May 31, 2019:
For the second time in history the U.S. Air Force has restored a retired B-52 bomber to active service. This was done by using one of the dozen retired B-52s stored at AMARC (Aerospace Maintenance and Recovery Center) where still flyable warplanes are retired, for use as spare parts and eventual scrapping or, if they are stored at a higher level of readiness (Type 1000 storage), for rapid restoration to flight status and a return to active duty. The U.S. is allowed, according to an arms control treaty, to maintain 76 B-52s on active duty. All of these aircraft are over fifty years old and, via regular upgrades and refurbishment, kept current and useful. When one of the 76 is lost due to an accident, an AMARC B-52 is revived. The first time this was done was in in 2015, when it took 70 days to get the AMARC aircraft flying again. These aircraft are then flown off to receive the latest updates and a final checkout by the air force AMARG (Aerospace Maintenance and Regeneration Group). This process will be completed for the latest revived B-52 by early 2020. The latest revival was a B-52 that was retired to AMARC in 2008 and it will replace a B-52 lost in a 2016 crash. The air force plans to keep its B-52s flying into the 2050s and more upgrades are planned, including new engines. The most recent revived B-52 was manufactured in 1962 and that design proved so reliable and flexible that they have continued to be useful as a heavy bomber ever since. This is especially true since the use of smart bombs became common in the 1990s. This made a single B-52 capable to supporting ground operations over a huge area (all of Afghanistan or Iraq) because it could refuel in the air and remain airborne for more than 12 hours at a time, speeding to distant locations and delivering the dozens of guided bombs it carried as needed by troops below.
B-52s are not the only aircraft revived from retirement. The United States has been storing retired aircraft in airbases located in dry desert areas since the 1940s when it had had the largest air forces on the planet. That meant lots of still usable aircraft could be retired inexpensively for later return to service or use as a source of spare parts. It was noted that since World War II most military aircraft ended up being scrapped, not shot down or lost in accidents. Some nations, particularly the United States, created an intermediate status for retired aircraft; storage. The main American storage site is AMARC. 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. Stored aircraft that are revived have to meet strict standards similar to those applied to new aircraft.
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, which 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. The basic technology bombers and transports are based on hasn't changed much. The best example is the B-52 bomber, which entered service in the 1950s and the ones still flying were built over fifty years ago. Another example of older but still flyable aircraft are the hundred or so DC-3 civilian transports that began flying in the 1930s.
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 "boneyard", just for spare parts. This has been a practice in combat from the very beginning of military aviation. Especially during World War I, when far more aircraft were lost to bad landings and takeoffs than to enemy action. Such wrecks became a source of replacement parts for airframes and engines of aircraft still in service. Thus the most efficient boneyard 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 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's 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 boneyard 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 of 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.