December 12, 2007:
U.S. Air Force F-15 fighters will be grounded for most of December. After two
weeks of no flying, and intense structural inspections in November, the U.S.
F-15 fleet was cleared to fly again on November 23rd. But then, on December
3rd, more inspections were ordered because closer study of the November
inspections indicated the possibility of more metal fatigue (and possible break
up of aircraft during violent maneuvers.) So over 500 F-15s are each undergoing
12-20 hours of more inspections. This now involves partial disassembly of the
aircraft, and removing some paint.
The U.S. Air Force had
halted non-critical flights of its F-15C (the interceptor version) fighters
after a National Guard F-15C crashed on November 2nd. It appeared that the
crash was the result of structural failure. Five years ago, an F-15C traveling
at high (over 2,000 kilometers an hour) speed crashed when its left tail fin
F-15Es (the two seat
bomber version) operating in Afghanistan were not grounded initially, but soon
were when it was realized that the problem may be a design flaw, not age, that
caused the 27 year old F-15C to go down. The F-15Es were restored to flight
status after about a week, once each aircraft had undergone an extensive
structural examination (taking about 13 man hours). Most F-15Es are less than
ten years old. But some F-15Cs are over twenty years old. The F-15E is still in
production for export customers like Singapore and South Korea. F-15Es were not
subject to the current round of inspections.
Structural failure is
more common in older fighters that have lots of hours (over five thousand) on
them. When originally designed, the F-15 was believed to have a service life of
only 4,000 hours. But new materials and design techniques increased that to
8,000. In peacetime, F-15s are in the air 250-300 hours a year. But because of
the 1991 Gulf War, the 1990s "no-fly-zone" patrols over Iraq, and the current
war, the F-15 fleet has piled up the hours more quickly, and many are
approaching the 8,000 hour mark.
If weak components are
detected, they can be replaced with stronger ones, made of materials not available
when the F-15 was originally built. But you want to find the weak components
before they fail. While scanning technology has improved, it's still not good
enough to detect all the F-15 components possibly weakened by years of use. As
a result, flying an F-15 is going to be a bit more stressful from now on. To
some in the air force, this situation has a bright side. One can now make a
more compelling case to build more F-22s, to replace F-15 that are wearing out
faster than expected.