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  • 8/10/2019 Acceptable Means of Compliance Para Hums CAA


    CAP 693



    CAA AAD 001-05-99


  • 8/10/2019 Acceptable Means of Compliance Para Hums CAA


    CAP 693



    CAA AAD 001-05-99


  • 8/10/2019 Acceptable Means of Compliance Para Hums CAA


  • 8/10/2019 Acceptable Means of Compliance Para Hums CAA





    1 Introduction 1

    2 Scope 3

    3 Purpose 4

    4 Background 4

    5 Means of Compliance 6

    6 Compliance by Method A 6

    7 Compliance by Method B 9


    A Additional Guidance for compliance by Method A 11

    B References 17

  • 8/10/2019 Acceptable Means of Compliance Para Hums CAA




    The Civil Aviation Authority has issued AAD 001-05-99, which became effective on

    7 June 1999. The AAD makes the installation and use of health monitoring systems

    (HMS) mandatory for United Kingdom registered helicopters issued with a Certificateof Airworthiness in the Transport Category (Passenger), which have a maximum

    approved seating configuration of more than 9 passengers. However this Directive is

    not applicable to helicopters certificated to BCAR 29 or JAR-29.

    This CAP provides operators with the basis for an acceptable means of compliance

    with the Directive.

    Figure 1 shows the operator options and timescales for compliance with this AAD.

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    CAA Health Monitoring



    Certificate of Airworthiness in the

    Transport Category (Passenger)

    with seating Configuration of more

    than nine

    Not Applicable to:

    Type Approvals


    BCAR 29 or JAR-29

    Design Assessment


    Is an approved system

    currently fitted?

    Comply by Agreed


    Comply by fitting an

    approved system

    YES NO

    1 year


    Comply with

    procedures and


    2 years

    Certificate and fit

    new system and

    comply with


    and practices.

    2 years


    FIGURE 1

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    2 SCOPE

    AAD 001-05-99 when referring to HMS or HUMS means a system utilising on

    board equipment for monitoring the health of helicopter rotor and rotor drive systemcomponents. Helicopters affected by this AAD will need to demonstrate an effective

    vibration health monitoring capability. Consequently much of this CAP relates

    specifically to vibration monitoring systems. However, it is accepted that many

    methods of health monitoring, such as transmission magnetic plugs, chip detectors, oil

    analysis, will already be adequately controlled by the helicopter constructors

    maintenance instructions. The monitoring techniques to be considered are;

    Vibration Monitoring System (VMS) which should monitor :-

    Engine to main gearbox input drive shafts Main gearbox shafts, gears and bearings

    Accessory gears, shafts and bearings

    Tail rotor drive shafts and hanger bearings

    Intermediate and tail gearbox gears, shafts and bearings

    Oil cooler drive

    Main and tail rotor track and balance

    Plus existing established techniques :-




    Physical Displacement (Rotor blade and tracking is a practical


    Debris generation via real time analysis in fluid streams (Oil and

    exhaust gases)

    Magnetic Plugs

    Spectrographic Oil Analysis

    Absence of any of the above techniques will need to be justified.

    It is acknowledged that existing health and vibration monitoring systems are complex,

    and that there may be many different methods to monitor a particular failure mode.

    For this reason this CAP is not prescriptive with respect to items such as the position

    of accelerometers, algorithms and threshold settings. The object of this CAP is to

    describe the level of monitoring required and give advice on ensuring that the

    monitoring is effective and reliable.

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    The purpose of this CAP is to provide guidance to industry on what the CAA consider

    to be an acceptable means of compliance with the subject AAD.

    The AAD has been raised in response to numerous AAIB rotorcraft accident/incidentreports which have highlighted the positive role that vibration monitoring systems

    could play in reducing the accident rate in the helicopter community.

    The benefits of vibration monitoring in the UK have so far been limited to North Sea

    operations of Transport Category (Passenger) helicopters, where the operators have

    fitted these systems in recognition of the benefits together with their desire to satisfy

    their customers safety objectives.

