The US Air Force will keep more than 170 F-15s grounded after finding structural defects that will force either major repairs or the retirement of the warplanes.
About 60 per cent of the 442 F-15 Eagles have been cleared to return to flight, Air Force Major Dave Small told Deutsche Presse- Agentur dpa .
The entire fleet of the F-15 A-to-D versions was grounded for inspections following a November accident in which one of the planes came apart in flight.
The inspections concluded that the airplanes suffered from a manufacturing defect in the "spine" of the ageing fighter jets.
The defects took place in production of the planes from the late 1970s to 1985, Small said.
The F-15 Eagles were build by McDonnell Douglas, which was acquired by Boeing in 1996 in one of the many post Cold-War defence industry mergers.
The F-15E Strike Eagles were not affected by the accident and the fleet of 217 remains active.