The Lockheed Martin F-35 is grounded while programme officials investigate a dual generator failure and oil leak on the AF-4 test aircraft on 9 March.
The in-flight power failure forced the test aircraft, which is dedicated to mission systems testing, to return and land at Edwards AFB, Lockheed says.
The joint programme office has “temporarily suspended” flights by the F-35 until a root cause is determined, the company adds.
“Once the cause is known, the appropriate repairs and improvements will be made before flight operations resume,” Lockheed says.
This is the second grounding order for the F-35 in six months. On 1 October, a software problem discovered in simulations caused the programme to park the test fleet for several days.
If not fixed, that glitch could have allowed a fuel pump to shut down at altitudes over 10,000ft.
At the same time as fuel pump problem was discovered, a weakness was also found in the hinge of the lift-fan auxiliary air inlet for the F-35B short take-off and vertical landing (STOVL) variant. That problem forced Lockheed to halt all vertical landing tests for more than two months.
But the generator failure and oil leak on AF-4 marks the first in-flight issue for a conventional take-off and landing (CTOL) variant since the 19 test flight in May 2007 by the AA-1 test aircraft.
That issue also involved a power system failure. An electrical short disabled the flight controls in the horizontal stabilizer, but former chief test pilot Jon Beesley landed the aircraft without incident.
Lockheed also has had problems before with the generator on the carrier-based version of the F-35. In 2007, Lockheed acknowledged the generator was sized to support only 65% of the power requirement for the F-35C.