    The CAA has been proactive in promoting the positive aspects of health monitoring

    and has adopted requirements for the certification of large helicopters which include arotor and transmission design assessment, in which health monitoring is an acceptable

    compensating provision. The decision to mandate the installation and use of health

    monitoring systems has been taken in response to the recent AAIB recommendations

    relating to incidents and accidents.


    In the early 1980s in response to a growing unease with the safety record of large civil

    transport helicopters, the Chairman of the CAA requested that the Airworthiness

    Requirements Board (ARB) establish a panel to review existing helicopterairworthiness requirements. This panel, the Helicopter Airworthiness Review Panel

    (HARP), reported in 1984 and confirmed that the fatal airworthiness accident rate for

    large twin engine helicopters was significantly higher than for comparable aeroplanes.

    From their review of failures, HARP recognised that the helicopter is different. So

    much of its critical mechanism, the rotors and rotor drive system, involved single load

    paths without duplication or redundancy. They identified a fundamental difference

    between the helicopter and fixed wing aircraft, the inability to guard against a failure

    by duplication.

    However, experience showed that although this machinery employed safe life rather

    than fail safe design, often defect propagation would occur for a period of time before

    failure occurred. This, coupled with the recognition that there had been important

    developments in health monitoring system (HMS) technology, encouraged the

    members of HARP to recommend the philosophy that where full redundancy has not

    been possible by design then warning of likely failure in a suitable time scale could

    provide an acceptable level of safety.

    The types in todays UK fleet are likely to remain in service for the foreseeable future.

    Therefore, all the really significant improvements to the airworthiness codes that the

    CAA/JAA has introduced over the past few years are effective for new helicopters,but are currently of no benefit to the existing fleet. Therefore, there is a need to

    improve the airworthiness of the current fleet.

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    Operational trials of health monitoring systems were carried out between 1987 and

    1991 over the North Sea. These trials demonstrated the technical feasibility of first

    generation HMS, including vibration monitoring, in what must be considered as a

    very testing environment. Today all large UK registered helicopters employed in

    North Sea operations have HMS fitted, as a customer requirement, rather than amandatory requirement, except for the S61, where a CAA mandatory AAD

    necessitates HMS for a specific component.

    The CAA has recognised that helicopter rotor and transmission systems are

    susceptible to potentially hazardous and catastrophic failure effects, due to the very

    nature of their design (single load path) and has for many years realised the benefit of

    the installation of HMS as a compensating provision, for new certifications.

    The CAA has sponsored, along with the UK government, a comprehensive safety

    research programme which culminated in successful operational trials of HMSequipment on four North Sea oil/gas support helicopters. The Authority has also

    managed extensive research programmes exploring the technical feasibility of HMS,

    including in-service trials and seeded fault tests.

    It is considered that the first generation HMS, which added comprehensive vibration

    monitoring to existing health monitoring techniques, has already demonstrated the

    ability to identify potentially hazardous and catastrophic failure modes, and has

    already reduced fatal accident statistics.

    First generation HMS (including vibration monitoring) has been shown to be both

    technically feasible and economically justifiable for existing operational helicopters.The CAA believes that it is technically feasible to extend the benefits from HMS to

    helicopters currently in service (helicopters issued with a Certificate of Airworthiness

    in the Transport Category (Passenger), which have a maximum approved seating

    configuration of more than 9 passengers).

    Several HMS systems are available for most types of medium and large helicopter.

    The systems employed by operators have generally been designed for retrofit into

    existing helicopters. The recent UK CAA requirement for CVFDR systems to be

    installed on helicopters above 2730 kg has allowed system designers to incorporate

    Health Monitoring Functions along with the mandatory CVFDR functions. Thiscombining of systems has reduced the overall design, implementation and weight

    burden of two separate systems. It follows that many UK operators complying with

    CVFDR requirement now operate their helicopters with health monitoring including

    vibration monitoring.

