November 23, 2007:
After two
weeks of no flying, and intense structural inspections, the U.S. F-15 fleet was
cleared to fly again . 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 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.
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 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.
Some countries, like Israel,
are making big bucks by upgrading older fighters, and this sometimes includes
structural mods that extend the useful life of the aircraft. Despite the
greater dependence on electronics, jet fighters still do a lot of violent
maneuvering, and this weakens key components of the aircraft structure. This
eventually causes aircraft to fail and fall from the sky. It's nothing new, and
has been happening since World War I. This problem, despite all the attention
it has gotten, will not go away any time soon.