Last November, a U.S. Air Force F-16 crashed when its jet engine came apart while taking off from Balad air base in Iraq. This sort of thing always makes pilots nervous, even though the pilot of the Balad F-16 was able to walk away from the aircraft before it burned up.
Air force investigators found the cause of this F-16 crash was undetectable cracks inside a disk fan. Jet engines consist of several sets of fans, and the blades, or disks, on the fans, are subject to enormous heat and pressure. The disks are carefully examined during manufacture and installation into the engine. The disks are periodically checked for surface cracks, and replaced if found with any. But in the case of the Balad accident, the cracks developed inside the blade, and there was nothing in the manufacturing records to indicate why this would happen.
Metallurgy and jet engine experts consider this a low probability event, but are checking the metal manufacturing procedures to see if they can detect any changes that can be made to avoid such difficult to detect failures in the future. The F-110 engines in the F-16 cost about $5 million new, and are well maintained and rebuilt over a decade or more of service. The F110, which comes in many versions, weighs two tons, is 118 cm (47 inches) in diameter and up to 590cm (about 19 feet) long.