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    5.1 Demonstration of Compliance

    The following two methods are acceptable for showing compliance with this AAD.

    These are;

    Method A : By utilising a health monitoring system which is already approved.

    Note : A list of AAN numbers defining currently approved health

    monitoring systems for different helicopter types is contained within

    the AAD.

    Method B : By obtaining CAA approval for an alternative health monitoring


    5.2 Continuation of Compliance

    Once compliance has been demonstrated the CAA will monitor compliance with this

    AAD by continued surveillance visits and direct input where the means of compliance

    has been predicated upon CAA involvement. It is anticipated that the degree of direct

    (day to day) CAA involvement will be minimal and will take the form of an audit

    function based upon satisfactory demonstration of capability. The CAA will therefore

    seek a management structure with appropriate safeguards and controls that will ensurethat all changes and decisions are made by appropriately trained and authorised

    personnel of the operator. This will include, where applicable, sub-contractors

    involved in maintenance of the helicopter as part of the overall quality function.

    Compliance with this Directive will affect maintenance activity and therefore will be

    audited under JAR-145 procedures.


    As detailed in section 1 of this CAP, the CAA AAD applies to helicopters issued witha Certificate of Airworthiness in the Transport Category (Passenger), which have a

    maximum approved seating configuration of more than 9 passengers.

    Compliance with Method A utilises HMS which are currently approved by the CAA.

    This can be applied to helicopters already fitted with such systems (North Sea

    operation) and also helicopters of the same types which are not yet fitted with HMS.

    Existing operators with approved systems will need to address all the aspects of this

    section, however this is unlikely to require additional testing or design assessment. It

    is considered that such systems / operator combinations have a proven capability with

    respect to an acceptable standard of transmission fault detection such that a degree of

    credit can be granted for their compliance with this AAD. As such the period for

    compliance has been set at 1 year from the effective date of the AAD.

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    Operators of helicopters not already fitted with a HMS, who elect to comply by

    installing an existing CAA approved system will need to demonstrate to the

    satisfaction of the Authority that their fitment and use of the system offers an

    equivalent level of safety to that currently afforded by existing operators of the

    system. Operators may also elect to incorporate, as much as possible, the controls andprocedures available from the vendor in order to facilitate demonstration of

    compliance. As such the period for compliance has been set at 2 years from the

    effective date of the AAD.

    In assessing compliance with this AAD the CAA are seeking procedures and practices

    associated with a well managed health monitoring system.

    It is anticipated that the procedures and practices highlighted in this section may well

    mirror that of some existing operators and therefore the degree of adjustment may be

    minimal. In showing compliance the CAA expects existing operators of approved

    systems to utilise, as much as possible, their existing procedures and workingpractices.

    6.1 Exposition / Handbook - Method A

    In order to show compliance with AAD 001-05-99 each affected operator will need to submit

    the following information with respect to the HM systems and procedures in place

    within their organisation. This may be submitted in the form of a stand alone

    document / handbook. Alternatively a cross reference to the relevant company

    procedures may be provided. This documentation will need to be accepted by the

    CAA during finding of compliance as a record of the operators methods of satisfying

    the CAA AAD.

    Below are the subjects which need to be addressed.

    6.1.1 Duties and responsibilities of HMS personnel

    This section should clearly define those personnel (management and technical) involved

    in all aspects of the health monitoring activity, giving a description of each of their

    roles and responsibilities. The scope of authority for decisions based on health

    monitoring information and data is extremely important and therefore should be

    clearly defined and understood. A flow chart detailing the process and personnel

    involvement in clearing the helicopter for flight may be useful.

    6.1.2 Organisational chart

    This should show the position of the personnel detailed in 6.1.1 above, within the


    6.1.3 General vibration monitoring process procedures

    General procedures covering, as a minimum, frequency, content and method of down

    loading data, troubleshooting, maintenance instructions associated with alerts,correlating alerts with their cause, review in the event of a persistent alarm,

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    Maintenance of HMS, interface with HMS supplier, interface with CAA, review and

    control of excessive false alarm rates, data storage etc.

