing, seats, flight controls, etc). Undoubtedly, if this information is available, then manufacturing drawings should be issued because all the information is known. This is never the case, therefore, from a practical viewpoint; it is more reasonable to route the harness directly in the prototype (study the possible routing in the prototype, build the harness with generous extra length, install the connectors at one side of the harness, and afterwards, route it inside the helicopter, when the installation has been finished, complete the installation of the remaining connectors at the other side of the harness) and document it. Harness routing As mentioned previously, the location of the harness along the helicopter has to be known by the electrician to install it, by the maintenance personnel to inspect or repair it when necessary, by the designer to make sure there is no interference (mechanical or electromagnetic) with any other helicopter element, and by the certification authority to ensure the design is safe. Besides, if the modification will be applied to other helicopters, all of them should have the same harness routing. Those are the reasons why harness routing drawings have to be produced. It is very hard, time consuming and expensive to reproduce the entire helicopter (fuselage, equipments and system routing) to design a reliable 3-D harness routing, therefore, it is customary to depict the routing in simple helicopter sketches (just showing the external contours in a lateral, plant or isometric view). It is good in order to have a rough idea what the electrical harness position is, but it is not useful from a practical standpoint, because the lack of detail (normally the harness is shown alone, and it is usually routed with other harnesses, no distances to structure or other elements are shown, no specific information about slack or harness bend radius is normally provided). Therefore, I suggest editing a first issue of harness routing drawings as mentioned above, but immediately after the prototype harness routing is completed, editing a revision of those drawings. This new issue will be made up of actual electrical harness photographs, and where appropriate, insert some dimensions or notes about bend radius and slack on the photos. Add as many photographs as possible and try to show the harness surroundings (frames, other nearby system routings, pieces of equipment, etc) in order to help in future installations or maintenance tasks. Before the electrical load analysis is dealt with, a few basic comments concerning electrical design will be included:

a) Breakers: Electrical breakers are provided in any installation to protect the wires, not the equipment. The equipment is protected internally from a voltage surge by a fuse; if they are not protected, then that particular equipment does not comply with aeronautical standards (both military and civil)

b) Electrical bonding: Follow the recommendations shown in MIL-B-5087: it will prevent many safety and functioning problems. Try to use, if possible, the bonding provisions of the helicopter. If this is impossible, design and install those needed and identify them (following the same philosophy as that of the existing bonding provisions) on the pin to pin drawings and the wiring manual supplement

Pag.34

c) Audio equipment: When an audio system is installed in a helicopter (new radio, satellite telephone, loud speakers, etc) it has to be connected to the helicopter audio box in order to be used by the crew or the passengers. It is quite frequent that the system does not work as expected (the volume is too low or too loud or there is too much noise, or simply nothing is heard). The reason is that the new system impedance has to be adapted to that of the helicopter audio system. Therefore, the first step is to check the new installation to make sure it has been designed and installed without any flaws. Then, check the helicopter audio box; most of the time the problem is there. The impedances have not been adjusted.

Electrical load analysis As mentioned previously, the new system has to be assigned to a specific helicopter electrical bus bar depending on its category. Then it is time to verify that there is enough power available to switch it on and operate it. The question is: how can we know it if the helicopter electrical consumption is unknown? That is what the next paragraphs will try to show. The first thing to do is to know how many KVA or Kw the helicopter’s electrical generation system is able to provide. This data is obtained from the maintenance manuals (how many generators are installed, what their power is, how many batteries exist, how many amperes per hour they are able to provide). Next action is to know the electrical consumption of the new equipment and how it is distributed. In other words, how many amperes are needed to switch it on and for how long, how many for warming up (if needed), and how many for nominal operation? At this point, two different situations can occur, namely:

a) The helicopter operator (customer) possesses the electrical balance report of their helicopter provided by the manufacturer

b) The helicopter operator (customer) does not possess the electrical balance report In the first case, the work to do is simple, because the helicopter’s electrical consumption (and the time for each condition) is known. So, add the new equipment electrical consumption for each particular condition (switch on, nominal operation, etc) to the table of the affected bus bar. The electrical balance report will mention how many amperes are available at this bar. Just subtract the total bus bar consumption for each condition, including the new equipment, and the result will show if there is enough power available to install the equipment in that bar. In the second case, the solution is not that easy, and two different methods can be used, but some uncertainty will remain until the ground, EMI/EMC and flight tests have been completed.

a) Measure the helicopter electrical consumption on the ground b) Make an assumption

The first method is easy to implement, the other is quite complex; it depends on the design of the helicopter electrical distribution system. And it is always very complex to do it in flight. It consists of operating all the equipment and systems connected to the same bus bars that could be working at the same time, in order to record their consumption. Because the total

Pag.35

generation power is known, the difference between generation and consumption will show if it is possible to install the new equipment in the selected bus bar: it implies that all the bars have to be analysed. Check that the electrical consumption of this bar, when the new equipment has been added, does not exceed the level of bus bar protection. It can happen that more equipment than that operated on the ground could be operated in flight and the measurement will not be accurate, even non-conservative. Therefore, set up functional and ground, EMI/EMC and flight tests to verify the bus bar is able to power the new equipment. The second method is an assumption that should be confirmed during the ground, EMI/EMC and flight tests. This assumption is based on the following procedure:

a) Search for the bus bar protections (breakers, relays, etc) b) According to their protection capabilities (how many amperes they can withstand), set

up the theoretical bus bar capacity, that of its protection device. This is not the actual bar capacity, because these devices are designed to protect the bar from voltage surges etc, but it will be assumed as the real bar capacity

c) Add the electrical consumption of all the equipment installed in that bar, including the new additions. Most of the time, the only available information concerning electrical consumption is that of the nominal operational condition

d) Make sure the overall equipment electrical consumption does not exceed 80 to 85% of the bus bar theoretical capability

It is evident that some degree of uncertainty exists, particularly because no information about equipment switch on consumption is known. Therefore, to be absolutely sure that the design is sound, set up tests in the functional and ground, EMI/EMC and flight test procedures. They are as simple as operating all the equipment connected to the same bus bar at the same time, if this is a realistic condition, or to reproduce in ground and flight the real operation conditions and check if the bus bar is able to power the new equipment. 6.4 AERODYNAMIC AND PERFORMANCE DESIGN Aerodynamics, except that concerning the rotors and tail strakes, is not so important for helicopters as for aeroplanes, because of the speed of the former. It implies that very sophisticated analyses are usually not required for most modifications; however, some care has to be exercised. There are two issues to be considered from an aerodynamic standpoint in a modification:

a) Drag b) Vortex trajectories

Drag:

Pag.36

Drag of devices installed externally will be determined for the worst (maximum aerodynamic load) case. Evidently, this load has to be worked out to design the structural supports and the appropriate reinforcements in the helicopter airframe. Lift of most devices is negligible, an exception to the previous statement are some external loads, so it can be ignored. There are many different types of equipment to be installed in the helicopter exterior with many different shapes, so it is impossible to define a single procedure to assess drag. However, some guidelines will be presented for some typical elements. Antennas: Drag is obtained from the very well known formula

Cv dSD 2

21 ρ=

Be conservative when applying it because many existing aerodynamic effects cannot be considered (aerodynamic interference, main rotor downwash effects, etc), and it is worthless to do it; therefore, although not strictly correct, use the sea level air density, helicopter never exceed speed, the maximum surface the antenna can see in any flight direction and the appropriate Cd; if Cd cannot be obtained in an easy manner, use a value of 2. If a more refined analysis is needed, the following guidelines can be regarded. In FAA AC 43.13-1B (Antenna installation) the stated formula can be seen in a generalised manner for any antenna type, disregarding its shape. Round and flat antennas (logarithmic antennas), typical of radar warning systems, can be assumed as flat plates, therefore, Cd = 2 Blade antennas can be analysed as wings without torsion. In order to determine Cd, the blade antenna airfoil has to be known; if this is not available, use a NACA 0012 airfoil, error will not be excessive. Bear in mind that helicopters can slip in any direction; therefore, the area this type of antenna can show to the incident air may be their lateral profiles. Laser warning sensors: These sensors (little cameras able to detect any laser beam) can be considered like flat plates from an aerodynamic viewpoint (Cd = 2) Missile launching warning sensors: These sensors can also be considered like flat plates (Cd = 2). They are cameras able to detect the ultraviolet radiation from the missile engine exhaust plumes. Rescue hoists: Some of them are endowed with aerodynamic fairings, others are not. Those fairings can be approximated, in many occasions, to geometrical shapes analysed in many aerodynamics’ text books (spheres, cylinders, cones, etc). Obtain the Cd and S values defined in those texts and multiply them by the sea level density, 1/2 and the helicopter never exceed speed. The result will be a good approximation of the actual value.

