F-35 Cleared For Flight; Turbine Crack Cause Identified

WASHINGTON: The entire F-35 fleet has been cleared to resume flying only one week after being grounded for the second time this year. In vintage Pentagonese, this is how the return to flight was announced today to Capitol Hill:

"Upon completion and compliance with the immediate action Time Compliant Technical Directive (TCTD) issued this week to borescope inspect the LPT stage 3 turbine blades, F-35 LRIP aircraft are returned to flight status. Additionally, a 25 Effective Flight Hour (EFH) reporting cycle of creep damage will now be implemented to monitor and limit turbine creep exposure."

Basically, it means that a potentially really serious problem -- turbine blades suffering from high cycle fatigue -- was not the cause of the half-inch crack which led to last week's highly publicized grounding. It also means that the Joint Program Office and Pratt & Whitney will be monitoring the turbine blades for what will probably be several weeks of test flights. Here's the press release version from the Joint Program Office which oversees the largest conventional weapons program in history: The F-35s "have been cleared to resume following engineering analysis of the turbine blade which developed a crack.

This decision concludes a cautionary flight suspension that began on Feb. 21 after a 0.6 inch crack was found on a 3rd stage turbine blade of a test aircraft at the Edwards Air Force Base F-35 Integrated Test Facility during a routine inspection." One of the interesting details in the JPO release is that no other cracks were found in any of the other F-35s flying. Pratt & Whitney, who make the engines and were the lead on investigating the cause of the crack, issued a release saying that: "We believe that a key factor for the blade crack stemmed from the unique operating environment in flight test. The engine had operated more than four times longer in the high-temperature environment of the flight envelope than the typical F-35 duty cycle. The exposure in this high temperature part of the envelope led to a separation of the grain boundary on this single blade."

The military, industry and Congress will keep watching this to make sure the problem is not systemic and does not repeat.