    It may also be necessary to define particular helicopter operating conditions that need to

    be established prior to acquiring data.

    6.1.4 HMS facilities

    This section should identify the ground based HMS equipment and its location on site

    with respect to flight crew and maintenance personnel accessibility.

    6.1.5 Helicopters affected

    This should list all the helicopters affected by the AAD detailing the helicopter type,

    registration and HUM system configuration. Also the standard of accelerometer

    supports, position and type should be controlled.

    6.1.6 Procedures for changing HMS equipment, software or maintenance practices

    The procedures detailed in the handbook / cross reference list will control HMS related

    activity within the company. As this documentation will be accepted by the CAA as

    showing the means of compliance with the AAD, any changes affecting the handbook

    must also be agreed by the CAA. A procedure for controlling these changes must be

    put in place.

    6.1.7 Supervision of subcontractors involved in HMS activity.

    Where any part of the health monitoring process is subcontracted to another

    organisation, the operator will be responsible for ensuring that all subcontract activity

    is in compliance with the appropriate procedures.

    6.1.8 Threshold setting and adjustment

    Current HMSs generate indicator values from vibration recordings taken from different

    positions on the rotor drive system. These indicators will show different pre-failure

    conditions such as gear tooth damage, shaft imbalance, shaft misalignment etc. Each

    indicator will have one or more threshold settings which, when exceeded by theindicator value will alert the maintenance staff to investigate the problem.

    All thresholds must be defined whether in the form of absolute signal values, number of

    standard deviations above the mean fleet value, or other means. Any change to these

    values should be agreed by the HMS system supplier and recorded along with the

    reason and justification for the change. All changes will be reviewed periodically by


    Some indicators may require trend monitoring, such as the AS332 input drive shaft,

    resulting from an AD or other source. These trends must also be defined. Again any

    change to these values should be agreed by the HMS system supplier and be recordedalong with the reason and justification for change. All changes will be reviewed

    periodically by CAA.

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    Where the constructor is prepared to offer advice, regarding the setting of thresholds, this

    information should be adopted.

    6.1.9 Minimum Equipment List

    The maximum allowable unserviceable period of any items of HMS equipment will

    need to be agreed with the Authority and defined in the Minimum Equipment List.

    The factors to be considered in establishing acceptable periods for HMS equipment

    unserviceability will be:

    Propagation rate of the failure mode being monitored

    Other means of monitoring the same failure mode

    Service history of similar failures on the helicopter type

    Any mitigating actions, such as checking previous HMS data to establish the

    indicator level and look for rising trends

    The Master Minimum Equipment List for each affected type will be amended


    6.1.10 Training

    Training must cover all personnel involved in HMS activity to ensure that the

    competencies necessary to ensure effective use of the systems can be achieved and

    maintained. CAA will periodically review training records, initially in order to accept

    compliance with the AAD and later during routine operation and maintenance

    approval audits.

    6.1.11 Event Reporting

    Any cases of successful VHM alerts, or defective components found where VHM failed

    to alert, should be notified to CAA. This will continue until system reliability has been

    confirmed and will not replace the need to submit an MOR where this is applicable.

    6.1.12 Quality System

    Describe links with company quality system Audit plan

    Remedial action

    Audit training

    More detailed guidance on some of the above subjects can be found in Appendix A.


    In demonstrating compliance using this method the applicant will also need to address

    the requirements of method A, (i.e. paragraphs 6.1.1 to 6.1.12 above and the guidancematerial of Appendix A). It is considered that normal means of compliance with this

    CAA AAD will be method A as it presents less of a cost burden to the operator. This

    section does not therefore repeat the method A requirement details.

