WASHINGTON (Reuters) - Pratt & Whitney is 99 percent sure the fan blade problem that grounded the Pentagon's 51 new F-35 fighter jets was not caused by high-cycle fatigue, which could force a costly design change, according to two sources familiar with an investigation by the enginemaker.
Company engineers have concluded that a 0.6 inch-long crack found on a turbine blade in the engine of an F-35 jet at Eglin Air Force Base in Florida was almost certainly caused by lesser issues, such as high heat exposure or a manufacturing problem, that would be easier to solve, the sources said.
"They're 99 percent sure that it's not the worst-case scenario of high-cycle fatigue," said one of the sources.
Flights of the single-engine, single-seat F-35 fighter could resume as early as this week if the Pentagon accepts the findings of Pratt, a unit of United Technologies Corp (UTX.N), after additional tests to be done Wednesday, said one of the sources, who was not authorized to speak publicly.
The Pentagon announced the grounding of all F-35 warplanes on Friday after an inspection revealed a crack on a turbine blade in the Pratt-built jet engine of an F-35 jet 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 built by Lockheed Martin Corp (LMT.N), the Pentagon's largest weapons program.
Military officials are eager to resume test and training flights as soon as the engine issue has been resolved.
It was not immediately clear if the Pentagon would order a one-time inspection of all F135 engines built by Pratt for the new F-35 fighter, or whether the incident would result in a new recurring inspection requirement. Some inspections of the other 50 fighters already in use by the Pentagon were underway.
Pratt began detailed tests of the engine on Sunday evening at its Middletown, Connecticut facility after the blade assembly was removed from the Florida test plane and shipped north.
Pratt spokesman Matthew Bates declined comment on any specific results or conclusions, but said the company was making good progress in its investigation of what caused the crack.
"We have made significant progress ... and believe we're very close to determining root cause," Bates said.
One defense official said it was premature to speculate about the cause of the crack until the full battery of structural tests had been completed.
The official, who was not authorized to speak publicly, said Pratt was expected to deliver a comprehensive analysis of the test results to Pentagon officials no later than Thursday evening.
Two sources familiar with the investigation said the fan blade tests would include a "destructive" test that would cut into the turbine blade to better understand how the crack developed.
Engineers believe the crack is either a "creep rupture along a grain boundary" that was caused by prolonged exposure to high heat, or that it was caused by an anomaly during the metal casting process, the sources said.
The F-35 program, initially meant to start operating in 2012, is overdue and well over its original budget, but defense officials say it is making progress. They argue that the current grounding -- and a separate issue involving the plane's temperature control unit -- are normal occurrences during the development phase of a any new warplane.
The delays are causing problems for countries like Australia, which was due to buy 100 of the radar-evading F-35s, but is now considering whether to buy 24 more Boeing Co (BA.N) F/A-18 Super Hornets instead.
(Reporting By Andrea Shalal-Esa; Editing by Paul Tait)