Pag.37

For those rescue hoists without fairings, consider it is a flat plate (Cd = 2) and determine the area (S) of a cross section normal to the forward flight direction. Multiply Cd and S by the sea level density, 1/2 and the helicopter never exceed speed and an acceptable drag value will have been obtained. Rescue hoist inertial loads are usually much more important than aerodynamic ones, and they drive the structural design, but in any case, the check has to be done. Forward-looking infrared sensor: Those sensors are similar to a vertical cylinder ended in a half sphere. Cd can be obtained from any aerodynamic textbook for a shape like that. External fuel tanks: Elements with shapes similar to those of external tanks have been extensively studied in many aerodynamic texts. It is quite simple to determine drag loads for these elements because of the available literature. For several reasons, these tanks may be released in flight. And although extensive flight tests are required to validate their behaviour when released, make some theoretical analysis to be sure, at least as sure as possible, that they do not create lift. Otherwise, when released from the helicopter they could impinge on the airframe due to the aforementioned lift force. Torpedoes and missiles: The same as above apply to these components. However, the release process is more complex than for fuel tanks because of missile engine start interaction. Wind tunnel tests and CFD (Computer Fluid Dynamics code) analysis are required to gather reliable aerodynamic data and flight tests are needed to validate the torpedo and missile release process. Vortex trajectories: To determine vortex trajectories is not an easy task, a CFD code is needed and even with the help of this powerful tool the results are sometimes only approximations of reality, because some geometrical or aerodynamic conditions are hard to properly model (particularly the main rotor influence). Vortices can be dangerous if they impinge on the tail rotor, the horizontal stabiliser or the vertical stabiliser. Therefore, in the light of the external element size and position and helicopter configuration, some engineering judgement and experience is needed to decide if the work is worth being done or can be skipped. If any doubt exists, flight-test the problematic configuration. It is obvious that tandem rotor helicopters are less prone to be affected by vortices because they do not have either vertical or horizontal stabilisers; besides, the downwash influence of both rotors deflect vortices away from them.

Pag.38

For conventional design helicopters, it is this author’s experience that any piece of equipment installed under the area washed by the main rotor is quite unlikely to cause vortices problems. If possible, place the voluminous items on the other side of the tail rotor. In any case, define some manoeuvres to be achieved during the flight tests to be sure no problems arise. These manoeuvres are descends at several descend speeds including autorotation, turns at several bank angles, etc. The documentation substantiating the aerodynamic analysis is usually a report called “Aerodynamic analysis report”; it will contain the following information, as a minimum:

a) Installation description b) Assumptions and simplifications (helicopter geometry, etc) c) Analysis cases d) Finite element model description and checks (if applicable) e) Drag load values and directions f) Vortex trajectory picture, if applicable g) Conclusions

Note: As mentioned in the first paragraph of this subchapter, rotors and tail strakes are excluded from these guidelines, since they require a very detailed aerodynamic analysis and many flight test hours to be validated. Those analyses involve CFD and wind tunnel tests. Performance of a helicopter is defined by the formulas found in any undergraduate text. From a practical standpoint, for most modifications it is very hard, if possible at all, to appreciate any change in the helicopter’s performance. There is a noticeable exception to this, which is the Vne. This speed varies appreciably in some cases. Some examples are the forward looking infrared cameras installed in many military and Para public (Police, fire fighting, etc) helicopters. This speed is reduced between 4 to 10% compared with the non-modified helicopter Vne, depending on the camera size, installation arrangement and helicopter configuration. It is not worthwhile to determine theoretically this possible reduction; the most straightforward manner to obtain it is to fly, progressively increasing the forward speed: when the vibration level inside the helicopter is considered to be unacceptable, that will be the new (reduced) Vne. If deemed necessary due to the modification, a performance analysis can be issued. This report will include the following information, as a minimum:

a) Brief installation description b) Assumptions and simplifications c) Cases analysed d) New values obtained for the parameters studied (Vne, descend speed, ascend speed,

etc) e) Conclusions

Pag.39

Helicopter stability cannot be assessed analytically unless a lot of engineering information is available (helicopter inertia matrix, mass distribution, static and dynamic stability flight test results, etc). In other words, only the helicopter manufacturer possesses that information and it is extremely unlikely they share it with any other company. Therefore, if static and/or dynamic stability is a concern because of the modification undergone by the helicopter, the only practical action to take is to flight test the aircraft. Static and dynamic stability flight tests will be defined in the flight test procedure. Static stability is the tendency of the helicopter to return to its undisturbed equilibrium position when the disturbing force causing the departure from that equilibrium position has been removed. Static stability determines the short-term response of the helicopter to a disturbance. Dynamic stability determines the long-term response of the helicopter to that disturbance. In other words, a helicopter is dynamically stable if after removing the disturbing force, it comes back to the equilibrium position. Therefore, to assess static stability, specify tests where disturbances are caused by forward speed variations (increased speed) and pilot induced manoeuvres (collective control pull up) and verify the helicopter tendency to return to its equilibrium position. The same applies to dynamic stability assessment: induce disturbances by control inputs, release them and check the helicopter behaviour until it reaches the equilibrium position again. If the helicopter does not return to its equilibrium position after a time ranging from a few seconds to a few minutes, depending on the stability mode checked, the modification has caused some instability and it has to be further investigated to solve the problem or, if not possible, to set up certain limitations to be collected in the flight manual supplement. 6.5 SYSTEMS DESIGN Many helicopter systems can be modified, and some of them will be installed as a complete new system instead of modified (ie, installation of air conditioning systems is usually asked for by the customer, because most helicopters are not endowed with them when they leave the factory). It is impossible to write in detail about all the potential modifications every system can undergo, a specific book would be needed for each one. Therefore, only some basic comments will be done for some selected systems.

6.5.1 Fluid systems (hydraulic, air conditioning, anti-icing, fuel, etc) The first consideration to be taken into account is to make sure the helicopter is able to provide enough power to operate the modified or new systems. If not, some limitations have to be set up in the flight manual supplement or additional power sources need to be envisaged (this can lead to dramatic changes: for example switching the current electrical generators for more powerful ones. It can require changing most of the helicopter electrical system, so apply engineering judgement when necessary). When installing air conditioning systems the above statement is particularly true. These systems can be electrically or mechanically (from the main transmission) powered. In hot climates the engine performances are degraded, therefore, some bitter surprises might occur:

Pag.40

depending on the engine and the air temperature, in certain conditions the air conditioning cannot be used (usually when it is hotter outside). Hydraulic systems are seldom modified; it is more likely they are needed to operate new equipment (like a big radar antenna, for instance). If this is the case, design a complete independent hydraulic system with its own pump, deposit, heat exchanger, etc in order to avoid affecting the original helicopter hydraulic systems. Independent system actually means as independent as possible because the new system has to be powered from the electrical helicopter system. Apply the same judgement as mentioned for air conditioning systems. The remainder of fluid systems do not require so much power, but in any case, the power availability has to be checked.

A document describing the system components, system design, system operation, limitations (if any) and growing capacity (design always for some extra performance) has to be issued. This document is the system functional analysis (hydraulic, pneumatic, fuel, fire extinguishing, etc). Besides, system structural, safety, electrical, fire protection, component qualification etc analyses have also to be produced.

6.5.2 Mechanical systems These systems have also to be structurally analysed, and of course, all other considerations have to be taken into account (component qualification, safety analysis, etc), but it is also very important to perform a kinematical analysis for two main reasons:

a) To make sure they deploy as foreseen (some deployment positions may have been overlooked and they might cause some problems)

b) To assure no interferences will occur between the moving mechanisms and other equipments or structures (ie, a retracting landing gear will not impinge and deform any valve, hydraulic hose, door, etc) causing unpredictable consequences

3-D design programs (Catia V5, or V4 and any other similar) are very useful and easy to use when performing those analyses. These analyses have to be documented in a system kinematical analysis report. As mentioned at the beginning of chapter 6, those modifications affecting the rotors, transmissions and other flight controls should be done with the cooperation of the helicopter manufacturer.

6.6 NOISE It is quite unlikely that most helicopter modifications can cause noticeable noise level increases; helicopters are noisy enough by themselves. Effective noise reductions can be attained during the helicopter development phase (main rotor design and materials, main gear box design, fuselage, etc) and this is out of the scope of this book.

