The U.S. Navy found cracks in two of its older F-18A/B/C/D series of aircraft. The apparent cause was a missing fastener. But to be on the safe side, all 622 of these F-18s are being inspected. The navy has been watching its F-18 carefully, because as aircraft age, they develop unexpected cracks. And the F-18 fleet has been aging fast.
Over the last decade, the U.S. Navy found that their older F-18C Hornet fighters were wearing out faster than planned for. This was sort of expected with the F-18Cs, which entered service during the late 1970s and early 80s. These aircraft were to last about twenty years. But that was based on a peacetime tempo of operations, with about a hundred carrier landings (which is hard on the airframe) per year. There have been more than that because of the 1991 Gulf War (and the subsequent decade of patrolling the no-fly zone) and the war on terror. So to keep enough of these aircraft operational until the F-35 arrives to replace them in the next decade, new structural components (mainly the center barrel sections) are being manufactured. This is good news for foreign users of the F-18C, who want to keep their aircraft operational for longer. But if the tail cracks problem is not related to missing fasteners, that's another matter.
Two years ago, the U.S. Navy discovered that part of the wings on their F-18E (officially the "F/A-18E/F Super Hornet") were wearing out faster than expected. But an inspection of 476 F-18Es, only ten more were found to have cracks. The cracks indicated that, instead of lasting 6,000 flight hours, the portion of the wing that supports the pylons holding stuff (bombs, missiles, equipment pods or extra fuel tanks) is now expected to be good for no more than 3,000 flight hours. The metal, in effect, is weakening faster than expected. Such "metal fatigue", which ultimately results in the metal breaking, is normal for all aircraft. Calculating the life of such parts is still part art, as well as a lot of science. The navy will modify existing F-18s to fix the problem, which is a normal response to such situations. Sometimes these fixes cost millions of dollars per aircraft, but this particular fatigue problem will cost a lot less to fix.
The problem does not occur with the older F-18s (the A, C and D models) because, while they are also called F-18s, they are not the same as the F-18 E, F and G models. That's because, when the navy decided to build a replacement for the earlier F-18, they found they could get away with calling it an upgraded F-18 model. Thus, instead of it being called the F-24 (the next number available since the start of the Department of Defense's standard designation system in 1962) it could be called the F-18 E and F. While the F-18F looks like the original F-18, it is actually quite different. The F-18E is about 25 percent larger (and heavier) than the earlier F-18s, and had a new type of engine. By calling it an upgrade, it was easier for the navy to get the money from Congress. In the early 1990s, Congress was expecting a "peace dividend" from the end of the Cold War, and was slashing the defense budget. That's when the "F/A" designation was also invented, ostensibly to indicate that the aircraft was a fighter (the "F") and light bomber (the "A" for "Attack"). There was a lot of commonality between the two F-18s, but they are basically two different aircraft.