The F-35 Lightning II test fleet has been cleared for flight, but the U.S. Air Force's production aircraft at Eglin Air Force Base, Fla., are still grounded, the Pentagon announced Aug. 18.
An Air Force safety investigation board is continuing its investigation of the failure of the AF-4's Integrated Power Package on Aug. 2, which led to the grounding of the entire fleet of 20 aircraft. The AF-4 is the fourth conventional takeoff and landing variant produced by Lockheed Martin.
A government and contractor engineering team determined that flight operations of the test aircraft could continue after reviewing data from ground and flight tests, and revised the test monitoring procedures that govern the IPP. Ground operations of the test fleet resumed Aug. 10.
"The root cause investigation indicates that an IPP valve did not function properly," a release from the F-35 Joint Program Office states. "Monitoring of valve position is a mitigating action to allow monitored operations. A permanent resolution is in work."
The IPP, which is built by Honeywell International, combines the functions performed by an auxiliary power unit, emergency power system and environmental controls.
The Air Force's test F-35s are at Edwards Air Force Base, Calif., with U.S. Navy and U.S. Marine Corps' variants based at Naval Air Station Patuxent River, Md. The Air Force's aircraft at Eglin, which do not have test instrumentation, will be grounded until the investigation is finished and any required corrective actions are completed.