Pag.41

However, when for any reason the helicopter passenger cabin internal liners are removed and replaced by new, different liners , the helicopter owner sometimes expects some noise reduction, especially if the helicopter is very noisy. Unfortunately, although some decibels may be reduced, marginal noise reduction can be achieved by this method, that is, utilising passive components (such as the stated liners). The reason is that the required thickness, apart from a careful selection of the isolation materials, of such a sound screen is too big (impractical for any aircraft) to get appreciable noise reductions. The only effective method is the active one, that is, a system detecting the noise emitted by the helicopter and generating the opposite sound wave in order to cancel it. The operation concept of this method is evident, but the practical implementation is not that easy, and although much research is being done in this field, a lot of work has still to be carried out. If required, record the noise inside the passenger cabin before the modification is implemented, and after it has been achieved, and issue a “noise analysis document” where both records will be compared. Unfortunately, not much can be done yet, as far as noise is concerned, when a helicopter is already in use, unless active noise control systems be utilised, and even in that situation the results may be not as foreseen. 6.7 ADDITIONAL DOCUMENTS FOR CERTIFICATION There are some documents difficult to classify in any of the above subchapters but necessary for the modification certification process. These documents are:

a) Modification description b) Master drawing list c) Helicopter configuration report d) Equipment qualification report e) Modification check list f) Service bulletin

Modification description: This document depicts the modification. It states in which helicopter the modification takes place, which new equipment is to be installed (models and part numbers), what their mechanical and electrical interfaces with the aircraft are, where they are located, what their weight is, how they work, what their electrical consumption is, which is the electrical bar that they are connected to, how they are protected (electrical breakers), and any other relevant information. It is very helpful for a clear understanding to provide as many sketches or drawings as possible in that document. This document is one of the first to be sent to the certification authority. It will help them to become familiar with the proposed modification.

Pag.42

Master drawing list: This document is basically a list with all the drawings (from subassembly level to the modification top drawing). The list states the drawing number, drawing title and drawing issue. This document is the one defining the approved modification configuration. Helicopter configuration report: This document describes the whole helicopter; that is, what type of engines it has got, what fuselage part number, what blades, rotors and dynamic components, landing gear, avionics, electrical generators, battery(ies), lights, and any other system (hydraulic, APU, etc). Every element has to be listed and identified by its part number. Do not forget to include any optional that could be compatible with the modification (rescue hoist, cargo hook, NVIS, EAPS, skis, armament, etc). The certification authority, when issuing the modification approval certificate (STC for civil helicopters), will mention that the modification is only valid for helicopters having this exact configuration. Equipment qualification report: It has to be demonstrated that the new installed equipment is able to withstand the helicopter operational environmental and EMI conditions. This is realised in this document. A comparison has to be established between the environmental and EMI conditions seen by the helicopter and the tests passed by the equipment. If no data about helicopter environmental conditions are available, use those of MIL-STD-810 for military helicopters and DO-160 for civil ones. This is the reason why it is so important to demand from the equipment supplier all the information possible concerning the equipment qualifications (vibration levels, humidity, temperature, etc). If it is not demonstrated that the equipment can withstand or exceed the helicopter conditions, the certification authority may demand the performance of some tests of the equipment with a high cost and unpredictable consequences. Modification checklist: When the modification airworthiness has been demonstrated to the certification authority, in order to wrap up the process, it is customary to issue a document where all the paragraphs used to certify the modification are compiled and the documents where the compliance is demonstrated explicitly referenced. A sheet of this document may be similar to the one shown below:

Pag.43

COMPANY LOGO

DOCUMENT TITLE DOC. Number: DOC. Issue: DOC date:

Certification paragraph: Write down literally the applicable paragraph Reference documents: List all the documents utilised to demonstrate the aforementioned certification paragraph Remarks: State briefly how the certification paragraph has been demonstrated Modification company project manager signature

Modification company system responsible signature

Blank Certification authority project manager signature

Certification authority specialist signature

Service Bulletin: This document describes how the modification has to be installed in a helicopter. When it has been approved by the certification authority, the modification can be applied to any helicopter defined in the “Applicability” paragraph. Normally, applicability is established by mentioning the helicopter serial number or saying that “this service bulletin is applicable to all helicopter having the same configuration as that defined in document number XXXX (the helicopter configuration report)”. For more details about this document see subchapter 7.1 Service bulletin. 6.8 MANUAL SUPPLEMENTS The manual supplements to be issued in any modification are:

a) Flight manual supplement b) Maintenance manual supplement c) Wiring manual supplement d) Illustrated Part Catalogue supplement

Pag.44

Flight manual supplement: The flight manual supplement will follow the same pattern as the original flight manual. If there were no change in any particular section, it will be explicitly mentioned that there is no change at all, and the original flight manual has to be followed. A flight manual supplement structure could be something like:

a) General b) Limitations

a. Helicopter configuration limitations b. Minimum crew c. Altitude limitations d. Temperature limitations e. Speed limitations

c) Warnings d) Normal procedure

a. Checks before flight b. Equipment basic operation

e) Weight and balance f) System description

a. General b. System capabilities c. Location

g) Basic servicing

General: It could be a very brief description of what new equipment has been installed and what its main purpose is. Limitations: Any limitation obtained during the certification process (flight test, EMI/EMC test, analysis, etc) has to be explicitly stated in this chapter. Some typical limitations are: Helicopter configuration: For certain operations some determined configurations are mandatory, otherwise, the helicopter is not allowed to perform those operations. Minimum crew: For some operations two pilots (pilot and co-pilot) are required Altitude: Some particular equipment might limit the maximum altitude a helicopter could fly Temperature: Some particular equipment might limit the maximum or minimum temperature a helicopter could operate in Speed: External loads and protuberances (forward looking infrared cameras, for instance) limit the Vne (Never exceed velocity) of the helicopter

Pag.45

Warnings: Any precautions the flight crew should heed Emergency procedures: This chapter describes how to proceed in case an emergency took place on board the helicopter due to the new systems or equipment installed. Normal procedures: This chapter describes how to proceed under normal conditions. Check before flight: It has to be stated what tasks (checks and how to achieve them) have to be carried out before any flight can take place, due to the installation of new equipment. Equipment basic operation: It will be described how to switch on and off the new installed equipment. Weight and balance: A complete list, including part number, of each and every piece of equipment, structural (supports, fixed provisions, et.) and electrical (harness, relays, etc) component has to be included in this chapter. The weight and moment caused by each component has to be defined, to show the total modification weight and moment. System description: A brief description has to be included in the flight manual supplement, but bear in mind that this document is to be consulted by the crew in flight, so it has to be very concise. General: It can be a general description (what equipment has been incorporated in the aircraft and what for). System capabilities: All the capabilities of the new equipment have to be described in this subchapter, and briefly, how the equipment is operated to attain the aforementioned capabilities. For comprehensive information, the equipment operational manuals have to be referenced. Location: Where the equipment is located around the helicopter. It has also to be mentioned if any access exists to check the equipment in flight. Basic servicing: If any basic servicing task can be done by the flight crew to improve the equipment’s performance (eg, cleaning the lens of a camera), it should also be mentioned. These tasks are not maintenance operations.

Pag.46

Maintenance manual supplement: This supplement will allow the customer to keep continuous airworthiness of the modified helicopter. The information this supplement should contain is the following: Equipment: What the equipment maintenance concept is, that is, if they are “On condition” or have scheduled maintenance or it is based on retirement life. What maintenance tasks have to be performed, with what means and how they have to be implemented for “O” (organisational) maintenance level. For “I” (intermediate) maintenance level, what maintenance tasks have to be performed and with what means, if this level can be achieved by the customer. If not, the address and contact person of the companies able to perform this maintenance level (they are normally the equipment manufacturers). For “D” (depot) maintenance level, the same information as for “I” level. Structure and mechanical components: The maintenance manual supplement has to inform about the inspections the new structure has to undergo. These inspections intend to detect cracks and corrosion problems the structure may encounter during its life. As far as corrosion is concerned, the type of inspection, the inspection periods, the means to perform these inspections and the levels (location and extension) of negligible, repairable and unacceptable corrosion damage will be defined for all relevant structural components. For repairable damage, the procedures (actions and means) to achieve the repair have to be defined. Crack inspections are derived from the structural analyses carried out during the modification structural design phase and tests. In the same way as for corrosion, the type of inspection (visual, NDT, etc), the inspection periods, the means to perform these inspections and the levels of negligible, repairable and unacceptable damage will be defined for all relevant structural components. Procedures will be set up for repairable damages. Mechanical components will provide the same information about corrosion and crack inspections as that of structural elements; also wear out limits have to be defined. Any additional action to be taken in those components (greasing, or any functional test, for example) will be defined. To define such actions means to set up the action to be achieved, the moment to do it (before the flight, after the flight, etc), the required means (material and human resources), and the criteria to know if the check has been successfully passed.

Pag.47

Electrical harness and components: Electrical components are usually “on condition” elements, but it has to be explicitly mentioned. It has to be defined what damage (location and extension) is negligible, repairable or unacceptable for each and every electrical harness due to the modification. When the damage is repairable, the procedures to do the repair have to be clearly set up. Actions to be taken under special conditions: What to do after a lightning strike or a collision of a truck with the helicopter and so on. Wiring manual supplement: This supplement will gather all the pin-to-pin diagrams elaborated for the modification. It will allow the customer to know all the new electrical connections and be able to keep continuous airworthiness of its helicopter from an electrical standpoint. Because there is much information on any pin-to-pin diagram, try to edit them in a suitable format to ease the reading of any code (particularly those of the wires and the equipment connections). Illustrated part catalogue supplement: The IPC (Illustrated Part Catalogue) supplement will be a collection of exploded 3D isometric views of the new installation. Each element will be labelled with a number and in an attached list; those numbers define the element part number and its title, as a minimum.

