We've talked about the Air Force's looming financial difficulties before, but it looks like the USAF's aircraft fleet may get quite a bit older as well as smaller in the coming years.
(Another) Aircraft Shortage
According to Brig. Gen. Michael J. Schmidt, the executive officer at the Fighters and Bombers Directorate, based at Wright-Patterson Air Force Base, a shortage of modern fighters may force the F-15 Eagle to stay in service through 2040.
The F-15 is one of the older aircraft in the Air Force's fleet - having first entered service in the 1970s. The F-16 and A-10 Thunderbolt - both still widely used by the USAF - also first flew in the 1970s. With the average age of Air Force aircraft at 27 years, these three fourth-generation aircraft are serving longer than anticipated.
F-15's Assist With Raptor Shortage
Currently, the Air Force is testing the F-15 to see if the aircraft's structure is wearing at the expected rate. This fatigue testing will determine if the aircraft is capable of extending its designed life span by up to three times the original planned service life. This extended duty is necessary since the F-22 - which was planned to replace the F-15C as an air superiority fighter - was bought in much lower numbers than originally planned.
Originally, a fleet of approximately 750 F-22 Raptors was planned, but the final purchased number was a measly 187. The last F-22 was finished in 2011, and many of the manufacturers of components for the aircraft - such as Pratt & Whitney, who made the engines - no longer produce those components. This makes the recommendation by Congress to consider restarting Raptor production more expensive than it would have been if the production run had never been shut down.
Covering for the F-35
The F-16 and A-10 are also nearing (or past) the end of their planned lifespan. Both were expected to be replaced by the F-35 Lightening, although the ballooning cost and slower-than-expected progress in the F-35 program mean that contingency plans are necessary.
This puts the Pentagon in a difficult position. Since budget cuts have already been harmful to the USAF fleet, spending additional money on extending the flight time of older aircraft may be seen as a waste. However, with the F-35 severely hampered in its ability to use all of its planned ordnance and advanced sensor capabilities, something is needed to "plug the gap" in tactical capabilities.
The Problem: Fatigue and Survivability
In many cases, upgrading the older aircraft to have advanced sensors, jamming equipment, and weapons is not the problem. The real challenge faced by the Air Force is much more basic. Years of stress on airframes leads to metal fatigue, especially for aircraft carrying heavy loads or performing high-G maneuvers. In 2007 a Missouri ANG F-15C came apart in mid-air and this resulted in the grounding of the F-15 fleet for months.
Additionally, while both the F-16 and especially the A-10 have been quite effective against ISIS and other terrorists in the Middle East, there is a real concern over survivability in hostile airspace. Most of the air strikes that took place in Iraq, Afghanistan, and other recent countries were over uncontested airspace, so pilots and planners didn't have to worry about large, integrated air defense systems. In a conflict with a more advanced nation, though, even with the latest radar and electronic countermeasures, there is doubt that non-stealthy aircraft like the F-16, F-15, and A-10 would be able to operate effectively and survive.