SHM QUALIFICATION PROCESS AND THE FUTURE OFAIRCRAFT MAINTENANCE

F. Dotta∗, R. P. Rulli∗, P. A. da Silva∗, L. C. Vieira∗, A. K. F. Tamba∗, G. O. C. Prado∗, D.Roach∗∗, T. Rice∗∗

∗Embraer S.A., Brazil , ∗∗Sandia National Labs, USA

Keywords: Structural Health Monitoring, Qualification Process, CVM, LW, Aircraft Maintenance

Abstract

Structural Health Monitoring (SHM) is a toolthat has the potential to revolutionize aircraftmaintenance. When compared to current NonDestructive Test (NDT) technologies, SHMcan reduce the amount of time and burden ofthe inspection tasks, it can provide facilitatedstructural damage detection in areas with re-stricted access with early detection of flaws, andalso allow the reduction of maintenance costsdue to less time-consuming and less complexmaintenance procedures.

There are two different types of applicationfor SHM technologies, Scheduled StructuralHealth Monitoring (S-SHM) and AutomatedStructural Health Monitoring (A-SHM), whichare commonly recognized by the SHM com-munity. Installation, integration and operationrequirements depend on each type of application.Aircraft industry requires reliable SHM systems.In order to determine systems’ reliability, Em-braer selected two different SHM technologiesfor a more in-depth study.

These two SHM technologies were exten-sively investigated through ground tests withmetallic and composites structural componentsand assemblies and through tests on-board of aflight test aircraft. After demonstrating strongresults on ground tests and in the flight testaircraft, Embraer decided to develop a projectusing the S-SHM approach for the qualification

of Comparative Vacuum Monitoring (CVM) andLamb Waves (LW) technologies and to validatethe performance of such systems in real-lifeoperational environment. This project includedlaboratory tests for the assessment of detectioncapabilities and tests with systems installedon a number of operator’s aircraft to checkoperational behavior.

Detection capability was demonstrated interms of Probability of Detection according tothe One-Sided Tolerance Interval methodology.SHM sensors and cables were installed into fiveaircraft operated by an airline, and were periodi-cally assessed, demonstrating their survivabilityand durability in real operational environment.

A formal process for S-SHM implementationwas established, allowing the next steps for SHMdamage detection systems application. ReliableSHM systems will allow time and cost reductionof structural inspections without affecting, oraffecting positively aircraft safety.

1 Introduction

Aiming to meet Embraer’s roadmap for theimprovement of aircraft maintenance, SHMtechnologies such as CVM, Electro-MechanicalImpedance (EMI), Acoustic Emission (AE)LW have been studied by Embraer Researchand Technology teams [1], including applicationscenarios for structural damage detection and the

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improvement of inspection tasks.

According to Airlines for America (A4A)MSG-3 document [2] and the ARP-6461 doc-ument from SAE International [3], there aretwo different types of application for SHMtechnologies: S-SHM which means the use ofSHM devices for inspections at an interval setat a fixed schedule; and, A-SHM that relieson the SHM system to inform maintenancepersonnel that action must take place. In theEmbraer perspective, S-SHM application typeconsiders the installation of SHM sensors, cablesand connectors into the aircraft with periodicscheduled structural inspections being performedon-ground with ground support equipment. And,the A-SHM application type will be the installa-tion of not only sensors, cables and connectors,but also SHM interrogators and data recordersinto the aircraft. In such a case, the structuralinspections would be performed automatically atany time in small time intervals or continuously,what would require to the system to have, atleast, a power supply during the regular aircraftoperation. These two types of SHM applicationhave different installation, integration and oper-ation requirements depending on the presenceof components, such as interrogators, installedin the aircraft, eventually powered up duringflight and connected to avionic systems for datatransfer.

For both S-SHM and A-SHM applications,reliable SHM systems are required. Two SHMdamage detection technologies were selected byEmbraer for a more in-depth study, which areCVM and LW.

2 Embraer Background on SHM

Embraer has performed laboratory tests withCVM such as the application of the technologyfor the continuous inspection of rivet holes ina metallic R&D barrel test, the application indifferent locations of the Full-Scale Fatigue Testof the company’s E-Jets aircraft - such as shear

clips, splice joints, windows frames and rivetholes - and application in many other structuralcomponents and assemblies [4]. Figure 1 showsexamples of CVM sensors installed in both theR&D barrel test and in the E-Jets Full-ScaleFatigue test.