Pag.48

7. PROTOTYPE INSTALLATION

When the modification has been designed and the parts manufactured, the next step is to install all these parts and the new equipment (radios, cargo hooks, engine air particle separators, countermeasures systems, stretchers etc). From this author’s standpoint, this is the most stressful part of the whole project for several reasons, among others:

- The raw materials will not be on time, they will come late (very late, in fact) and some will not be the part numbers you required

- If the equipment is not off-the-shelf, in other words, it is a new development, be absolutely sure that it will not work by the time the supplier has assured you, several times, that it would be handed in to you

- If the equipment is imported, customs will take a longer time than expected to clear the documents

- For electronic equipment, by this time you will realise that some piece of equipment that you had never heard about is absolutely necessary to test or operate the electronic equipment and it costs a lot and has an unacceptable lead time

- Be sure somebody will make a mistake (will drill the wrong skin, will install the right support in the wrong place, etc). So time has to be spent in repairing the damage and manufacturing a new part

- The customer will change the prototype helicopter in the last minute due to operational reasons, and the new helicopter will be similar to the one you used to design the modification, but similar is not the same as identical. In other words, many parts will have to be adapted (more time and cost)

- Because there are usually some economical milestones associated with prototype completion, everybody forgets that the installation is on a prototype (some changes should be expected) and any delay is a sin or a crime.

- Customer anxiety. Now it can be seen that the modification is a real thing and the modified helicopter becomes the most important of the fleet. It has to be operative yesterday

- Any others you can figure out (and as you guess there are a few) Concerning the documentation that has to be produced, it is restricted to two documents, namely:

a) Service Bulletin b) Work Order

Service Bulletin: This document has to be issued for two reasons. The first one is because the certification authority approves it and it means that the modification can be applied to all the helicopters having the same configuration as that of the prototype. The second is a commercial reason; once the service bulletin is approved, it can be offered to other customers with helicopters with the same configuration as that of the prototype.

Pag.49

Work Order: This document describes all the tasks to be carried out in order to implement the modification. This is an internal document of the modification company and it is for exclusive use of the workshop. Therefore, all the installation tasks have to be achievable with the means existing at the company. If the work is subcontracted to another company, the service bulletin is the document required to achieve the job, because it states what actions have to be done to accomplish the tasks, but it does not say anything about the specific means to do it. An example will make clear this point. Suppose some skin has to be riveted to a number of frames, the service bulletin will say that the skin P/N XXXX has to be riveted to the frames P/N’s XXXX using so many rivets P/N XXXX according to the distribution shown on drawing XXXX. The work order will mention all of that plus the procedures to install the rivets (it could be manually according to the company procedure XXXX, or manually according to the procedures shown on FAA AC 43-13-1B or using an automatic riveting machine, if available and cost permits it). As stated above, some companies have developed internal procedures to perform several workshop tasks (riveting, painting, electrical harness construction, etc). Therefore, when writing the work order these procedures have to be mentioned; others do not have those internal procedures; in that case, it is acceptable to mention those of FAA AC 43-13-1B. 7.1 SERVICE BULLETIN The structure of a service bulletin is well defined and it has to contain, at least, the following information:

a) Scope: What the goal of the service bulletin is b) Applicability: In which helicopters (serial numbers) the service bulletin can be applied c) Description: A brief description of the modification d) Category. It can be Optional, Mandatory, etc e) Approvals: What certification authority has approved this service bulletin f) Continuous airworthiness: What maintenance manual supplement has to be applied to

assure continuous airworthiness of the aircraft g) Reference documents: All documents needed to complete the information contained in

the service bulletin (installation drawings, for instance) h) List of materials: All the materials and equipments needed to carry out the

modification, including consumable items i) List of special tools: All the special tools (if any) required to achieve the modification j) Test equipment: List all the test equipment needed to check the proper functioning of

the new installed equipments k) Work instructions. Describe, in a very detailed manner, all the tasks to be carried out

to complete the modification from all standpoints (structural, electrical, tests, etc) l) Verification tests: Specify the required test to validate the modification in any series

helicopter. These tests are not the same as those undergone by the prototype. They are usually functional tests only

m) Weight and balance information: Provide information of the weight increase of the modification and its impact in the weight moment of the helicopter. Also mention any possible restriction

n) Electrical consumption information: Provide information of the new equipment electrical consumption, in stand by mode, nominal operation, switch on period and any other relevant operation state

Pag.50

o) Log book: It has to be mentioned how (the exact wording) the service bulletin has to be recorded on the log book

7.2 WORK ORDER A work order has basically the same structure as that of a service bulletin, but there are some differences. The information to be provided on a work order is the same as that on a service bulletin but in more detail, because it has to take into account the means of the company. The chapter regarding work instruction has to describe how to perform the tasks mentioned in it, and a three-column table has to be issued. The first column describes the task to be performed (ie, to paint some areas of the helicopter), the second column is a blank area for the technician’s signature (the painter in this case), and the third column is another blank area for the quality assurance responsible validating that the task has been properly achieved. An additional chapter, compared to a service bulletin, has to be added to describe the inspections to be carried out in the helicopter when delivered by the customer but before the modification starts, in order to reveal any discrepancy and inform the customer about it. It is also needed to set up the type of inspections to be carried out when the modification is finished and prior to delivery of the helicopter to the customer, in order to detect any FOD or discrepancies of any kind. Finally, another chapter is required to list all the deviations and concessions that may have arisen. Deviations: Deviations are those departures from the design documentation (drawings) defining the modification. Those departures can arise for many reasons (there are some delays in receiving the specified bolts, rivets, nuts, electrical wires, etc and an alternative has to be used, there are mechanical differences in the helicopter for whatever reason, etc). These deviations have to be accepted by the customer, or at least, they have to be informed. Concessions: You can be sure that in a prototype installation some error will take place. Concessions are authorisations by the customer to use those damaged parts not detrimentally affected in their airworthiness capability. When the work order is issued (and the installation drawings), the parts are manufactured and all the special tooling, if required, is available, the process is to follow the work order instructions and to accomplish the modification. All the processes (riveting, sealing, painting, electrical harness manufacturing, electrical harness routing, etc) are exactly the same as those performed in the assembly line or during maintenance works. The customer often requires the modification to be able to be applied to several helicopters of the same type. This requirement is interchangeability; that is, any piece of equipment or support that can be detached from the helicopter (mobile provisions) has to be installed in any other helicopter without any additional work, apart from that of tightening the fixations. It implies that any equipment support has to be the same, and has to be placed in the same location in all the helicopters involved in the modification project. This is achieved using

Pag.51

special tooling (laser tracker is valid to assure the orientation of certain equipment like antennas, but it is quite cumbersome to fix the attaching points compared with a drilling tooling). This means that if interchangeability is required, tooling has to be designed and regarded from the beginning of the project, and it has to be used adequately during the installation on the prototype and the series helicopters.

Pag.52

8. PROTOTYPE TESTS

Once the modification installation is finished, it is time to verify it works as expected. This verification is usually carried out by the following tests:

a) Functional and ground tests b) EMI/EMC tests c) Flight tests

Note: if for whatever reason, a test has to be performed in a laboratory, this has to be approved by the certification authority. Ask the laboratory for the approval documentation before going there. If they are not qualified, look for another one. Tests performed in non-approved facilities may not be accepted by the certification authority and they may have to be repeated. 8.1 FUNCTIONAL AND GROUND TESTS The purpose of these tests is multiple. Firstly, they intend to demonstrate the proper functioning of the newly installed equipment integrated in the helicopter. Secondly, these tests will show compliance with all the certification paragraphs mentioned on the certification plan to be demonstrated by ground test or inspection (ie, helicopter evacuation time, aisle width, decals properly visible, etc). The documentation needed to define and record the tests is:

a) Functional and ground test procedure (or protocol or plan or many other names): This document describes how the tests have to be performed, step by step, which data has to be recorded and what are the acceptable values (tolerances) of the data to be collected. An example of a functional and ground test procedure is subsequently included.

b) Functional and ground test result report: As its name implies, this document records the test results and states any possible deviation occurring during it.