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    Compliance by method A is predicated upon existing CAA approved health and

    vibration monitoring systems. These systems have already been assessed with respect

    to failure modes, which in most cases were assessed without the input of the

    constructors. The analysis of these failure modes and the positioning of sensors was

    however part of an extensive study over many years involving the CAA and theequipment providers. The algorithms and thresholds were established on the basis of

    analysis, component testing and a period of helicopter validation testing.

    In complying with method B the operator would be expected to base their application

    on the current JAA requirements which cover the aspects of health monitoring. The

    applicable requirements and their relevant advisory material can be found in:

    JAR 29.547(b) Main And Tail Rotor Structure;

    JAR 29.917(b) Rotor Drive System; and

    JAR 29.1309 Equipment, systems and installations.

    The above requirements necessitate that a design assessment including top downas

    well as bottom up(FMECA) techniques be carried out in order to determine all the

    rotor and rotor drive system components for which functional failure would prevent

    continued safe flight or safe landing of the helicopter. Once these components are

    identified any health monitoring provisions which are considered to be both

    technically feasible and economically justified must be identified and used to form a

    health monitoring specification.

    The helicopter health monitoring system must then be designed, manufactured anddemonstrated against this specification. Currently draft JAR / FAR advisory material

    is contained in NPA 29-18 which details the considerations necessary in order to

    claim safety credit for a health monitoring system.

    The CAA accepts that this task will be more difficult without the input of the

    constructor. Where this input cannot be obtained the CAA will take this into

    consideration in the demonstration of absolute proof against certain failure modes.

    It should be made clear that the purpose of the design assessment is to establish the

    failure modes, for which the likelihood of occurrence must be minimised. The CAA

    consider that, for conventional design helicopters (i.e. those which rely heavily uponsingle load path critical parts), in order to achieve the minimiseobjective above,

    vibration monitoring would be necessary.

    Approval of a new system design, as required for compliance by Method B, will

    require detailed discussion with the CAA in order to establish a satisfactory

    methodology for performing the design assessment, agree the basis of approval of the

    system hardware and software and demonstrate monitoring technique performance.

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    Management procedures within the organization should be established to include groundstation hardware and software support procedures, backup, archiving and retrieval of data. At

    bases where operators fly large numbers of helicopters, expanded ground station capability

    may be required to allow more than one user to access health and vibration monitoring data at

    any one time.

    Training - Initial and Continuing

    With the introduction of health and vibration monitoring the appropriate staffing levels and

    skills are necessary. Some of these skills such as aircraft system maintenance can be handledby existing staff with additional training. However, organizations unfamiliar with advanced

    health and vibration monitoring techniques or large computer based systems may require the

    employment of appropriate personnel.

    Experience with health and vibration monitoring to date has indicated that training is needed

    to make full use of the system. This can range from basics such as computer familiarity and

    keyboard skills to detailed training in the vibration algorithms and their significance. Such

    training is necessary as an ongoing task to maintain skills, implement improvements in the

    use of existing health and vibration monitoring capabilities, or implement added capabilities.

    Improving the ability of the system to analyse the data acquired and improving the interface

    with the system, the operator can reduce the training required to use the monitoring system.Integration of the monitoring system and its outputs with the maintenance and logistic support

    systems will also require training of the personnel involved to make full use of such


    The introduction of health and vibration monitoring to an operators fleet will demand new

    skills of that operator, in particular from the maintenance personnel who in most operations

    will be the principal interface point. These new skills should be identified and appropriate

    training carried out. In some organizations a cell of specialists is already in existence for

    condition and vibration monitoring.

    The helicopter constructor (if possible) and equipment supplier should work with the intended

    user to carry out a study of the available manpower and the abilities present in the personnel

    who will be tasked with carrying out health and vibration monitoring functions in the

    organization, with a view to identifying areas where additional training is likely to be

    required. Assessment of this training and the availability of a training plan and records will

    form part of the CAAs assessment for compliance with this AAD.

