US clears F-35 fighters after grounding over turbine crack

The Pentagon on Thursday lifted its order grounding the new F-35 jet fighter since 22 February, allowing them to get back on service operations.

The grounding came after a routine inspection discovered a crack in the turbine blade of one plane. But investigators said this was a one-off incident which occurred because it was overused in test operations.

The program office that oversaw the investigation said no other cracks were found in inspections of the other engines made so far, and no engine redesign was needed.

It said the engine in which the blade cracked was in a plane that ''had been operated at extreme parameters in its mission to expand the F-35 flight envelope.''

The program office added that ''prolonged exposure to high levels of heat and other operational stressors on this specific engine were determined to be the cause of the crack.''

There are currently 64 F-35s in operation. The Pentagon on Thursday made a down payment of $333.7 million to manufacturer Lockheed Martin for a further 35 aircraft.

Pratt & Whitney, which makes the engines, investigated the problem with military experts. The company, a unit of United Technologies, said on Wednesday that the crack occurred after that engine was operated more than four times longer in a high-temperature flight environment than the engines would in normal use.

The F-35, a supersonic jet meant to evade enemy radar, is the Pentagon's most expensive programme and has been delayed by various technical problems. The programme could cost $396 billion if the Pentagon buys 2,456 jets by the late 2030s, as it envisages.