A suggested table of contents for a functional and ground test procedure is the following:

a) Purpose b) Applicability c) Applicable documents d) Helicopter approved (by the certification authority) documents affected by the tests e) Applicable certification requirements f) Prototype configuration g) Required authorisations h) Test personnel i) Instrumentation j) Location and test conditions k) Safety regulations during the tests l) Weight and balance configuration(s) m) Test preparation

a. Required equipment b. Test sequence

Pag.53

c. Test acceptance criteria d. Preparation of systems to test e. Schedule

n) Test procedures o) Data sheets

Purpose: It has to be mentioned that the document is written to describe how to perform the functional and ground tests of the modification concerning the applicable equipment on a helicopter of a defined type and model. Applicability: This paragraph has to define the prototype helicopter where the tests are going to take place. It has to state the type, model, serial number and tail number. Applicable documents: Those documents needed to achieve the tests, such as equipment operation manuals, modification description, etc. Helicopter approved documents affected by tests: Those documents likely to be modified (issuing supplements) after the modification is completed (flight manual, wiring manual, maintenance manual, etc). Applicable certification requirements: This is a list of all the certification paragraphs (excerpt from the certification plan) intended to be demonstrated by functional and ground tests. Prototype configuration: Define the exact configuration the prototype helicopter needs to have in order to perform a representative tests. Do not forget the optionals. If the test is carried out in summer at 35ºC, it is almost certain that the helicopter will not have the skis installed, particularly if they are not absolutely necessary for the test. So, in order to ensure that the new installed equipment is compatible with the skis, although they are not installed for the test, it is necessary to demonstrate it in this paragraph of the test procedure, stating why (there is no mechanical, electrical, etc. Interferences, weight and balance are acceptable when skis and new equipment are both present, etc). By doing so, you will prevent the bitter surprise of receiving an airworthiness certificate not including several optionals compatible with the modification. In other words, once the modification is approved by the authority, the helicopter operator would not be able to fly the aircraft with the modification and the optionals at the same time, and it would require an additional certification process. Required authorisations: The authorisations required to perform functional and ground tests depend on the type of helicopter (civil or military), the country’s regulations; and in Europe, for civil helicopters, if the modification company is a DOA (design organisation approval) or not. It implies that these authorisations range from an installation certificate of conformity issued by the modification company (declaring the modification has been achieved according to the

Pag.54

design documentation, and the helicopter is ready to fly because all the pertinent maintenance tasks have been achieved and there are no mandatory service bulletins pending to apply) to several documents (certificate of conformity + authorization by the military representative, etc). Unfortunately there is not a homogeneous and unique rule for all type of helicopters applicable in any country. Test personnel: List the personnel needed to achieve the test; do not forget personnel from the certification authority. For example: One pilot and co-pilot One test engineer (from the modification company) One test engineer (from the certification authority) Etc. Instrumentation: If any instrumentation is needed to perform the test, this is the paragraph to list it. The name, part number and specific requirements (electrical power, weight, etc) of the instrumentation equipment has to be recorded and a drawing showing its installation on the helicopter has to be annexed in the functional and ground test procedure. The reasons to use this instrumentation and the parameters to be recorded with it have also to be mentioned. Location and test conditions: It is obvious that the place where the test will be carried out has to be explicitly mentioned, as well as the proposed time to start it. As far as test conditions are concerned, the atmospheric conditions (outer air temperature, humidity, no snow or rain present, airport altitude, required light conditions (day, night, sunset, sunrise, natural light (helicopter outside hangar) or artificial light (helicopter inside hangar)), and any other relevant requirements have to be clearly set up in this paragraph. Safety regulations during the tests: Safety is the first and most important issue during any test (and helicopter operation). Therefore, apply common sense and your company regulations for tests (if they exist). As a minimum, the following has to be enforced:

a) Place the helicopter under test in an isolated area, far away from any other aircraft or building

b) Highlight the test area conspicuously (by using red and white tape similar to that utilised by police or fire-fighters to spot a particularly dangerous location, for instance, or any other means)

c) Allow only and strictly the minimum required personnel to perform the test inside the helicopter

d) Keep spectators as far away as possible e) Do not allow any work in the helicopter prior to or during the test

Pag.55

As mentioned before, bear in mind that safety is always the first priority, even though it may imply the cancellation of the test and some delay may be caused. Weight and balance configurations: Define if any particular configuration of weight and balance is required to perform the ground tests. If so, the helicopter has to be loaded (or unloaded) accordingly before the tests start. Test preparation: Under this headline the following paragraphs are included: Required equipment: All the auxiliary equipment needed to perform the test has to be listed here in order for it to be obtained and set up when the test starts. It may include: GPU (ground power unit) for electrical supply (mention the acceptable limits of the voltage provided by the GPU; for some equipments, these limits need to be very narrow) VOR simulators, IFF simulators, etc: It is wise, depending on the test, to have available a mechanical technician and an electrical one, with standard tools, in case any eventuality occurs during the test and repairs are required to allow the test to continue or reinitialise it. This will prevent cancellation of the aforementioned test. Test sequence: The order to perform the several tests has to be clearly established to prevent confusion. It is advisable to begin with the inspections (aisle width, ergonomics, decals, etc) and afterwards, start with the tests requiring equipment activation and performance. Test acceptance criteria: This paragraph will indicate when a test is accepted or rejected, and how it has to be recorded on the data sheets (for example, a “√” is written down in the “acceptance” column or, on the contrary, a “X” is inserted in the “rejection” column). A test is normally accepted when the results are according to the expected value of the parameter being measured (tolerances have to be regarded) and is rejected otherwise. Expected values for any parameter plus tolerances have to be shown on the data sheets. Preparation of systems to test: The systems to test have to be totally installed (mechanically and electrically) and they will be operated according to the pertinent operation manual (state (title, number and issue) which those manuals are). Schedule: The expected schedule for the ground tests has to be described and communicated in advance to the certification authority and the customer.

Pag.56

Test procedures: This paragraph has to describe in full detail and step by step how the tests have to be performed. No doubt should arise during the test if this paragraph has been read in advance. An example will try to make clear this point. Assume that a VHF radio has been installed and is going to be functionally tested. The test procedure would be something similar to:

1) Make sure the radio control is in the “OFF” position 2) Insert the VHF radio breaker (located in position XX in breaker panel number

XX) 3) Switch on the radio control (place the control knob in the “ON” position) 4) Select the frequency XXXX (operating the selection buttons according to

manual XXX) and check the reception is loud and clear. Take note in the data sheets

5) Select the frequency XXXX and check with the control tower (or any other station) reception of emitted message. Take note in the data sheets.

6) And the same procedures can be written for as many frequencies as deemed necessary to check. Finally, the radio control has to be switched off (indicating how to do it) and the breaker pulled out

Data sheets: Data sheets are the formats where the test results are recorded. They summarise the test procedure and show the expected value of any parameter to be evaluated and its tolerances. An example is included below.

Pag.57

Test VHF radio functional test

Identification FGT-1 Requirement Emission and reception quality.

Procedure 1) Make sure the radio control is in the “OFF” position. 2) Switch on the radio control 3) Select frequency XXX and check reception is loud and clear. 4) Select frequency XXX and emit a message. Check reception. 5) Switch off the radio and pull out the breaker.

Parameter Value Tolerances Recorded A R3) Reception Reception loud and clear --------- 4) Emission Emission loud and clear --------- --------- --------- --------- Notes: A: Accepted R: Rejected

Date Time Test operator Certification engineer

All the above paragraphs comprise, as a minimum, a functional and ground test procedure. Once this procedure has been approved by the certification authority, it is time to carry it out. The procedure will be followed step by step and the data recorded, but unfortunately, on several occasions malfunctions or unexpected happenings take place and somebody has to decide if the test may continue or it has to be cancelled. This is the responsibility of the test director. This person (an employee of the modification company) is in charge of the test, and he or she has to decide what to do (with the agreement of the certification authority present at the test site) when things go wrong.

Pag.58

Finally, when the functional and ground tests have been completed, a test result report(s) has(ve) to be issued collecting all the results and events of the tests and mentioning if the functional and ground tests have been successfully passed or they have not. This test result report is sent to the certification authority for approval. Once the ground tests have been concluded, it is time for the electromagnetic tests to start, if any equipment able to emit or transmit has been installed in the helicopter, or is electrically powered. 8.2 EMI/EMC TESTS There are three EMI/EMC test types that a modified helicopter and its equipment can undergo. These are:

a) Equipment level tests (only for equipment and performed in a laboratory) b) Interoperability test (applicable to the whole helicopter with the new installed

systems/equipment on board the aircraft and working in normal conditions) c) Allowable radiation level (applicable to the whole helicopter working under radiation)

Equipment level tests These tests are carried out by the equipment manufacturer in order to obtain certification of its equipment. The test procedures, acceptable or rejection conditions, test parameters and so on are defined in several specifications (eg MIL-STD-461 (for military equipment), DO-160 (for civil equipment) and similar developed in different countries). The modification company has to demand the equipment manufacturer shows evidence (Declaration and design of performance, qualification test reports, etc) that the selected equipment complies with the appropriate EMI/EMC standard prior to installing them in the helicopter. Interoperability test The purpose of this test is to verify that the newly installed equipment does not adversely affect the existing equipment on the helicopter and vice versa, the operation of the existing helicopter equipment does not adversely affect the new items. It implies that any equipment has to be considered as a PNS (possible noise source), and also as a potential victim of the operation of all the others. This test is performed on the ground with the helicopter operative. The test consists of switching on, operating and switching off the PNS and checking the effects on the operation of all the other equipment. Bear in mind that ALL the systems powered by electricity or activating any electrical device have to be checked, including collective and cyclic levers, pedals, fuel pumps, hydraulic systems, etc. A simple inspection is usually enough to verify that the equipment under test is affected or is not by the operation of the PNS. This inspection includes a visual check of the system