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    Computer Skills Training

    Both existing and intended potential health and vibration monitoring users have reported that

    personal computer (PC) operation is an area where basic skills are likely to be lacking. In

    many cases this may be the first use of computers as such by flight line personnel. While

    stores stock and technical records computers have been in use in this environment for sometime, the interfaces tend to be simpler and less intimidating to non-computerpeople.

    Many experienced line technicians have limited PC skills. Conversely skilled PC users tend

    to have less hands onhelicopter experience. Condition monitoring specialists may already

    have useful PC and data analysis/handling skills which will be a sound basis for advanced

    training. Any computer skills training may well need to be structured at two levels: basic,

    covering routine operations needed to transfer and assess data at line level, and advanced,

    covering additional tasks such as software and operating system installation, text editor usage,

    disk management, and communications/networking operations. Operators without previous

    experience in house may need to bring in specialist advice in these areas.

    One operator has reported that computer training specifically avoiding use of jargon has been

    effective in gaining acceptance of HMS and PCs as an engineering tool rather than a

    specialist function. Another operator has assisted staff in the purchase of PCs for home use.

    The resultant increase in computer literacy and awareness was considered to be a good

    training investment. Targeting and winning over engineers is seen as essential in successful

    health and vibration monitoring implementation.

    Data Interpretation Training

    Health and vibration monitoring data may require interpretation by the operator, as systemscurrently available tend to present much processed gearbox data in the form of esoteric defect

    indicators derived from gear vibration. The principles behind the indicator extraction and the

    significance of the indicators themselves will have to be taught to line operators who may be

    required to make go / no-go decisions on the basis of data produced by a system where there

    is an element of manual interpretation. The depth of training should obviously reflect the

    extent of exposure and decisions expected of the individual.

    Case studies will be required to assist training in diagnostics and prognostics. Threshold

    setting is a potentially controversial area. If the intention is to devolve authority to change

    thresholds, thorough training in data interpretation and the principles applied to establishingthresholds must be provided. This process and the associated controls will form part of the

    AAD compliance documentation submitted.

    Awareness Training

    Higher levels of engineering management, commercial departments, and pilots should have

    an understanding of how health and vibration monitoring operates and how the organisation

    uses it. While these people may not be exposed regularly or be required to carry out data

    interpretation they should have a basic knowledge of the system functions, capabilities and

    limitations. They should also be informed of any administrative or organizational changes

    which may be required for its use.

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    The operator will have to arrange maintenance training classes for his personnel in the use

    and intended applications . The training will prepare the operatorspersonnel to perform 1st

    (field), and 2nd (routine) level maintenance to the level of scheduled and unscheduledmaintenance based on HMS advisory messages.

    Training classes will have to cover the use of health and vibration monitoring data to schedule

    inspection, cleaning, replacement/repair, and for performing adjustments, operational checks,

    and troubleshooting of the system and monitored components.

    Two different philosophies have been reported regarding the approach to training. One is to

    instigate a formal classroom based course lasting several days, while the other is to carry out a

    series of less formal one day teach-ins, on a one-to-one basis.

    The classroom based training offers the potential of training to a deeper level of knowledge

    but can be lacking in practical content unless very carefully structured. This approach may not

    suit an operator who chooses to introduce system functions gradually, as training may well be

    forgotten before the opportunity to apply it is available. One day teach-ins have the advantage

    that training content will always be up to date and can be readily re-structured. If presented

    one-on-one, this training will also be able to focus on an individuals particular needs. This

    could, however, become a burden as it may not make the most efficient use of an instructors


    External courses run by equipment suppliers have been used by most system operators.

    Where the operator is not the equipment supplier, this has so far been the approach to trainingin data interpretation by the operator. Equipment suppliers courses have been reported, in

    general, as being too deep and too expensive for large numbers of line personnel. The

    approach has generally been to send monitoring specialists and supervisors on such courses

    and use them to train line personnel in house.


    Most operators using existing health and vibration monitoring, to date, have had a significant

    input into the installation design and in many cases have performed the installations in house.This experience has been invaluable in terms of building knowledge of the system and

    evolving fault diagnostic procedures.

