By Marina Malenic

F-35 Joint Strike Fighter test flights resumed last week after it was found that faulty maintenance procedures led to an engine generator failure earlier this month, the Joint Program Office said last week.

“Maintenance documents have been amended to provide improved engine starter/generator servicing,” a JPO spokesman said via e-mail March 25.

The fleet was grounded after a malfunction of one of the test articles (Defense Daily, March 14). The aircraft, designated AF-4, is one of 10 F-35 System Development and Demonstration (SDD) aircraft. On March 9, AF-4 experienced a dual generator failure and oil leak while conducting flight test operations out of Edwards AFB, Calif. The aircraft’s backup system returned it safely to base.

As a safety precaution, all F-35 flight operations were temporarily suspended at that time. Experts from the JPO and prime contractor Lockheed Martin [LMT] investigated the incident.

The configuration of the generator on AF-4 and the other new F-35s is different than the installation on the original test aircraft. The problem was initially traced to the newer configuration. Test aircraft with the earlier configuration had been cleared March 14 to resume flight. The three new-configuration test aircraft–AF-4, BF-5 and CF-1– remained grounded, along with the first two low-rate initial production F-35As, while the investigation continued.

The investigation revealed that the maintenance procedure for the alternate engine starter/generator configuration allowed excess oil to remain in the generator’s lubrication system, according to the JPO spokesman.

“Previous maintenance procedures could allow a small amount of extra oil to remain within the generator following servicing,” he said. “Under some conditions, the extra oil that is churning inside a narrow air gap within the AES/G could cause internal temperatures to increase. It was assessed that high temperatures led to the generator failures.”

There was no significant impact to the SDD program or production operations, according to the JPO.

“The F-35 test programs at Edwards AFB, Calif., and Naval Air Station Patuxent River, Md., remain ahead of their monthly flight test schedules,” the spokesman added.

The A-model is undergoing testing at Edwards, while the B- and C-models are flying out of Patuxent River. The two production jets and 10 preproduction test aircraft have completed over 700 flights since testing began in December 2006, according to Lockheed Martin.