P&W F135 proof test needed before Lockheed F-35 JSF returns to flight

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First F-35 will return to flight if trial proves successful following August ground failure of F135 turbine blade

The remaining hurdle to returning the first Lockheed Martin F-35 to flight is a proof, or "dwell", test to check whether its Pratt & Whitney F135 is susceptible to the turbine blade cracking that caused the failure of an engine during ground testing in August.

Although Lockheed resumed ground runs on aircraft AA-1 in October using the first flight test engine (FTE 1), the Joint Strike Fighter programme office (JPO) has elected to proof-test FTE 3. If that engine passes the test, it will be installed in the aircraft.

F135 ground testing was halted in late August when a turbine blade failed. "Pratt & Whitney discovered a combination of factors that, when present in an engine, could lead to high-cycle fatigue cracks in the third-stage turbine blade," says the JPO.

The issue is related to a combination of operating conditions, material properties and manufacturing tolerances, says Bill Gostic, P&W vice-president F135 programmes. "We inspected 1,000 third-stage turbine blades for cracks and found none," he says.

"We feel strongly the root cause is a combination of factors," says Gostic. The engine dwell test will deliberately excite high-frequency vibration of the turbine blades, and if the same combination of factors is present fatigue cracks will develop, he says. If no cracks form, then the engine is not susceptible to the phenomenon, P&W believes.

FTE 3 is planned to be proof-tested and inspected this week, and if clear of cracks a safety review will be conducted to release the engine for flight test in AA-1, says Gostic. P&W, meanwhile, has delivered the first short take-off and vertical landing flight-test engine to Lockheed for fit checks in the first STOVL F-35B.

All flight-test engines will be proof tested and turbine blades eddy-current inspected, Gostic says. Ground-test engines will be checked using a non-contact stress measurement probe that will look at the blade frequency in the turbine rotor as the engine operates. A change in frequency will indicate a crack has formed, he says.

The blade problem halted F135 ground testing for two months, but Gostic says there is sufficient margin in the STOVL engine flight release programme to still meet a schedule that calls for F-35B hover pit tests to begin in April 2008, leading to a first flight the following month.