Pag.59

indicator (if any, such as engine oil pressure indicator, etc), verification of control binding or friction increase, perception of audio levels (noise) for radio communications, etc. As for any test, it is required to set up a test procedure. A proposed table of content for such a document is shown below:

a) Purpose b) Applicability c) Applicable documents d) Helicopter approved (by the certification authority) documents affected by the tests e) Applicable certification requirements f) Prototype configuration g) Required authorisations h) Test personnel i) Instrumentation j) Location and test conditions k) Safety regulations during the test l) Weight and balance configuration m) Test preparation

a. Required equipment b. Test sequence c. Test acceptance criteria d. Preparation of systems to test e. Schedule f. Definitions and acronyms

n) Test procedures

a. New systems affected by the existing ones b. Existing systems affected by the newly installed ones

Purpose: It has to be mentioned that the document is written to describe how to perform the EMI/EMC test of the modification concerning the applicable equipment on a helicopter of a defined type and model. Applicability: This paragraph has to define the prototype helicopter where the tests are going to take place. The type, model, serial number and tail number have to be stated. Applicable documents: Those documents needed to achieve the test, such as equipment operation manuals, modification description, etc. Helicopter approved documents affected by tests: Those documents that are probably to be modified (issuing supplements) after the modification is completed (flight manual, wiring manual, maintenance manual, etc)

Pag.60

Applicable certification requirements: This is a list of all the certification paragraphs (excerpt from the certification plan) intended to be demonstrated by EMI/EMC test. Prototype configuration: Define the exact configuration the prototype helicopter needs to have in order to perform a representative test. Do not forget the optionals. Required authorisations: The authorisations required to perform an EMI/EMC test depend on the type of helicopter (civil or military), the country regulations; and, in Europe for civil helicopters, if the modification company is a DOA (design organization approval) or not. In any case, it is mandatory that the functional and ground tests have been accomplished successfully to be able to start the EMI/EMC test. Test personnel: List the personnel needed to achieve the test, do not forget personnel from the certification authority. For example: One pilot and co-pilot One test engineer (from the modification company) One test engineer (from the certification authority) Etc. Instrumentation: If any instrumentation is needed to perform the test, this is the paragraph to list it. The name, part number and specific requirements (electrical power, weight, etc) of the instrumentation equipment has to be recorded and a drawing(s) showing their installation on the helicopter has(ve) to be annexed in the EMI/EMC test procedure. The reasons to use this instrumentation and the parameters to be recorded with it also have to be mentioned. Location and test conditions: It is obvious that the place where the test will be carried out has to be explicitly mentioned as well as the proposed time to start it. As far as test conditions are concerned, some atmospheric conditions (outer air temperature, no snow or rain present, no thunderstorms forecasted), helicopter outside the hangar (because the test has to be carried out with engines on (and rotors turning)), and any other relevant requirements have to be clearly set up in this paragraph.

Pag.61

Safety regulations during the tests: Safety is the first and most important issue during any test (and helicopter operation). Therefore, apply common sense and your company regulations for tests (if they exist). As a minimum, the following has to be enforced:

f) Place the helicopter under test in an isolated area, far away from any other aircraft or building

g) Highlight the test area conspicuously (by using red and white tape similar to that utilised by police or fire-fighters to spot a particularly dangerous location, for instance, or any other means)

h) Allow only and strictly the minimum required personnel to perform the test inside the helicopter

i) Keep spectators as far away as possible j) Do not allow any work in the helicopter prior to or during the test

As mentioned before, bear in mind that safety is always the first priority, even though it may imply the cancellation of the test and some delay may be caused. Weight and balance configurations: Any weight and balance configuration is valid to perform this test. Test preparation: Under this headline the following paragraphs are included: Required equipment: All the auxiliary equipment needed to perform the test has to be listed here in order for it to be obtained and set up when the test starts. It may include: VOR simulators, IFF simulators, etc It is wise, depending on the test, to have available a mechanical technician and an electrical one, with standard tools, in case any eventuality occurs during the test and repairs are required to allow the test to continue or reinitialise it, and hence, to prevent cancellation of the aforementioned test. Test sequence: The order to perform the test has to be clearly established to prevent confusion. Test acceptance criteria: This paragraph will indicate when the effect of the PNS is accepted or rejected, and how it has to be recorded on the data sheets (for example, a “√” is written down in the “acceptance” column or, on the contrary, a “X” is inserted in the “rejection” column). The acceptance criteria have to be set up in this chapter because there are several conditions when interference can be accepted. For example: If the interference is caused during the engine start process If the interference lasts less than one second during any switch activation If the interference causes a change less than 20% in the ambient noise level

Pag.62

Preparation of systems to test: The systems to test have to be totally installed (mechanically and electrically) and they will be operated according to the pertinent operation manual (state (title, number and issue) which those manuals are). Schedule: The expected schedule for the EMI/EMC tests has to be described and communicated in advance to the certification authority and the customer Definitions and acronyms: Make a clear statement (definition) of what is a Possible Noise Source (PNS), electrical interference, noise level, malfunction and unacceptable response. And, of course, state what any acronym means. PNS: A possible noise source is any equipment able to cause an electromagnetic interference on any other equipment Electrical interference: Commonly known as noise, it is any non-desired electrical signal causing a malfunction on equipment Noise level: It is the system inherent noise (due to thermal effects, microphones, radio frequency environment, etc). This level is used as a reference to accept or reject the effect of interference. Malfunction: It is a system failure due to an electromagnetic interference Unacceptable response: It is an abnormality in the system functioning; although it is not a malfunction, it detrimentally affects the system operation. Test procedures: This paragraph has to describe in full detail and step by step how the test has to be performed. No doubt should arise during the test if this paragraph has been read in advance. The procedure has to set up how to proceed when the newly installed equipment is checked against potential interferences caused by other equipment, and in another chapter, how to proceed when the newly installed equipment is the potential source of electromagnetic interference. There are many ways of doing this: one method is to create a data sheet describing the steps to follow and with a table to record the results, as those shown below:

Pag.63

Test New system as PNS Identification NS-1 Requirement Check the proper operation of existing systems when the new systems (mention

which systems ) are operated. Procedure State step by step how to operate the new systems and what to check in the existing

systems

PNS (new system) Position lights Landing light Weather radar

VHF radio Notes: A: Accepted; R: Rejected

Date Time Test engineer Certification engineer

And when the newly installed systems can be perturbed by the existing systems, the data sheet may be slightly different, for instance:

Pag.64

Test Existing systems as PNS

Identification ES-1 Requirement Check the proper operation of new systems when the existing systems are operated.

Procedure State step by step how to operate the existing systems and what to check in the new systems (in this example the transmission and emission quality levels (noise))

PNS VHF radio Position lights Landing light Weather radar Fuel transfer pumps

Etc Notes: A: Accepted; R: Rejected

Date Time Test engineer Certification engineer

When the EMI/EMC test is finished, it is time to write an EMI/EMC test result report describing the results attained during the test and any deviation that might have taken place.

Pag.65

This report has to say if the EMI/EMC test has been successfully passed or not. Allowable radiation level These tests are seldom used for most of the modifications a helicopter will undergo along its operational life, therefore, not much detail is required They consist of radiating the helicopter on the ground until unacceptable interferences are encountered on the helicopter systems and equipment. Those radiation limits are recorded and limitations are mentioned in the applicable flight manual supplement with sentences similar to “do not fly in areas where the electrical field intensity is higher than XXXX V/m” (ie, high voltage lines, etc). Insurance policy for functional and EMI/EMC tests The functional and ground tests and EMI/EMC tests have to be covered by an insurance policy, issued by the modification company, in case some damage might be caused to the helicopter or any personnel involved in those tests. These policies vary from country to country; therefore, it is difficult to set up general rules. In the year 2006 it is reasonable to contract a 6 million euros policy for civil helicopters and 12 million euros for military ones (at the time of writing this book 1 euro = 1.27 US dollars approximately), although it is widely dependant on the type of helicopter. 8.3 FLIGHT TESTS When the functional (ground) and EMI/EMC tests have been successfully accomplished, the next step is to test the modification in flight. Before starting the test, several administrative tasks have to be cleared; apart from issuing a test procedure (to be described later), the following aspects need to be considered:

a) Authorisation to fly. This authorisation varies from country to country and it is also depends on the helicopter classification (military or civil). In any case, it is absolutely necessary to have a flight test procedure issued by the modification company and approved by the certification authority prior to receiving the authorisation to start the flight test.

b) Insurance policy: This also depends on the country’s regulations where the flights take place. In some countries it is mandatory for the modification company to issue an insurance policy covering potential damage to third parties (people and their belongings on ground, and the cost of the test helicopter in case of crash). This author suggests checking the liability responsibilities applicable in the country of interest (ask the civil airworthiness authority or its military counterpart). In addition to this policy, it is mandatory to issue another one covering the people involved in the flight test (crew, test engineer(s), certification authority(ies)). It is the responsibility of the modification company and it is charged to their expenses. Minimal coverage and other details vary widely from country to country.

