In early 2017 Israel revealed that it had saved over $30 million on the cost of obtaining another D-15D (the two seater F-15C). This was accomplished by uniting the intact front end of a two-seat F-15D with the undamaged rear of an older single-seat F-15C. The F-15C had been retired and kept around just in case. The engines were still good but the electronics and such were elderly or absent. The front end came from a 2011 accident that had the two-seater make an emergency landing after the engines and rear fuselage caught fire because of ingesting birds. The availability of these two wrecks got IAF (Israeli Air Force) engineers and maintainers speculating on a merger. The American manufacturer said the crashed two seater was unrepairable but Israeli engineers had a tradition of improvising and never throwing anything away. They calculated that the more modern two seater could be rebuilt by detaching the intact front end (which included the cockpit and most electronics) and merging it with the back end of the older F-15. They ran the idea past the manufacturer and were basically told that they were not sure. The implication was that if the Israelis went ahead they were on their own. Off the record it was agreed that this was workable in theory. So the Israeli went ahead and did it. Total cost was about a million dollars. A new or used replacement two-seater F-15D would have cost over $30 million.
The Israelis have used ingenuity to deal with other maintenance issues, often resulting in new methods and tools that can be sold to the manufacturer or other users to do inspections or repairs more quickly and cheaply. Since Israel was founded right after World War II they have had to improvise to equip their armed forces. That means rebuilding tier older vehicles and other gear and adapting captured enemy tanks and other weapons. Aircraft are especially attractive for this sort of thing because aircraft are so expensive.
Like most air forces the Israelis keep many wrecked or otherwise unflyable aircraft in storage for spare parts. The climate of Israeli desert areas (where most air bases are) is perfect (dry) for this. Other nations with similar climates, and needs, have made the news for rebuilding wrecked or otherwise unflyable warplanes. This is the case with Iran, which has kept 40 year old F-14s, F-5s and F-4s operational. Recently various factions in the Libyan civil war did the same thing with older Russian warplanes. But without much publicity some of the most modern air forces also do in. In particular Israel has, since the late 1940s, been improvising like this in a major way but it is rarely news, in part because it has long been their custom.
The United States has taken the concept a step further, in part because America has airbases in desert areas and since the 1940s has had the largest air forces on the planet. That means lots of still usable aircraft are retired. It was noted that since World War II most military aircraft ended up being scrapped, not shot down. Some nations, particularly the United States, created an intermediate status for retired aircraft; storage. The main such site in the United States is AMARC (Aerospace Maintenance and Recovery Center). This is the “boneyard”, and aircraft stored at AMARC would, if armed and operational, be the third largest air force in the world. This facility, at Davis-Monthan Air Force Base out in the Arizona desert, stores nearly 5,000 military aircraft no longer needed for active service. Every year, some are recalled, refurbished and sent back to work. But most get "harvested" for spare parts, until what's left is chopped up and sold for scrap. It is possible to do what the Israelis did but there has never been much need for going that far. Plus there are the legal liabilities of doing that in peacetime. Even stored aircraft that are revived has to meet strict standards similar to those applied to new aircraft. Israel, on the other hand, has Arab neighbors who have refused to make peace since the late 1940s meaning that Israel is always in a state of war and such improvisation is a matter of life or death.
AMARC isn't the only storage site, just the largest (in the world). Many other air bases in dry climates have room for some aircraft that might be needed again. Back in 2010 the U.S. Marine Corps recently took an old AV-8 Harrier vertical takeoff fighter that had been in storage for sixteen years at one of its air bases, and restored it to duty as a two seat trainer. The marines didn't think they would need that old AV-8. But the new F-35B, which is to replace the AV-8 was late in arriving and operations in Afghanistan have worn down the existing AV-8s. So reinforcements have been called up from storage sites.
This points out one of the major problems with modern warplanes; that some models have remained in service far longer than anyone expected. This happened partly because modern aircraft are built to last, and used engineering advances that worked out better than expected. Engineers tend to overbuild when they can or must. For example, 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 over forty years ago. There are also over a hundred, 70 year old, DC-3 civilian transports still in the air as well.
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 is often cheaper to scrap aircraft and buy a new design. But 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. Thus the most efficient bone yard in the world is Americas AMARC. While some of the aircraft stored there are recalled to active service every year, all are liable for disassembly to provide parts for aircraft that are still flying. But other nations with smaller boneyards and more urgent needs can take the basic boneyard concept as far as they can get away with.
AMARC fills 500-2,000 spare parts orders each month. Not just for American military aircraft, but for those of allies as well. Australia keeps its 1960s era F-111's flying with spare parts from old U.S. 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 manufacturer ability to produce them. In this case, AMARC delivered parts for the F-18 and continues to do so for other heavily used aircraft.
AMARC was set up in 1985, consolidating bone yard operations already there and from other locations in the United States. In that first year, 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.