One of the major problems with modern warplanes is that some models have remained in service far longer than anyone expected. This happened partly because modern aircraft are built to last. Commercial transports are very sturdy beasts, as they have to fly up to 12 hours a day for weeks at a time. Military aircraft fly less often, although their sturdiness is also meant to deal with the violent maneuvers of combat. But heavy bombers and transports can go on and on, as they don't fly as much as the civilian transports and the basic technology they are based on hasn't changed much. The best example is the B-52 bomber, which entered service half a century ago and the ones still flying were built forty years ago. Most warplanes are in production for a decade or less. Once the manufacturing stops, it starts to become difficult to get spare parts. The tools and equipment used to make the aircraft components are usually scrapped. Making the parts from scratch is so expensive that it would be cheaper to scrap aircraft and buy a new design. And a new aircraft is often more than the budget can bear as well. The solution to this problem is cannibalization. That is, using some aircraft, either those wrecked in accidents or older models retired to the "bone yard" just for spare parts. This has been a practice in combat from the very beginning of military aviation. Especially during World War I, when more aircraft were lost to bad landings and takeoffs than to enemy action, the wrecks became a source of replacement parts for airframes and engines of aircraft still in service. The most efficient bone yard in the world is Americas AMARC (Aerospace Maintenance and Recovery Center). This facility, out in the Arizona desert, stores aircraft no longer needed for active service. Some are recalled to active service every year, but all are liable for disassembly to provide parts for aircraft that are still flying. 

Some 4,500 aircraft are stored at AMARC, and 500-2,000 spare parts orders are filled each month. Not just for American military aircraft, but for those of allies as well. Australia keeps it's 1960s era F-111's flying with spare parts from old U.S. and British F-111s stored at AMARC. The U.S. Air Force A-10, built in the 1970s, and not a popular air force candidate for a new model, is kept flying (because it's so damn useful) with parts from AMARC. Even when parts are still in production, a wartime surge, as was experienced during the Afghanistan campaign, will outstrip the manufacturers ability to produce them. In this case, AMARC delivered parts for the F-18. AMARC was set up in 1985, consolidating bone yard operations there and from other locations in the United States. In 1985, it delivered spare parts worth half a billion dollars. While the airframes, stripped of all their more valuable parts, are worth only about 25 cents a pound as scrap, some of the parts are worth their weight in gold. Engines, which often comprise a third (or more) of an aircraft's value, are the most valuable single items. And each engine consists of thousands parts, some of which are worth quite a bit, even if the engine is no longer in use by any aircraft. Other nations cannibalize their retired or obsolete warplanes, but few have organized the operation as efficiently as the United States.