The entire F-35 fleet has been grounded pending an investigation into what caused a dual generator failure and an oil leak during flight tests of AF-4 at Edwards Air Force Base.
"The jet returned safely to base. As a routine safety precaution, the Joint Program Office (JPO) has temporarily suspended F-35 flight operations until a team of JPO and LM technical experts determines the root cause of the generator failure and oil leak," Lockheed Martin F-35 spokesman John Kent said in a press statement.
The aircraft in question arrived at Edwards in late January and was the fifth F-35A conventional takeoff and landing aircraft to ferry there for testing.
The grounding appears to have occurred because of the potential for loss of control posed by such a combination. The F-35's flight control surfaces are controlled by electro-hydrostatic actuators made by Moog. If they don't have power then the pilot can lose control. In this case, the back-up power system -- the Integrated Power Package which also serves as the starter and air conditioner -- kicked in as designed, allowing the pilot to return to base.
Kent noted that the F-35 has now flown 657 flights and this appears to be the first time a flight has encountered this problem. "Once the cause is known, the appropriate repairs and improvements will be made before flight operations resume," he said.