The U.S. Air Force has retired the last of its 384 F-15A fighters. Long flown only by reserve units, these are old aircraft, all built in the 1970s. Air force reserve units got the F-15As in the 1980s and 1990s, as active duty units got the new F-15C. But now the F-22 is entering service, and more F-15Cs are going to the reserves. Many of those F-15A flew for over 30 years.
Unfortunately, the later model F-15s are not aging well. Two years ago, the air force grounded all of its 442 remaining F-15As and Cs (and the smaller number of two seat B and D trainer models) for 18 days, then grounded them again, all because of suspicions that portions of the aircraft structure have been weakened by stress (lots of maneuvering during combat training).
Earlier, the U.S. Air Force has halted non-critical flights of its F-15C (the interceptor version) fighters after a National Guard F-15C crashed. It appeared that the crash was the result of structural failure. Seven 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 might 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 each). 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. This time around, the F-15Es were not grounded, because metal stress in the older F-15s would not occur in the F-15E, which is somewhat different in its internal structure.
Structural failure is more common in older fighters that have lots of flight 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.
This component failure problem is not unique to the F-15, and has been occurring with increasing frequency among aging fighter aircraft all over the world. The end of the Cold War in 1991 led to the cancellation of many warplane replacement programs. Air forces were compelled to make do with thousands of increasingly older aircraft. Whenever an aircraft goes down because of a structural failure, you have to ground all planes of that type until you know exactly what caused the loss, and made any needed repairs to other aircraft of that type. Pilots are a pretty sharp lot, so governments don't dare try to play games with this. If the pilots suspect they are being set up to fly dodgy aircraft, they will not fly them, or not fly them in a useful (stressful) way.