Fig. 1 Examples of CVM sensors installed in ametallic R&D barrel test and in the E-Jets Full-Scale Fatigue test

Laboratory tests with LW technology havealso been performed in metallic and compositestructural components (Figure 2) and assemblies,such as coupons, E-Jets Full-Scale Fatigue Test,barrel tests and others [5].

Besides the laboratory tests, ComparativeVacuum Monitoring and Lamb Waves technolo-gies were installed in the Embraer-190 flighttest aircraft in 2010, as showed in Figure 3. Inthis study, only sensors, cables and connectors

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SHM QUALIFICATION PROCESS AND THE FUTURE OF AIRCRAFT MAINTENANCE

Fig. 2 Examples of LW application for damagedetection in metallic and composite materials

were installed in the aircraft. Inspections wereperformed periodically using ground supportequipment.

3 Qualification of CVM and LW Technolo-gies

After demonstrating strong results on groundtests and in an Embraer-190 flight test aircraft,Embraer decided to step forward. In an effort tomove S-SHM into routine use for aircraft main-tenance procedures, a project was developed forthe qualification of CVM and LW technologiesand to validate the performance of such systemsin real-life operational environment. The work

Fig. 3 Examples of LW and CVM sensors in-stalled in the Embraer-190 flight test aircraft

aimed to develop and carry out a qualificationprocess for SHM damage detection systems,which includes laboratory tests (Figure 4) forthe assessment of detection capabilities in termsof Probability of Detection (POD) and to verifydurability, and tests with systems installed ona number of operator’s aircraft to check opera-tional behavior, survivability and stability of thesystems.

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Fig. 4 Mechanical tests for CVM and LW sys-tems installations

3.1 Laboratory Tests

The aim of the laboratory tests was to evaluatetwo major aspects of the SHM technologies,Detection Capability in terms of POD, andDurability. Considering specific scenarios forCVM and LW solutions, the target for detectioncapability was to determine crack length having90% probability of detection with 95% of confi-dence. Specimens used in those tests representedregions of an Embraer aircraft structure. TheFigure 5 shown a crack detected by CVM sensorduring a POD testing program.

Fig. 5 Specimen with a crack detected by CVM

The document MIL-HDBK-1823A [6, 7]presents an approach for POD determinationwhich was originally intended for traditionalnon-destructive testing. Another approach forassessing the system’s detection capability is theOne-Sided Tolerance Interval methodology [8],which was selected for POD determination inthis project.

As part of the durability investigation en-vironmental tests were performed with CVMand LW sensors and cables installed in platesmimicking actual aircraft installations (Figure6. These plates were submitted to hot-wet plusfreezing cycles aiming to provide informationabout the durability of systems’ componentssubjected to harsh environment.

Fig. 6 Environmental Tests and Test Aparatus

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3.2 In-service Aircraft Tests

Both CVM and LW technologies were installedinto five aircraft operated by Azul Airlines inselected regions in order to verify the opera-tional behavior, survivability and stability of thesystems (sensors and cables) subjected to a realoperational environment, for a period of time ofat over 24 months. No damage detection wasenvisaged in those aircraft.

Data acquisition has been performed peri-odically during overnight intervention (Figures7 and 8) allowing accumulation of a very rep-resentative amount of data. The SHM systeminstallations involved a total of 32 CVMs sensorsand 26 LW Smart Layers. Installations wereperformed in the facilities used by the airlinefor regular maintenance according to ervice Bul-letins (SB) issued by Embraer on Passenger DoorSurrounding and Central Fuselage II. Sensorsand cables of Comparative Vacuum Monitoringand Lamb Waves damage detection technologieswere installed in different opportunities.

Fig. 7 Accessing the interface connector by re-moving a single panel

Fig. 8 Data acquisition with a ground equipment

3.3 Results of the Qualification Project

SHM detection capability for the selected struc-tural components was obtained with laboratorytests for both Comparative Vacuum Monitoringand Lamb Waves systems through the determi-nation of Probability of Detection.

