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Appendix B CorrespondenceâAir Transport Association of America 99
Air Transport Association ata OF AMERICA 1709 New York Avenue, NW Washington, DC 20006-5206 Phone (202) 626-4000 December 19, 1986 Mr. Bernard Haggin Aeronautics and Space Engineering Board JH 413 National Research Council 2101 Constitution Avenue Washington, D.C. 20418 Dear Bernie: The purpose of this letter is to provide you with further information for use in the NRC AD hoc Committee final report to NASA on the Status and Viability of Composite Materials for Aircraft Structures. At your suggestion, we asked several ATA member airlines to update the 1977 SAMPE paperl/. As you will recall, three airlines have made earlier comments on the NRC study. The airlines generally believe that notwithstanding efforts by the airframe manufacturers, their most recent technology transports are still showing problems that indicate any future R&D program recommended by the NRC Committee should include detailed attention to conditions experienced by the operators of new technology aircraft. The current list of problems is not much different from those discussed in the SAMPE paper. One way to put it is: can new technology reduce the overall cost-of-ownership? From listening to the DOD briefers who operate advanced aircraft which incorporate composite materials one would conclude that their operating and maintenance difficulties are similar to those of the civil operators. Airlines operating the most recent domestic technology aircraft provided ATA with the following specific comments. "We have observed the following problems in our present aircraft composite structure which are basically graphite/epoxy and graphite/kevlar/epoxy construction: I/ "Today's Non-Metallic Composite Airframe Structure -- An Airline Assessment" by H. H. Kuperman and R. G. Wilson, of United Airlines.
- 2 - 1. Paint and resin matrix cracking leading to water ingestion and freeze/thaw delamination 2. Lightning strike damage 3. Inadequacy of aluminum flame spray lightning protection 4. Abrasion/erosion damage 5. Foreign object impact damage We have observed these problems over a period of 4 years. We believe 3-5 years are sufficient to disclose operating problems pertaining to composite structure. One general problem with composite panels is erosion of the leading edge on external panels. The worst erosion is seen on fan cowl doors, landing gear doors and wing leading edge panels. Erosion typically starts at the forward edge and extends back one quarter inch or more, involving several plies of material. If damage is not too severe, the panel edge may be smoothed by chamfering, then applying an epoxy resin to seal the exposed grain. A possible production improvement would be to wrap the edges of composite panels with a strip of fiberglass so that the end-grain is not exposed to wind and moisture. Another problem inherent to Kevlar composite panels is water ingestion. Although we have had no discrepancies reported on one new technology aircraft to date, we do have experience on another new technology aircraft to draw from. Kevlar panels must be topcoated with a flexible polysulfide sealer to prevent water ingestion. Unsealed panels can ingest detrimental amounts of water after only 12 to 24 months in service. Refinishing Kevlar panels previously topcoated with polysulfide sealer is another problem. It is difficult to scuff-sand the panel without sanding into the sealer. When this happens, the entire sealant coat must be sanded off. Sanding pads must be changed frequently since the sealer tends to gum up the pads. We did not want to use a sealant top coat in anticipation of the refinishing problem. However, in order to preserve the warranty provisions, we have continued to use the sprayable sealant topcoat. Another potential problem with composites was discovered recently during our initial ultrasonic inspections of rudders and elevators using recommended procedures. The ultrasound signal was attenuated (absorbed) over much of the inspection area to the extent that the inspection could not be completed. The aircraft manufacturer recommended that we revert to visual and coin-tap inspections.
- 3 - By design, composites tend to be dry (having the minimum acceptable amount of resin) in order to conserve weight. This may create tiny voids or air pockets which may attenuate an ultrasound signal and/or make the panel more susceptible to moisture ingestion and leading edge erosion. The composite ultrasound calibration standards provided by the manufacturer were manufactured with generous amounts of resin and yield excellent test reading. This particular comment points out the importance of establishing and maintaining quality control in manufacturing, which may be more difficult to achieve in composites. In order to enhance future applications of composites, manufacturers should emphasize quality control, reliability and maintainability. Weight savings loses its significance if the structure cannot be maintained." Another area mentioned by the airlines is the infrequent necessity, but costly in terms of the lost revenue, to ferry an aircraft from a field station to a major repair facility having appropriate capabilities to repair damaged composite structures. A previous ATA letter to you discussed some safety considerations worth examining in future R&D for use of composite materials in major fuselage and wing structures -- crash-impact dynamics and fireworthiness. As in other new technology areas, the excellent safety record of existing technologies should be maintained or enhanced, if economically possible. It is our understanding that the existing NASA composites program contains little, if any, specific safety content. On the other hand, the FAA has a limited safety program devoted to composites. The NRC Committee may wish to consider a recommendation for development of a joint NASA/DOD/FAA- industry (manufacturers, vendors, airlines, DOD) program that encompasses pertinent maintenance and safety aspects in addition to performance objectives. ATA member airlines appreciate NRC consideration of the above comments and request the opportunity to comment on future R&D endeavors the NRC may recommend on this subject. Please let us know if we, or the ATA members, can be of further assistance. Sincerely, ~ Dick Tobiason Director - Engineering Safety Technology DT:jp
