Pentagon says to resume F-35 flights

WASHINGTON (Reuters) - The Pentagon said on Thursday it would resume flights of its F-35 fighter jets following a week-long precautionary grounding imposed after a crack was found on an engine blade on a test plane in California.

'F-35 flight operations have been cleared to resume,' Pentagon spokeswoman Kyra Hawn said in a statement. She said flights could begin as early as Friday, depending on weather conditions.

No additional cracks were found during inspections of engines on the remaining 50 planes in the Pentagon's fleet, or any spare engines, Hawn said.

The order allowing flights to resume is good news for the $396 billion F-35 Joint Strike Fighter, the Pentagon's biggest weapons program which is bracing for big cutbacks when automatic across-the-board budget cuts take effect on Friday.

It comes two days after the Pentagon's F-35 program chief blasted enginemaker Pratt & Whitney and prime contractor Lockheed Martin Corp for trying to 'squeeze every nickel' out of the Pentagon and not shouldering enough risk on the program.

The order ends the program's second engine-related flight ban in less than two months. The Marines Corps version of the plane was grounded for nearly a month from mid-January because of a faulty hose in the engine, later blamed on manufacturing errors.

Matthew Bates, a spokesman for Pratt & Whitney, which supplies the engines for the planes, said the Pentagon's F-35 program office had decided to rescind the grounding order after extensive tests and analysis of the cracked turbine blade, which was discovered on February 19 during a routine inspection conducted after every 50 hours of flight time.

Bates said Pratt engineers had been working around the clock with Pentagon experts to determine the cause of the crack in the engine blade, including a 'destructive' test that cracked open the blade.

The tests showed that the crack resulted from the 'unique operating environment' in flight tests -- many of which tested the engine's powerful after burners -- rather than a high-cycle fatigue crack, which would have required a design change.

Bates said the engine in question had operated at high temperatures for more than four times longer than a typical F-35 flight, which led to a separation of the 'grain boundary' of this particular blade.

The Navy order rescinding the flight ban, or so-called 'red stripe,' said that engine had experienced the most 'hot engine time exposure' of all the engines in the developmental program. It said it would now require reports to monitor and limit similar damage after every 25 flight hours.

Pratt recommended the resumption of flights and ground operations of the engine earlier on Thursday after its engineers ruled out a high-cycle fatigue crack.

The Pentagon said the investigation concluded that the 0.6-inch long crack was caused by 'prolonged exposure to high levels of heat and other operational stressors.'

Pratt, a unit of United Technologies Corp, supplies the engine for the single-engine, single-seat fighter plane, which is built by Lockheed Martin Corp.

(Reporting by Andrea Shalal-Esa; Editing by Stacey Joyce and Richard Pullin)