mark vellacott – sms steve hall – cac · 2 overview ¡brief overview of widespread fatigue...

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SAFE AND ECONOMIC MANAGEMENT OF WIDESPREAD FATIGUE DAMAGE (WFD) USING

PROGNOSTIC / DIAGNOSTIC STRUCTURAL HEALTH AND USAGE MONITORING

Mark Vellacott – SMSSteve Hall – CAC5th DSTO HUMS Conference

Melbourne, Australia, 20/21 March 2007

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OVERVIEW

Brief Overview of Widespread Fatigue Damage (WFD)

The implications of WFD in the context of the Aging Aircraft Environment

How the FAA is proposing to address WFD and the Operational Implications of their approach

Basis for an Alternate Means of Compliance (AMOC) for addressingWFD using Prognostic/Diagnostic Health and Usage Monitoring

Overview of the Structural Health Monitoring (SHM) & ComparativeVacuum Monitoring (CVM) Technologies

Conclusions

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A PIVOTAL EVENT!(Aloha Flight 243 – April 28 1988)

A major portion of the upper crown skin and structure of section 43 separated in flight causing an explosive decompression of the cabin. The damaged area extended from slightly aft of the main cabin entrance door, rear ward about 18 feet to the area just forward of the wings and from the left side of the cabin at the floor level to the right side window level.

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WFD – A TWO HEADED MONSTER!

Immediately following the Aloha incident the concept of Multi-Site Damage (MSD) was identified

Multiple damage propagation within a single structural elementFor example multiple cracking located at holes within a single row of rivets

Subsequently it was realized that there was also the possibilityof Multiple Element Damage (MED)

Damage propagation in adjacent structural elementsFor example damage occurring within two adjacent bays of a wing box

WFD can occur as a result of MSD, MED or both

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OVERVIEW





Overview of the SHM & CVM Technologies

Conclusions

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AGING AIRCRAFT - AN ISSUE THAT CANNOT BE IGNORED

In response to the Aloha Airlines incident the FAA initiated the Aging Aircraft Program and formed the Airworthiness Assurance Working Group (formerly the Airworthiness Assurance Task Force)

The AAWG implemented a number of programs for pre damage tolerance certified aircraft aimed at providing an equivalent level of inspection and maintenance scrutiny as that applied to damage tolerance certified aircraft

The initial programs developed by the AAWG were:

Mandatory Modification Program (MMP)Corrosion Prevention and Control ProgramGeneric Structural Maintenance Program guidelines for aging aircraftSupplemental Structural Inspection Documents (SSID)Repair Assessment Program (RAP)

Subsequently the AAWG were also tasked to develop

Recommended procedures to preclude the occurrence of WFD within the transport aircraft fleet

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DAMAGE TOLERANCE VERSUS WFD

Damage Tolerance

Primarily deals with localized cracking as a result of a manufacturing or design flaw

Generally looking at relatively stable and detectable crack growth

As it is considered that with the application of appropriate techniques such damage can be detected the regulatory approach adopted is one of “problem management”

Inspection and maintenance intervals can be calculated and applied accordingly

Widespread Fatigue Damage

Primarily deals with the wearing out (fatiguing of a structure)

Possibility of small cracks, often barely detectable at multiple locations

Sudden linkage of these cracks can cause an “instantaneous” catastrophic failure

Due to the uncertainty associated with the reliability of detection, the regulatory approach adopted is one of “problem avoidance”

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THE IMPACT OF WFD ON THE OPERATIONAL AGING AIRCRAFT FLEET

While regulatory agencies, OEMs and operators all recognize that WFD is a challenge of which everybody has to remain cognizant; it is interesting to note that there has yet to be acatastrophic accident that is directly attributable to WFD

Even in the Aloha Incident WFD was only cited as a contributory factor with poor maintenance and inspection being cited as the other contributory factors

While the FAA have rightly stated that there is currently no program that has been implemented under the aging aircraft program that specifically addresses WFD, the AAWG (industry experts constituted by the FAA) note that the current aging aircraft programs do appear to be effective in detecting WFD

As evidenced by the lack of incidents attributed solely to WFD

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OVERVIEW






Conclusions

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WFD - MANAGEMENT BY AVOIDANCE

“IMPLEMENT STRUCTURAL MODICATION/REPLACEMENT IN A TIMEFRAME THAT WILL ENSURE THAT THE PRINCIPAL STRUCTURE IS REASONABLY FREE FROM CRACKING

(WFD)”

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HOW DO YOU ENSURE THAT WHAT YOU CANNOT SEE ISN’T THERE?

