In August 2014 the U.S. Air Force grounded 82 F-16D jets after cracks were discovered in longerons (metal support beams inside the forward fuselage, which hold the cockpit in place). The D version of the F-16 is the two seater used for training. Some 16 percent of the 969 F-16s in the U.S. Air Force are the D model and these are all at least 24 years old with more than 5,500 hours in the air. The longeron design for the D model is different than that for the single seat models because the D model has a longer cockpit canopy. The air force is making repairs and replacements to put the grounded F-16Ds back in service.
There was a similar problem in 2013 with some older F-15s. In this case the longerons in question were eventually discovered to be defective, not suffering from old age. The inspections of the older F-15s found substandard parts in 182 aircraft. Boeing (the manufacturer) eventually admitted that it had built over a hundred F-15s using longerons that were not 2.5mm thick, as specified, but in some cases only one millimeter thick. That apparently worked when the F-15s were new but age was not kind to the thinner metal and it eventually broke. Thicker stuff apparently would not have failed because of age.
The U.S. F-16 fleet is rapidly aging. The average age of existing F-16s is over 25 years, and the average aircraft has over 6,000 flight hours on it. Back in 2009 the first Block 40 F-16 passed 7,000 flight hours. In 2008 the first of the earliest models (a Block 25) F-16 passed 7,000 hours in the air. The F-16C was originally designed for a service life of 4,000 hours. But advances in engineering, materials and maintenance techniques have extended that to over 8,000 hours. Because of the wars in Iraq and Afghanistan, F-16s sent to these areas will fly over a thousand hours a year more than what they would fly in peacetime.
The U.S. F-16 is one of the most modified jet fighters in service. The most numerous F-16 is the C model. The first version of this, the F-16C Block 25, entered service in 1984. The original F-16, as the F-16A Block 1, entered service in 1978. While most F-16s still in service are the F-16C, there are actually six major mods, identified by block number (32, 40, 42, 50, 52, 60), plus the Israeli F-16I, which is a major modification of the Block 52. Another special version (the Block 60), for the UAE (United Arab Emirates) is called the F-16E. The various block mods included a large variety of new components (five engines, four sets of avionics, five generations of electronic warfare gear, five radars and many other mechanical, software, cockpit and electrical mods.)
The F-16 is the most numerous post-Cold War jet fighter, with over 4,200 built, and still in production. During The Cold War, Russia built over 10,000 MiG-21s, and the U.S over 5,000 F-4s, but since then warplane has plummeted about 90 percent. One exception, since the end of the Cold War, has been the F-16, which has been popular enough to keep the production lines going.
The F-16 can also function as a bomber and ground attack aircraft, although not as effectively as the air force folks would have you believe. It can carry four tons of bombs, and has been very effective using smart bomb. In air-to-air combat, F-16s have shot down 69 aircraft so far, without losing anything to enemy warplanes. Not bad for an aircraft that was originally designed as a cheaper alternative to the heavier F-15.