Pentagon to resume F-35 fighter jet flights Matthew Bates, a spokesman for Pratt & Whitney, which supplies the engines for the planes, said the Pentagon's F-35 program office had decided to rescind the grounding order after extensive tests and analysis of the cracked turbine blade, which was discovered on Feb. 19 during a routine inspection conducted after ever 50 hours of flight time. Mr Bates said Pratt engineers had been working around the clock with Pentagon experts to determine the cause of the crack in the engine blade, including a "destructive" test that cracked open the blade. The tests showed that the crack resulted from the "unique operating environment" in flight tests - many of which tested the engine's powerful after burners - rather than a high-cycle fatigue crack, which would have required a design change. Mr Bates said the engine in question had operated at high temperatures for more than four times longer than a typical F-35 flight, which led to a separation of the "grain boundary" of this particular blade. The Navy order rescinding the flight ban, or so-called "red stripe," said that engine had experienced the most "hot engine time exposure" of all the engines in the developmental program. It said it would now require reports to monitor and limit similar damage after every 25 flight hours.