Originally this was based on fatigue test evidence which could include one or more of:

A Full-Scale Fatigue Test (FSFT)Service Experience (Basis for many of the SSIDs)Other evidence such as numerical/analytical analysis

Latest Proposed Regulations (NPRM FAA-2006-24281) Are gravitating towards the FSFT being the only way of demonstrating that WFD will not occurAdvocating that only a Design Approval Holder (DAH) who has access to all the design data can establish when WFD may or may not occur

Original Equipment Manufacturer (OEM)Current Type Certificate Holder

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EXAMPLES OF FATIGUE TESTS

Definite benefits to having an FSFT result

Identification of failure mechanismsDevelopment of transfer functions

However, the cost of implementing such a test for aircraft mid-late life will in many cases be without any FSFT Requirement

Pre FAR 25.571 Amendment 45 (1978)

Need to rememberFSFT provides one test pointResult is dependant on:

Representative spectrumRepresentative Configuration

May or may not be representative of design role usage in 20 -30 yearsQuestionable relevance for aircraft being used in non-design roles

Special Mission Aircraft (Often low-level)

S-2 Tracker Circa 1980’s

S-3 Viking Circa 1990’s

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DAH ESTABLISHMENT OF INITIAL OPERATING LIMIT

The FAA consider that the required Initial Operating Limit (IOL)can only be established by the DAH

On the basis they have all the design dataNote that an Extended Operational Limit (which may require as much expertise/knowledge as establishing an IOL) can be undertaken by a third party

Challenge is what happens if the DAH decides they do not want to establish an IOL for non-Technical reasons?

From their perspective it is not economically viableThey are concerned about liability issuesFor commercial/competitive reasons they no longer wish to support the aircraft

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DEFINING AIRCRAFT INITIAL OPERATIONAL LIMITSIn an attempt to facilitate the implementation of their proposed WFD Regulations, the FAA attempted to define Initial Operational Limits for current aging aircraft fleets

There are three main areas of concern with their approach

Many of the limits that have been defined have already been significantly exceeded by a number of operational aircraft

Validity of these estimates as if adhered to they would require immediate grounding/action of actual operational aircraft even though their service records provide little to no basis for such action

L382 (Civil C-130) is a good example

The definition of what should be counted (monitored) is technically inconsistent (FAA has only specified take-off/landing cycles )

For fuselages it is logical to specify take-off/landing cycles (pressurizations) as this is the dominant factorFor wings it is logical to account for the occurrence and frequency of individual load excursions as the loads applied to the structure can vary significantly within the context of a given take-off/landing cycle

Beware of implicit spectrum assumptions!

The limits are only being applied to aircraft with Gross Take-off weights of 75,000lbs or greater. Lower weight aircraft (primarily regional jets) are excluded on the basis that their average age is less and therefore WFD is less likely

Ignores one of the major lessons of Aloha and other incidents, ie: the significant factor in fatigue is the load to which a structure has been subjected not its chronological ageThis violates the FAA’s own “One-Level” of Safety Rule

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IMPACT OF THE PROPOSED WFD REGULATIONS

Significant financial on the aircraft re-sale marketReduce the re-sale value of many aircraft being adapted for cargo or special mission roles (eg: aerial firefighting), particularly for those aircraft whose designs were certified pre FAR 25.571 Amendment 45 (requirement for structural fatigue testing)

Cost of converting and/or acquiring converted aircraft would be significantly increased as operators would forever be captive tothe DAHs

Who might not want to establish new IOLs for non-Technical reasons

Many of the aging aircraft affected provide vital infrastructure for remote communities/communities in crises

Even if the funding were available to replace these aircraft (in many cases unlikely) in a number of instances there are currently no suitable aircraft that could replace them

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MAINTAINING PERSPECTIVE

To address a problem, WFD, that to date has not yet been the sole contributor to any commercial aircraft accident/incident

It is important to realize that WFD is a real problem that couldbecome far more prominent if left unaddressed

The purpose of this paper is not to advocate complacency with regard to safety

However, given the immediate and ongoing economic and socio-economic impact of the approach that is being proposed it is necessary to ask:

Is there an alternate approach, economically viable, approach that can be used to address and manage WFD without compromising operational aircraft safety?

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OVERVIEW






Conclusions

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ALTERNATE MEANS OF COMPLIANCE (AMOC)

The FAA already supports the concept of Alternate Means of Compliance (AMOCs)

Exist in recognition that Advisory Circulars (AC) define one means but not the only means of regulatory compliance

Of particular interest is the possibility of obtaining an Equivalent Level of Safety (ELOS) finding

Intended for circumstances when direct compliance with regulations is not possibleThe identification and demonstration of the elements that ensure an equivalent level of safety to that intended by the regulations is a key to a successful submission

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Overall Structural Health Management of an Aircraft

Certified, Safe and

Economically Viable Aircraft

Inspection, Maintenance and Overhaul Intervals

Fatigue and Damage

Tolerance Analysis

Critical Area Identification

Previous Flight or Full-Scale TestsPast Service History

Maintenance Records in Different Roles

Aircraft Configuration and Model Variants

Loads Actually Experienced by the Aircraft

in Critical Areas

Critical Area Geometry Factors

Relevant Materials Data

Parameters to be Monitored

Recorders and Instrumentation

Operational Data Acquisition and

Validation

Data Analysis and Dissemination

Development of New Techniques

Depot LevelField Deployable

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WFD CONCERNS AND ADDRESSING THE WFD REGULATORY INTENT

ConcernsAs aircraft “age” they will become susceptible to WFDWFD may not be detectable prior to it linking up and causing catastrophic failureIf WFD cannot be consistently and reliably detected the likelihood of it occurring has to be avoided

