AIRLINES are desperate. With jet fuel over $4 per gallon and still climbing, American, United and other major carriers are raising fares...
AIRLINES are desperate. With jet fuel over $4 per gallon and still climbing, American, United and other major carriers are raising fares, cutting flights, trimming fleets and laying off pilots. They’re also ordering fuel-efficient Boeing 787s and Airbus A350XWBs — the new generation of plastic planes.
These new aircraft promise 20-percent-lower fuel consumption. Replacing heavier traditional aluminum alloys, 50 percent of their skins, panels and load-bearing structures are comprised of lighter, stiffer carbon-fiber-reinforced-plastic (CFRP) composites. Then add the latest, most fuel-efficient engine technology. Sounds good.
But beneath these advantages danger lurks — novel maintenance challenges for which neither airlines nor the Federal Aviation Administration (FAA) are prepared. Overall, today’s jetliners have reached a plateau of such aerodynamic and propulsive efficiency that individual Boeing 737s or Airbus A300 aircraft often spend two or three decades in service, as will new 787s and A350s.
Regarding fleet recapitalization, that was good news for the airlines’ bottom line — and for their stockholders — until fuse pin metal fatigue allowed engines to fall off the wings of 747s and corrosion caused the 1988 explosive decompression of an Aloha 737 at 24,000 feet, as the top of the fuselage peeled back and sucked out a flight attendant.
Now, composite aircraft components have also begun to rain from the sky.
Shortly after takeoff in November 2001, the entire composite vertical fin of American Airlines’ Flight 587 was ripped from the A300’s fuselage; 265 people died. On March 6, 2005, an AirTransat Airbus A310 barely made it back to Cuba after its all-composite rudder disintegrated at 35,000 feet, possibly triggered by an uncommanded rudder pulse similar to one that may have doomed Flight 587. This spring, a composite panel detached and tumbled to Earth from the trailing edge of the left wing of an older US Airways 757.
The list goes on. Stiff, rustproof composite components may be lighter than metal alloy parts they replace, but they can then fail catastrophically. We have begun to learn, too, that they have a unique “fatigue plateau.” After tens of thousands of flight and pressurization cycles, they can undergo hidden disbonding, delamination and ply separation from impacts or stresses from in-flight upsets.
Moisture (humidity, hydraulic leaks) can intrude between ply layers, followed by expansion-contraction, freeze-thaw cycles across the 150-degree temperature range often encountered during jetliners’ multiple daily flights at altitudes up to 37,000 feet.
Following the American’s 587 disaster (and as late as 2005), Airbus recommended only external visual inspection or an inadequate surface tap test for composite structures suspected of internal flaws. Trying other methods, American Airlines found additional A300s with damaged fins. Yet only now, according to a leading industry publication, is the FAA-sponsored National Institute for Aviation Research beginning to look “into fluid ingression damage mechanisms in composite sandwich structures.”
It may have results by 2010 — when such research should have begun 30 years ago. With the flying public facing increased danger as composite structures age, questions need answering: Do airlines stock the range of new nondestructive testing technologies needed to test panels and struts of differing thicknesses and composite composition? Are airline and outside technicians fully trained on these machines?
When will the FAA and its European counterpart, EASA, mandate specific inspection intervals for differing composite parts? Honeycomb sandwich panels and load-bearing CFRP attachment lugs require different time intervals. Flight 587 crashed, but in 1991 and 1997, an Airbus A310 and an A300 had suffered upsets in which their vertical stabilizers endured stresses higher than even ultimate load projections — and survived.
When will EASA and the FAA mandate that Airbus relabel its ultimate load projection as its in-service limit load — consequently requiring that it strengthen by 50 percent all A300 fins in airline fleets? What about upgraded testing standards for new Airbus A380 and A350XWB designs?
Earlier Boeing 777s and Airbus A320s have many composite components beginning to reach the fatigue plateau cited above. Moreover, with the superjumbo A380 now in service and the largely composite 787 and A350XWB both soon to fly, airlines and the FAA must quickly address new and complex challenges of composite structure testing and maintenance — before disaster strikes again.
Lee Gaillard’s articles on aviation issues have appeared in Airways magazine, as well as numerous newspapers and magazines. In 2005, he served on a panel of consultants for “Airline Cracks,” an ITV-West (Bristol, United Kingdom) telecast examining the safety of composite structures in commercial jetliners.