When a five-foot hole opens up in the fuselage of a commercial airliner, it gets the media’s and public’s attention. In general, the possibility that metal fatigue in older aircraft can result in such hull failures comes as no surprise to airline personnel and aviation experts. The stresses imposed on aircraft fuselages by repeated pressurization/depressurization cycles, landings and takeoffs, bad weather, pollution and aerial maneuvers are enormous. Combine these factors with the natural effects of the aging of metals and other components and you get the potential for parts failure. This is why commercial aircraft undergo such rigorous inspection regimes.
What is alarming about the recent incident involving a Southwest Airlines 737 — an aircraft type with an extremely good safety record — was that the tear appeared in the middle of the fuselage, rather than at a joint where stresses are thought to be greater. This suggests that even after we have been flying metal-skinned aircraft for some 80 years there are still surprises left.
The stresses imposed on commercial airliners are nothing compared to those experienced by military aircraft, particularly those designed for combat. Military aircraft don’t fly as often as commercial aircraft but the way they fly tends to put additional stress on airframes and engines. No commercial airliner has the requirement to fly at supersonic speeds or pull multi-G maneuvers. There are also the little things like the requirement to land on the moving deck of an aircraft carrier or at an expedient airfield.
What should really scare us is that the average age of U.S. combat aircraft, fighters and attack planes, is over 20 years. This is the average; some are so old that the sons of their original pilots are now in the cockpits. Both the tankers and the long-range bombers in the joint fleet average over 40 years of age. Virtually all these aircraft have undergone life extension programs, sometimes several times. These programs can involve taking them down to the bare skeletons and rebuilding them with new fuselages, engines, sensors and avionics. However, these programs can take years to complete during which the older aircraft can be placed on flight restrictions or even grounded.
Current plans to recapitalize the Air Force, Navy and Marine Corps fleet are inadequate to prevent further aging. The principal reason for this is that DoD is not planning on buying new aircraft at a rate that will replace old with new planes rapidly enough. The newly awarded KC-46 tanker program will take place over decades during which the Air Force will still have to fly — and maintain — the aging KC-135. The eldest portion of the KC-135 fleet is already precluded from conducting missions in war zones. Similarly, because the Pentagon has moved procurement of the F-35 farther into the future and reduced the numbers of aircraft bought in the initial years, the Air Force has had to spend money on upgrading legacy F-15s. The first new strategic bomber will not even be built until around 2020, at which point the youngest of the remaining B-52s will be some 60 years old.
As the recent Southwest Airlines experience shows, there is still room for surprises. The possibility of a sudden, unanticipated airframe or system failure keeps military aircraft maintainers up at night. So do the mounting costs of maintaining aging and obsolescing aircraft. Ultimately, the only way to insure against such an outcome is to fund aircraft modernization programs adequately.
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