Pag.66

Before describing a flight test procedure, it is worth saying a few words about the flight test director. This person is responsible for the flight test and he or she has to decide what to do in the case of an unexpected event (cancel, continue or reinitiate the test). It is also important to remember that when the helicopter is flight tested, it “belongs” to the modification company. This implies that any damage caused to or by the helicopter is the responsibility of the modification company, not the customer: that’s why the insurance policy is required. Another important factor is the flight test pilot(s). He or she should be part of the modification company workforce, but because it is almost impossible for any company to have a permanent set of pilots qualified in all helicopter types, in many cases they will be hired for an agreed number of flights. Their qualifications are also very important. For most modifications (communications, navigation equipments, etc) any pilot with a valid licence for the helicopter type to be tested is acceptable, but when performance tests (particularly those in the flight envelope limits and engine performance) are envisaged, it is advisable, although not mandatory, to hire a flight test pilot, and those are not very abundant. By flight test pilot, it is understood to be those that have passed a flight test course (usually over one year long) in helicopter flight test schools like Patuxent river (USA), Boscombe Down (UK), Istres (France), or any other equivalent. The military usually have several such pilots; therefore, if they are the customer, it is wise to ask for their assistance and mention on the contract that the test pilots will be provided by the Army, Navy or Air Force, as applicable (economical impact is disregarded, but this choice is usually much cheaper than any other alternative). As far as a flight test procedure is concerned, the contents are similar to those of the functional and EMI/EMC tests; that is, a description of means and requirements to be fulfilled, and evidently, what to do to complete the test. A suggested table of contents is the following: a) Introduction

a. History b. Test goals

b) Helicopter and equipment to be tested description

a. Helicopter description b. Equipment to be tested description

c) Test scope

a. Tests and test conditions i. Meteorological conditions

ii. Miscellaneous

b. Flight test envelope i. Limitations

c. Authorisations d. Test configurations

Pag.67

e. Test specifications d) Test methodology

a. Test methods b. Instrumentation and data reduction/data processing c. Support requirements

e) Schedule f) Safety requirements g) Test management

a. Human and material resources b. Facilities c. Required equipment

h) Test responsibility i) References j) Annexes

a. Test matrix b. Test configurations c. Cooper Harper Rating (HQR, Handling Quality Rating Scale)

History: It is nice to commence the flight test procedure mentioning briefly what the modification intends (eg to improve the helicopter communication capability, to adapt the helicopter to new threat environments (for military), etc) Test goals: This is a paragraph to state the main test objective in a very concise manner ( eg to check the proper functioning of the new system, to verify the performance of the helicopter in certain conditions, etc) Helicopter description: This paragraph is a courtesy to the certification authority because they are usually specialised personnel, and the one to flight test the modification may not have been involved in the rest of the certification process. So, a basic helicopter description is appreciated, to allow the certification authority to be familiar with it. Equipment to be tested description: Due to the same reason as in the previous paragraph, some concise description of the new systems or equipment (what they look like, where they are placed, how they work, what their basic performances are, etc) is advisable. Meteorological conditions: The conditions to be required are specified in this paragraph (temperature, humidity, snow, rain, etc). For most flight-tests it is very advisable to specify VMC (Visual Meteorological Conditions) to perform the tests. Obviously, there are some exceptions, such as icing tests, etc Miscellaneous: Any other condition deemed worth mentioning Limitations: The flight test envelopes have to be clearly defined in this paragraph (normally, this envelope is that of the approved flight manual, but there are exceptions, particularly if the

Pag.68

modification intends to expand these limits. In that particular case, use the progressive approach, that is, approach the desired new point in the extended envelope successively, in several attempts, pushing the limit step by step, so the helicopter behaviour can be analysed and safety assessed before trying a new attempt flying beyond the tested point. Besides, any other applicable limitation (for instance, all the flight tests have to be strictly performed in VMC) needs to be set up. Authorisations: As mentioned for the functional and EMI/EMC tests, the required authorisations vary from country to country, but in any case, no flight test should be tried until both the functional (ground) and EMI/EMC have been successfully completed. Several certification authorities demand an “airworthiness statement”. This document, issued by the modification company, states that the helicopter with serial number XXXX, tail number XXXX, with engines serial number XXXX, has been checked (all its systems have been verified (structure, mechanical devices, electrical system, hydraulic system, landing gear, etc) and they are in flight condition), and all the maintenance tasks have been properly accomplished while the helicopter has undergone the modification; in addition to that, there are no service bulletins pending to apply. In other words, the helicopter is ready to fly in a safe manner, at least, as safe as can be assured, because the impact of the modification has to be assessed, and that is the flight test goal. Test configurations: The helicopter configuration as far as weight, centre of gravity and landing gear position (if retractable) have to be defined according to the flight phase (takeoff, cruise, approaching, landing, etc) and the type of flight test. Test specifications: This paragraph collects the entire certification standard paragraph to be demonstrated by flight test. It is a list including standard paragraph number and title. Test methods: This paragraph describes what to do, step by step, and what data has to be recorded in each and every flight test. It has to set up what is the power to be applied to the engines, what weight and balance configuration, what is the flight altitude, what are the manoeuvres to be done (turns, level flight, autorotation, etc), in which order, which speeds, which bank angles, when applicable, and what data has to be collected and for how long. Instrumentation and data reduction/data processing: It will be mentioned if the data is recorded from the helicopter indicators (just reading them and recording the data in a FTC (Flight test card)) or some instrumentation will be used. If so, the instrumentation part numbers, models, location and parameters to be recorded have to be described. These FTI (Flight test instrumentation) will have been previously approved by the certification authority (location, electrical consumption, cooling requirements, etc). When FTI is used, thousand of pieces of data are recorded; the process to cope with this huge amount of information and obtain valid test results has to be outlined in this paragraph. Support requirements: It is wise to specify that a mechanical technician and an electrician be available during the flight test in order to correct any possible minor failure and proceed with the flight test, otherwise, it would have to be cancelled. Schedule: The date when the flight test will start and the foreseen days needed to accomplish have to be mentioned in this paragraph.

Pag.69

Safety requirements: They are basically the same as for any other test (see functional and ground test or EMI/EMC test safety requirements), but in this particular case, pay more attention to spectators. Keep them as far as possible away from the testing site. Follow strictly the ground and flight procedures for the particular helicopter under test summarised in the flight manual and company, or customer, procedures. Human and material resources: All the personnel and material required to perform the test will be listed in this paragraph. It includes pilot, co-pilot, flight technician (if applicable), test engineer, test engineer (certification authority), any other person deemed necessary and, as far as material is concerned, safety belts for all the occupants, life jackets and rafts (if the test will be carried out over water), etc. Facilities: where the helicopter will be during the flight test has to be specified (modification company premises, military base, etc). If the flight test is carried out in a remote area, fuel is a very important consideration; refuelling stations have to be considered. Water is also important to wash the helicopter engines if the test is performed over sea, and for people attending the test, if it is carried out in a hot, isolated area. Required equipment: This is a list of all the support equipment needed to perform the test, such as a GPU (ground power unit) to start the engines, to standard tools or very specific equipment (ie if a countermeasure system is being flight tested, a radar tracking station and MANPADS (infrared portable missiles) simulators are required). Test responsibility: This is a plain, clearly written paragraph; the flight test is always the responsibility of the modification company, but it is advisable to mention it explicitly to prevent any misunderstanding or confusion. References: List all the documents used to issue the flight test procedure. Annexes: As many annexes as necessary can be included in the flight test procedure, for this example three have been selected: Test matrix: This is a summary, in a table format of all the tests to be performed. It should be something similar to the following example. FLIGHT NUMBER

TEST WEIGHT AND BALANCE CONFIGURATION

ALTITUDE (FT)

SPEED (KIAS)

FAR PARAGRAPH

3 Cartographic navigator functional test

C7 500 70 29.771, 29.773, 29.777, 29.1301, 29.1309, etc.

Test configurations: This is a summary of the weight and balance configurations to be used during the flight tests. An example is shown below; this is a fictitious example.

Pag.70

CONFIGURATION GROSS WEIGHT (Kg)

CG (m) NEW EQUIPMENT INSTALLED

C1 10000 4,25 Yes C2 10500 4,00 No Handling Quality Rating Scale: This is a scale intended to help the pilot to classify any task when his/her qualitative judgement is required. This scale can be found in any flight test book.

