Pentagon officials ended the six day grounding of the F-35 Thursday evening following the completion of an investigation into a broken engine fan blade on one of the aircraft.
The investigation done by the F-35 program office and Pratt & Whitney, makers of the F-35 engine, found the crack was caused by "prolonged exposure to high levels of heat and other operational stressors," said Kyra Hawn, the Joint Strike Fighter program spokeswoman, in a statement.
Inspectors and engineers didn't find any additional cracks in the engine of the aircraft in question or the rest of F-35s being tested, program officials said.
"The engine in question is part of the F-35 test aircraft fleet and had been operated for extended time in the high-temperature environment in its mission to expand the F-35 flight envelope," Hawn said in a statement.
Seventeen test aircraft and 34 operational aircraft make up the F-35 fleet that includes three versions and stretches across the Air Force, Navy and Marine Corps.
Tests on the Pratt & Whitney F135 will continue as the F-35 returns to flying status, Hawn said.
Earlier in the week, Lt. Gen. Christopher Bogdan, F-35 program chief, publicly slammed both Pratt & Whitney and the industry lead on the Joint Strike Fighter, Lockheed Martin. He criticized their decision making and their lack of urgency to pursue cost controls over the life of the program.