Alternative Means For Addressing Regulatory IntentEstablish the structural status of large transport category aircraft (ie: where are they in their life cycle)Establish the point in the life cycle where WFD can be expected to occur (IOL)Establish analytical and predictive methods that establish when structure needs to be replaced to avoid WFDOn going SHM and damage monitoring of aircraft

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OVERVIEW






Conclusions

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TWO KEY COMPONENTS IN THIS APPROACH

Ongoing Structural Health Monitoring (Prognostic Component)Allows an accurate assessment of an aircraft’s ongoing structural health status based on actual usage as opposed to basing it on what it was assumed the aircraft might be doing some 20 – 30 years previously

Effectively stops operators “flying blind”

Detecting Damage at critical locations as early as practicable (Diagnostic Component)

The in-situ NDI sensor technology Comparative Vacuum Monitoring provide the potential to identify cracking at multiple locations during the early stages and replace structure as appropriate

Combining these two technologies can provide a powerful tool that can be used to manage the concerns related to WFD in an operationally viable and cost-effective manner

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OperationalBase # 1

OperationalBase # 2

OperationalBase # N

Central ProcessingFacility

EngineeringAnalysis

StructuralAnalysis

CorrosionAnalysis

Life-CycleManagement

Inspection andMaintenance

OperatorFeedback

InspectionsCompleted

Inspections/Maintenance

Required

InstrumentationMaintenance and

Support

“Day-to-Day”Management

Medium/Long-Term

Management

ONGOING STRUCTURAL HEALTH MONITORING

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C-130A AERIAL FIREFIGHTING AIRCRAFT (3,000 gallon)

Flight Test at U.S Army Test Range in Yuma Arizona

End February 2003Defined Profiles (No Fire)

Calibration FlightsTypical Firebombing Terrain

Level and MountainousTwelve Continuous Parameters

AccelerationsStrainsControl Positions

Eight Discrete Parameters

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door closeddoor open

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COMPARATIVE VACUUM MONITORING - CVM

Sensor is comprised of alternate galleries of vacuum and air and can be customized for individual structures

Self adhesive sensors with simple surface preparation similar to that used for a strain gauge

90/95 PoD Demonstrated Crack Length Detection Capability below 2.5mm (0.100”)

Applied to surface using pre-applied pressure sensitive adhesive (12 hour cure)

External sensor shown, but embedded sensors have been developed

Multiple sensors can be coupled together to provide larger area coverage

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CVM SYSTEM

CVM System consists of three primary components

Sensor (Elastomeric so it can conform to shaped surfaces and go over fasteners etcFluid flow meterStable source of low vacuum

Surface of the component of forms one part of the vacuum manifoldSystem works for both metal and composite structural substrates

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CVM SENSORSSurface Molded Silicone

ElastomericConformable to compound curved surfaces

Surface SheetVery low mass polymer (0.005” / 125um thick)Long length / large areaLaminated adhesiveLaser ablated automated manufacture

Integral SheetLap and butt joint monitoringRepair monitoring

Load BearingBolt hole crack initiation monitoringWasher replacement sensor

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Boeing - Common Maintenance NDT Practices Manual

Boeing has agreed to include CVM technology into its common maintenance NDT practices manual for a range of inspection applications on Boeing civil aircraft.SMS is working with Boeing & US airlines to enable CVM technology for an initial set of service bulletin inspections.

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Airbus – Joint Development Agreement

Sensor Network

CVM technology ready for introduction on new and existing Airbus aircraft

Development of CVM for an in-flight SHM system Use of CVM for A380 Full Scale Fatigue Testing

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Military Aircraft – Qualification Programs

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SAFAIR – Hercules Lead Fleet Installation

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C-130 RAINBOW FITTING

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OVERVIEW






Conclusions

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CONCLUSIONSAs a result of the Aloha Airlines structural Failure and recent FAA New Proposed Rule changes WFD is gaining increasing prominence

Proposed regulatory action aimed at addressing WFD could have profound and detrimental economic and socio-economic implications for many current operational aircraft fleets

Given the number of incidents that have been solely attributed to WFD it is considered that there might be more economically viable approaches to managing WFD

It is proposed considered that Alternate Means of Compliance based on an Equivalent Level of Safety Finding could be used to address WFD concerns. One practical method for developing an ELOS finding utilizes a prognostic/diagnostic approach to aircraft health management that combines Structural Health Monitoring and Comparative Vacuum Monitoring technologies

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QUESTIONS?

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CONTACT INFORMATION

Stephen HallCeleris Aerospace

+1 613 837-1161 x [email protected]

Mark VellacottStructural Monitoring Systems

+61 8 [email protected]