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 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 (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.