There is no need to include the Data Sheets, named FTC (flight test cards) for flight tests, in the flight test procedure. It is more convenient to issue them apart, even a few days before the flight tests start because it allows flexibility to change the order of the tests and some other minor change that could arise (and they always arise). The format of the FTC could be similar to those shown for functional and EMI/EMC tests. It has to mention, step by step, what to do and what data to record, when it is acceptable and when it is not (tolerances).

Pag.71

When the flight tests have been concluded and the data has been properly treated and analysed, a flight test result report will be issued by the modification company. This document will describe how the flight-tests were performed and if their results are acceptable or not. This document will be sent to the certification authority for its approval. As previously mentioned at the beginning of this subchapter, pay special attention to those flight tests concerning flight envelope limits and also those where external protuberances are involved (external loads when using cargo hooks or rescue hoists (some of them tend to rotate and/or lift), missiles, cannons, etc). Bear in mind that safety is always the first and most important issue. Provided that all documents forwarded by the modification company has been approved by the certification authority, they will issue a certificate (Supplemental Type Certificate for civil helicopters), technical certificate (for military aircrafts) or recommendation for certification if the certification authority is working on behalf of the certification authority where the helicopter is licensed (this is usual for military helicopters).

Pag.72

9. ENTRY INTO SERVICE

When the modification is finally concluded and approved by the authority, it is time to return the helicopter to the customer and to normal service. The documents that have to be delivered to the customer are:

h) Flight manual supplement i) Maintenance manual supplement j) Wiring manual supplement k) Illustrated Part Catalogue supplement l) Copy of the approved modification airworthiness certificate (this is not mandatory.

This certificate is called STC (supplemental type certificate for civil helicopters) and several names (according to the country) for military helicopters).

m) Completed helicopter log book n) For military helicopters in NATO countries: NSN’s (NATO Stock numbers of the

applicable parts) In addition, the customer has to sign off the acceptance document of the modification. Normally, from this moment, the warranties applied to the modification start.

Pag.73

10. SUMMARY

There are many different possible modifications and upgrades to be carried out in a helicopter, therefore, it is practically impossible to detail all of them, but whichever the modification might be, the same sequence of events has to be followed, namely:

a) Set up a clearly defined and honesty based relationship with your customer b) If the customer does not supply the systems and equipment to be implemented,

negotiate with the suppliers and demand all the documentation needed to carry out the design and certification processes

c) Make a safety analysis: it will influence the modification design, so the sooner the better

d) Work out a certification plan. Describe the airworthiness standard you wish to comply with, and make sure you are able to fulfil them. Then, agree the certification plan with the certification authority

e) Design the modification f) Procure the materials to manufacture the designed parts and to proceed with

the installation on the helicopter g) Perform the installation h) Test it i) Attain the airworthiness certificate, the flight manual supplement, the service

bulletin and maintenance manual supplement, signed and sealed by the certification authority

j) Make sure that all documents to be delivered to the customer are readily available the day the helicopter will be returned to them

All modifications are managed in the same way, the only difference is the complexity of the tasks to be achieved: it is much easier to flight test a new VHF radio than the helicopter behaviour when carrying heavy external loads, but the concept is exactly the same; that is, to demonstrate the safe operation of the aircraft when the new devices have been installed. Following the stated steps, any modification will be successfully accomplished from both technical and certification standpoints. An engineer working for a small or mid-size company needs to know many things, the more the better. Any specialist has got a much deeper knowledge of any particular subject, but it is unlikely he/she has a good understanding of the whole modification process. What you need to know depends on whom you work for. This book has intended to show this whole process: hopefully it has been useful.

Pag.74

REFERENCES ARP926: Fault/Failure analysis procedure. SAE ARP5580: Recommended failure modes and effects analysis (FMEA) practices for non-automobile applications. SAE DEF-STAN: Volume 2: Rotary wing DO-160: Environmental conditions and test procedures for airborne equipment. RTCA EASA 27/29: European aviation safety agency Parts 27 and 29 FAA AC 43.13-1B: Acceptable methods, techniques, and practices. Aircraft inspection and repair. FAA FAR 27/29: Federal aviation regulations Parts 27 and 29 MIL-B-5087: Bonding electrical and lighting protection, for aerospace systems. DOD MIL-HDBK-5: Metallic materials and elements for aerospace vehicle structures. DOD MIL-STD-461: Interface standard requirements for the control of electromagnetic interference characteristics of subsystems and equipment. DOD MIL-STD-810: Test method standard for environmental consideration and laboratory tests. DOD MIL-STD-1472: Human engineering MIL-W-5088: Wiring aerospace vehicle. DOD

Pag.75

GLOSSARY AC: Advisory circular ATP: Acceptance test procedure CAD: Computer aid design CDR: Critical design review CO: Carbon monoxide CRES: Corrosion resistant steel D: Depot maintenance level DDP: Declaration of design and performance DER: Design representative DOA: Design organization approval EAPS: Engine air particle separator EASA: European Aviation Safety Agency EMI/EMC: Electromagnetic interference/electromagnetic compatibility FAA: Federal Aviation Administration FAR: Federal aviation regulation FDR: Final design review FEM: Finite element model FMEA: Failure mode and effect analysis FOD: Foreign object damage FTC: Flight test card FTI: Flight test instrumentation GPS: Global positioning system GPU: Ground power unit HQR: Handling quality rating

Pag.76

I: Intermediate maintenance level ICD: Interface control document IFF: Identification friend and foe IPC: Illustrated part catalogue JAA : Joint Aviation Authority KVA: Kilo Volt Amperes Kw: Kilowatts m: meter MANPADS: Man portable antiaircraft system MTOW: Maximum takeoff weight NATO: North Atlantic Treaty Organisation NDT: Non destructive test NSN: NATO stock number NVIS: Night vision system O: Organizational maintenance level PDR: Preliminary design review P/N: Part number PNS: Possible noise source SAE: Society of automotive engineers STC: Supplemental type certificate UK: United Kingdom USA: United States of America V: Volt VHF: Very high frequency VMC: Visual meteorological conditions

Pag.77

VOR: Very high frequency omni directional range XXXX: Any generic name or value

Pag.78

INDEX A Aerodynamics: 2,35,36 Airframe: 36,37 Allowables: 25 AQAP 110: 6 ATP: 11,12,75 Audio: 34,59 C Cargo hooks: 48,71 CDR: 8,9,10,75 Certificate of conformity: 11,53,54 Certification bases: 15,16 Composites: 19,25,28,29 Corrosion: 7,8,9,29,46,75 Crack: 25,30,46 D Data sheets: 11,12,19,53,55,56,61,70 DDP: 11,12, 19,75 Drag: 35,36,37,38 Dynamic analysis: 28,29 Dynamic stability: 39 E EAPS: 20,42,75 Electrical harness: 31,32,33,47,49,50 Electrical system: 7,39,68 EMI/EMC test procedure: 8,9,14,60 Ergonomics: 8,9,20,55 External fuel tanks: 37 F Fatigue analysis: 14,25,29,30 FDR: 8,10,13,75 Fire protection: 14,20,40 Flight test procedure: 9,14,35,39,65,66,67,69,70 FMEA: 11,14,19,21,22,74,75 FOD: 50,75 Forward looking infrared: 6,38,44 FTC: 68,70,71,75 Functional and ground test procedure: 8,9,14,32,52,54,57 H Hazard analysis: 14,21 Helicopter geometry: 6,23,38 HQR: 67,75 Hydraulic system: 2,14,20,40,58,68

Pag.79

I ICD: 11,19,76 Insurance policy: 65,66 IPC: 23,47,76 ISO 9100: 6 L Laser warning: 36 M Main gear box: 20,40 Maintenance manual supplement : 9,10,15,18,43,46,49,72,73 Master drawing list: 14,16,41,42 Mechanical components: 46 Metallic materials: 19,28,29,74 Missile launching warning: 36 MTOW: 39,76 N NDT: 46,76 Non-approved data: 5 P PDR: 8,9,76 Pneumatic system: 14,33 PNS: 58,61,62,63,64,76 Project manager: 4,6,9,10,43 R Radar warning: 36 Radomes: 19 Rescue hoists: 36,37,71 S Service bulletin: 5,14,41,43,48,49,50,54,68,73 Specification: 6,7,10,11,12,13,19,58,67,68 Static analysis: 28,29,31 Static stability: 39 STC: 42,72,76 Stress distribution: 22,23,24,25,26,27,28,30 T Temperature: 7,11,40,42,44,54,60,67 Test engineer: 54,60,63,64,65,69 Test pilot: 66 V VMC: 67,68,76 Vortex: 35,37,38

Pag.80

W Weight and balance: 14,31,44,45,49,52,53,55,59,61,68,69,70 Wire: 19,32,33,47,50