    There may well be new or unfamiliar hardware in the HMS installation, e.g. the

    accelerometers, microdot connectors, and miniature co-axial cables used in the early

    production systems. These items require specialist handling and termination techniques which

    must be incorporated into initial technician training. Sensors themselves also present some

    new considerations with respect to testing and trouble shooting.

    From the helicopter operators point of view current HMS are complex systems. They

    interface with many other helicopter systems and are not self-contained in the way many otheravionic systems such as Flight Management System (FMS) or Electronic Flight

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    Instrumentation System (EFIS) can be. Successful trouble shooting may require a knowledge

    of the architecture of the system and software processes in use.

    Suggested topics for a users course are:

    (i) Cockpit Voice / Flight Data Recorder (CVFDR) system, associated components,Built In Test Equipment (BITE) indications, mandatory parameters, and data

    acquisition. Legal implications of data use (pilot confidentiality, etc), defect

    investigation, and rectification.

    (ii) Health and vibration monitoring sensors, interface/relationship to CVFDR,

    displays, and BITE indications. Gearbox / transmission monitoring principles

    and responses to indications. System troubleshooting and rectification.

    (iii) Rotor monitoring, diagnostics, and adjustments.

    (iv) Transmission trend and vibration monitoring.

    (v) Data transfer operations, ground station use including navigation through

    displays, and airborne system configuration if applicable.

    Personnel attending such courses will require background experience and knowledge of

    helicopter maintenance. There is some overlap of trade boundaries in the course requirements

    which will have to be considered and addressed by the course provider with respect to

    licensing and approval regulations applicable to individuals.

    Continuation Training

    HMS although not new is still a dynamic technology and as a consequence is changing

    rapidly. Continuation training is necessary as changes are implemented. Operators and

    equipment suppliers should hold regular reviews on the subject of such changes and revise

    training programmes to incorporate the effects of significant changes.

    Threshold Changes

    The refinement of the HMS limits will be an ongoing process accompanying the validation

    effort. Refining the limits requires an understanding of the relationship between the values ofthe various parameters or algorithms and the increase in the severity of the fault detected. In

    some cases some improvement in limits can be fairly easily made. Several instances of

    removal of a component for an indicated fault, followed by inspection of the component

    which shows that no fault or a level of damage which is insufficient to warrant removal of the

    component from service, would serve to indicate that some increase in the corresponding

    limit can be made. In general, however, one needs to have a history for the particular

    component to develop the relationship between the severity of the fault and the health and

    vibration monitoring indication or indications. In the cases of the existing approved systems

    this was developed for gearboxes by several research programmes sponsored by the CAA.

    The procedures and controls for these limits and any proposed changes will need to be agreed

    with the CAA and should be based upon past experience coupled with sound engineeringpractices.

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    Correlation of HMS data

    Correlation of health and vibration monitoring data with component tear down reports is an

    important part of the process of validating the vibration algorithms and limits. This requiresco-operation between the operator and the constructor (where possible, or equipment

    providers) or any other organization that overhauls / inspects the gearboxes or other


    The operator needs to obtain as much information as possible to relate the severity of the

    component damage to the health and vibration monitoring indications. This knowledge will

    permit better setting of criteria for defining fault indication levels which require action.

    Agreements between the operators and the constructors (where possible, or equipment

    providers) may be negotiated where information beyond that usually provided to the operator

    is requested.

    Flight Data Recorders

    Operators who use Flight Data Recorder sensors to gather parameters for HMS analysis are

    required to ensure that the data gathering systems related to those parameters are always


    Many operators who have begun to work towards JAR-OPS compliance will be aware of the

    5% rule in JAR-OPS which states that operators are allowed to have up to 5% of the

    parameters they are required to record inoperative before a data recorder is declared

    unserviceable. HMS operators should note that the 5% will not applyto any HMS parameters.The operability of all such parameters is mandatory unless agreed with the CAA under

    MMEL provision.

