January 9, 2008:
The U.S. Air Force expects to keep 442 older
F-15C/D fighters on the ground until the end of January. These aircraft were
originally grounded in early November.
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 far, over 400 F-15s
have undergone 12-20 hours of
inspections. Special attention is being paid to the longerons (metal support beams inside the forward
fuselage, which hold the cockpit in place).
The U.S. Air Force first 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 broke off.
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
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 (to pilot and aircraft) 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.