Bennett Croswell, president of Pratt & Whitney's military engine division, says a problem with an F135 engine that grounded the Lockheed Martin F-35 is due to thermal creep, and is unlikely to affect the aircraft further as it returns to flight.
The issue was a crack in a third-stage turbine blade on a single engine. As a precaution, the US military grounded all F-35 aircraft until a cause was discovered.
"During [an] inspection we found about 1/6-inch (4.2mm) crack on the turbine blade," says Croswell. "We felt we could continue to fly, and we took that recommendation to the [joint programme office], but on consultation with them we both came to the conclusion it was safer to suspend operations."
Thermal creep from high-temperature, high-intensity testing was found to be the cause of the crack. The engine, the tenth built, powers the second F-35A, was tested extensively at supersonic speeds and at low altitudes, generating significantly more heat than expected, says Croswell.
"It was operating at levels four times higher than an operational mission, and four times greater than the levels we had qualified the engine for," says Croswell. "That was very good news, you don't want something like high-cycle fatigue or low-cycle fatigue." The issue is not expected to impact operational aircraft for months or years, depending on how the aircraft are flown, he says.
Fatigue leading to turbine blade cracks has twice grounded the F-35 in recent years.
Pratt & Whitney has lately come under criticism from the US military's programme office for its attitude to the F-35 project.