    Laptop Computers

    Careful attention should be given to any commercially available lap top computer which may

    be required to be connected to the airborne health and vibration monitoring system and

    operated when the helicopter is flying. Unlike avionics equipment these laptops are not

    subject to the same rigorous Electro-Magnetic Interference (EMI) tests and there have been

    cases of laptops interfering with helicopter systems whilst in use.

    System and Data Considerations

    1) Who Is Going To Use The System ?

    There are two aspects to this, firstly, which departments should have access to the system and,

    secondly, what level of expertise do they have in using the system?

    The question of who should have access to the system is very important, especially if the

    outputs of the system are important to the analysis processes. Systems whose output is

    important have to be sufficiently protected to prevent data corruption or loss. This impliesthat the only people who should have access to it are those who are going to be doing or

    analysing the processing related to that system. Even if the possibility of malicious sabotage

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    is discounted, if other departments such as sales or even other engineering departments, are

    allowed to have access to the system (even indirectly via a network) there is the possibility

    that system data could be accidentally lost or corrupted. There are other aspects to protecting

    health and vibration monitoring analysis systems from accidental or deliberate damage and

    these are discussed later in this CAP.

    The level of expertise of the intended users is just as important as the restricted use of the

    system. Health and vibration monitoring analysis is detailed and complex and if the analysis

    team is presented with a tool they are unfamiliar with their job can only become harder.

    If a completely new system is being selected the associated training needs of the staff must be

    considered and provided before the tool is put in to use, particularly if there is any level of

    importance associated with its output.

    Equally, if a modification is being made to the system, the effect on how the system is used

    must be carefully considered. Human factors research indicates that, when a system ismodified in such a way that it appears to be largely unchanged, errors start to occur because

    system operators incorrectly assume that it will perform in exactly the same way as the

    previous version. This misjudgement can, and does, lead to serious errors. If the system in

    question is calculating or analysing something that has an airworthiness impact on the

    helicopter, this should be carefully considered.

    2) How Can Data Be Protected?

    Several steps can be taken to protect data from corruption or loss and some of these, such as

    careful scrutiny of the gathering, storage and transfer systems have already been discussed but

    there are other steps that can be taken.

    Firstly, once the required function of a system has been determined, all software other than

    that needed for the required functionality should be removed from the machine it is running

    on. This will ensure that there is no way any other software programs can accidentally corrupt

    it. The system should also be isolated wherever possible. i.e. not connected to networks,

    adjacent systems or the internet as this will help to protect it from misuse and viruses. It will

    also help to protect it from the accidental corruption that can occur when interlinked networks


    A set of procedures related to the access and use of the system and its related data should alsobe created to prevent accidental misuse. These procedures should also cover any applicable

    security aspects of data protection such as the application of disk drive and store room locks

    and a list of who has keys to them. The procedures should be reviewed on a regular basis to

    ensure their continued applicability.

    Finally a set of verification and validation procedures should be put in place to ensure that

    any accidental or deliberate corruption of data can be spotted in time to compensate for it.

    These procedures should also deal with the issue of what to do if corrupted data is found and

    how and if it can be of continued use. Some systems that monitor component usage deal with

    corrupted data by taking a deliberately pessimistic view of the analysed flight and assigning

    figures that represent severe use and wear for that flight, this is one possible way to deal withcorrupted or missing data.

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    JAR-29 Issue 1. Large Rotorcraft. JAA/INT/POL/27 and 29/1, Issue 2. Protection From The Effects of HIRF.

    EUROCAE ED-12B, RTCA DO-178B Software Considerations on Airborne Systems and

    Equipment Certification.

    EUROCAE ED-14C, RTCA DO-160C Environmental Conditions and Test Procedures for

    Airborne Equipment.

    Airworthiness Notice 45A Software Management and Certification Guidelines.

    CAAIP Leaflet 11-16 Computer Control - Records and Programmes.

    AMC / IEM 20-115B

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