The US Air Force grounded 452 Boeing F-15 fighter jets after investigating a crash earlier this month and finding defects in the aircraft's fuselage. The Air Force said Wednesday that its oldest F-15s, or more than 60 percent of the fleet, will remain grounded until each one is inspected, and possibly repaired. It was the second time this month that F-15s were grounded. The entire fleet was grounded following a November 2 Missouri Air National Guard training exercise in which a pilot safely ejected before the aircraft crashed. The Air Force subsequently returned the F-15s to flight status on November 21. The Air Force investigation, which is still under way, found there were "possible fleet-wide airworthiness problems" because of defects in metal rails that hold the F-15's fuselage together.