A $14 billion spares shortfall for such aircraft as the Lockheed Martin [LMT] F-35A fighter and the 1950s-era KC-135 tanker and B-52 bomber by Boeing [BA] is a major challenge for the U.S. Air Force. "Honestly, we've got a lot of work to do with spares," Air Force Lt. Gen. Linda Hurry, the head of Air Force Materiel Command, told a Mitchell Institute for Aerospace Studies virtual forum on Wednesday. "We've got a $14 billion hole in spares. Why? On…