UPDATE 1-Pentagon reviewing Pratt recommendation to resume F-35 flights
* F-35 fleet grounded for nearly a week
* Pratt investigation rules out worst-case problem
* Testing now points to excessive use at high heat
WASHINGTON, Feb 28 (Reuters) - The Pentagon said on Thursday it was reviewing a recommendation by Pratt & Whitney to resume flights and ground operations of the F-35 fighter jet after a week-long grounding prompted by a cracked engine blade, but no decision has yet been made.
Spokeswoman Kyra Hawn said officials from the U.S. Air Force, Navy and the Pentagon's F-35 program office were reviewing data from a comprehensive engineering investigation conducted by Pratt about the cracked blade discovered on a test plane in Florida on Feb. 19.
Pratt spokesman Matthew Bates confirmed that the F-35 Joint Program Office was assessing the company's recommendation to resume flights but declined to offer further comment.
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.
The Pentagon announced the grounding of all F-35 warplanes on Friday after an inspection revealed a crack on a turbine blade in the jet engine of an F-35 being tested at Edwards Air Force Base in California.
It was the second engine-related grounding in two months of the $396 billion F-35 Joint Strike Fighter, the Pentagon's largest weapons program. The Marines Corps version of the plane was grounded for nearly a month starting in mid-January because of a faulty hose in the engine.
The Pentagon said on Wednesday that no additional cracks have been found on F-35 fighter engines during inspections begun after the Feb. 19 incident.
Pratt began investigating the cracked blade on Sunday evening after the blade assembly arrived at its Middletown, Connecticut, facility, first through nondestructive testing such as X-rays, followed by a procedure that split open the blade for a closer examination.
Those tests have convinced the company's engineers that the problem with the turbine was not caused by high-cycle fatigue, which could force a costly design change, or a design defect, sources familiar with the investigation told Reuters earlier this week.
Instead, engineers now believe the crack is a "creep rupture" caused by the fact that the engine on that particular test plane had been run particularly hard at hot temperatures since it was used for after-burner testing, according to a source briefed on the Pratt recommendation.
The source, who was not authorized to speak on the record, said Pratt engineers were continuing to study the crack to better understand the root cause and develop "potential changes needed to mitigate future occurrences," said the source.
But it was clear that normal fleet use would not reach that degree of "hot time" for a period of years, the source added.