November 7, 2007: The U.S. Air Force has halted non-critical flights of its F-15C [PHOTO](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 [PHOTO](the two seat bomber version) operating in Afghanistan were not grounded initially, but soonwere 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. Until the exact cause of the F-15C crash can be determined, most of the air force's 700 F-15s will stay on the ground. 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.

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 to 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 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 (PHOTO), to replace theF-15 that are wearing out faster than expected.