Pratt & Whitney's F135 powerplant for the Lockheed Martin F-35 suffered a second turbine blade failure on 4 February, the same day the US Department of Defense tried for a third time to cancel the General Electric/Rolls-Royce F136 alternative engine.
The third-stage low-pressure turbine blade failed during proof testing of flight test engine (FTE) 6, the F135 scheduled to power the first short take-off and vertical landing F-35B, aircraft BF-1. First flight is expected to be delayed.
The first failure, on a STOVL F135 during ground testing in August 2007, was traced to high-cycle fatigue resulting from vibration excited by interaction of the blade with the wakes from vanes upstream of the third LP turbine stage.
P&W devised a proof test to deliberately excite the vibration and determine whether any turbine blades were susceptible to failure. Two conventional take-off and landing F135s, FTE 1 and 3, have been proof-tested and cleared for flight.
"FTE 6 was next in line. A single LP turbine blade responded to the vibration and broke," says Bill Gostic, F135 programme vice-president. "That was the intent [of the proof test]. We fully expected to find blades that cracked, but believed we could identify them before they broke. That was the surprise."
To replace the damaged STOVL engine, P&W planned to begin proof testing FTE 2 on 8 February, but the incident will delay the start of propulsion system testing on BF-1 at Lockheed. "The extent of any delay is still to be decided," says Gostic.
P&W is delivering one engine a month. "So the initial thought [on the delay] is nominally 30 days - less if we can expedite FTE 2," he says. BF-1 was planned to fly in late May at the earliest, following hover pit tests of the STOVL propulsion system.
The blade failure is not expected to delay flight qualification of the STOVL F135, Gostic says. Ground-test engine FX635 is finishing up a 1,000-cycle accelerated mission test, while FTE 5 is completing altitude testing. "We have completed all powered-lift performance testing," he says.
P&W thinks the problem is restricted to STOVL F135s, because the LP turbine works harder when powering the shaft-driven lift fan. Although the F135 is derived from the F119 engine powering the Lockheed F-22, the third LP turbine stage was added to power the lift fan.
P&W is redesigning the third-stage LP turbine vane, but plans to proof test all ground- and flight-test F135s. Gostic expects the improved design to be implemented beginning with the second low-rate initial production batch of STOVL engines.
The Pentagon, meanwhile, has eliminated development funding for the GE/R-R F136 from its budget request for the third year running. Previously, Congress has restored funding for the alternative Joint Strike Fighter engine.