All data were compiled and analyzed by Em-braer and then submitted for Brazilian regulatoryagency (ANAC) approval. A formal process forS-SHM implementation was established withthe close consultation of the ANAC, and thefeasibility and the durability of damage detectionsystems were also demonstrated to support thevalidation.

4 The Future of Aircraft Maintenance withSHM

In the short term S-SHM has the potential to be-come a reality, where damage detection systems’components (such as sensors and cables) will be

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installed in the aircraft for the accomplishmentof scheduled inspection tasks, providing an alter-native to traditional Non Destructive Inspection(NDI) methods, for instance.

During maintenance inspections groundsupport equipment will be attached to connectors(strategically placed in the aircraft in easy accesslocations/hatches), sensors will be interrogatedand results will be displayed immediately bythese ground equipment. This will expedite in-spections, providing accurate information aboutthe presence of damage, avoiding unnecessarydisassembly, reducing time and cost withoutaffecting, or affecting positively aircraft safety.

However, current available technologieshave different readiness levels for aircraft appli-cation. Demonstrating the reliability of SHMdamage detection systems adequately is veryimportant. For instance, considering the twoSHM technologies involved in this work, CVMdemonstrates a high level of maturity, havingnot so complex installation and operation proce-dures. It presented good performance during thetests, being considered by Embraer as a possibleoption for hotspot inspections (as an alternativemeans of compliance). Lamb Waves has alsodemonstrated good results on both laboratoryand in-service tests, but it requires further studiesin order to better understand variables whichaffect system responses and to develop morerobust installation and operation procedures.

In the long term, with the maturation of SHMdamage detection systems and their evolutionto the Automated Structural Health Monitoringconcept, these systems will be monitoring theaircraft structure and, when detecting an event,they will be capable to inform maintenance per-sonnel that an action is required, leading to therealization of the Condition Based Maintenanceconcept.

5 Acknowledgements

The authors wish to acknowledge the participa-tion and the support of the involved personnelfrom Embraer S.A., Sandia National Labs,Azul Airlines and Brazilian regulatory agency(ANAC).

6 Contact Author

For further information please contactfernando.dotta@embraer.com.br.

References

[1] Rulli R P and Silva P A. Embraer Perspec-tive for Maintenance Plan Improvements by Us-ing SHM, 3rd Asia-Pacific Workshop on SHM,2010.

[2] Airlines for America. MSG-3 Opera-tor/Manufacturer Scheduled Maintenance -Volume 1 - Fixed Wing Aircraft - Revision2009.1.

[3] SAE International. ARP-6461 - Guidelines forImplementation of Structural Health Monitor-ing on Fixed Wing Aircraft. 2013.

[4] Rulli R P and Silva P A. Overview of CVMTechnology Tests Performed by Embraer, 8thInternational Workshop on SHM, 2011.

[5] Dotta F, et al. Early Results of Lamb Waves Ap-proach to Assess Corrosion Damage Using Di-rect Image Path in an Aeronautical AluminumAlloy, 8th International Workshop on SHM,2011.

[6] Department of Defense. 2009. MIL-HDBK-1823A: Nondestructive Evaluation System Re-liability Assessment.

[7] Pado, L. E., J. B. Ihn, and J. P. Dunne. 2013.“Understanding Probability of Detection (POD)in Structure Health Monitoring Systems”, pre-sented at the 9th International Workshop onSHM, September 10-12, 2013.

[8] Roach, D. 2013. “Validation and VerificationProcesses to Certify SHM Solutions for Com-mercial Aircraft Applications”, presented at the

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SHM QUALIFICATION PROCESS AND THE FUTURE OF AIRCRAFT MAINTENANCE

9th International Workshop on SHM, Septem-ber 10-12, 2013.

Copyright Statement

The authors confirm that they, and/or their companyor organization, hold copyright on all of the origi-nal material included in this paper. The authors alsoconfirm that they have obtained permission, from thecopyright holder of any third party material includedin this paper, to publish it as part of their paper. Theauthors confirm that they give permission, or have ob-tained permission from the copyright holder of thispaper, for the publication and distribution of this pa-per as part of the ICAS proceedings or as individualoff-prints from the proceedings.

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