The U.S. Air Force has removed 29 KC-135E tankers from service because of metal fatigue problems with the pylons that attach each of the four engines to the wings. All 732 KC-135s were built between 1956 and 1965. The Boeing 707 commercial transport is actually a civilian version of the original KC-135 (which itself evolved from the World War II B-29 heavy bomber.) Over the decades, the KC-135 fleet has undergone constant repair and reconstruction. New engines, and new structural components have been added, as older items wore out, or showed signs of wearing out faster than anticipated. The problem with older aircraft is that you never know whats going to go next. The KC-135s are carefully scrutinized for metal fatigue, which is, so to speak, the silent killer of aircraft. Better diagnostic tools, like scanners that allow careful examination of components without tearing apart portions of the aircraft, as well as new sensors that can be installed in the aircraft itself, keep the KC-135s flying safely. With current technology, its believed that the KC-135s could be kept going at until 2040. The 145 ton aircraft can carry up to 37 tons of fuel. Over 500 of them are still in service.