January 11, 2008: The U.S. Air Force has allowed 260 of its 442 older F-15C/D fighters back into the air. But another 180 may be grounded permanently. Inspections have found fatigue cracks in the longerons (metal support beams inside the forward fuselage, which hold the cockpit in place) of eight of the 180 older aircraft. Moreover, most of these aircraft have longerons that were not built to specifications.

The F-15s are needed mainly for air defense of North American and for clearing the air of enemy fighters in a combat zone. That would be a risky business in an aircraft with metal fatigue problems. F-22 enthusiasts in the U.S. Air Force see a bright side to all this, as the sudden shortage of flyable F-15Cs might motivate Congress to come up with cash to buy more F-22s (at about $150 million each.) Meanwhile, F-16s are good air superiority aircraft, and with the proliferation of smart bombs, fewer fighters are needed for bombing missions.

All F-15C/D fighters 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.

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. While the defective F-15s can be repaired, many air force generals would prefer to build more F-22s instead.