F-35: Cleared to Fly

The F-35 joint program office late Friday night announced the Joint Strike Fighter test flight program -- suspended more than two weeks ago following the in-flight failure of a key component -- is resuming in full:

The three remaining F-35 test aircraft with the production alternate engine starter/generator (AES/G) configuration, as well as the first two production aircraft, will resume flight operations.

This action follows a thorough analysis of a March 9 in-flight dual generator failure on AF-4 during a test flight at Edwards AFB, Calif.  During the March 9 flight, the aircraft backup electrical generator came on line as designed and the test pilot was able to return the jet safely back to base.

Flight clearance was reinstated after an investigation and test data indicated a maintenance procedure resulted in excess oil remaining within the AES/G lubrication system. Previous maintenance procedures could allow a small amount of extra oil to remain within the generator following servicing.  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. Maintenance documents have been amended to provide improved engine starter/generator servicing procedures.

There are two primary generator configurations used on the F-35 - the ES/G (Engine Starter / Generator) which is used on AF-1/2/3, BF-1/2/3/4 and a newer production item known as AES/G (Alternate Engine Starter/Generator) which was installed in AF-4, BF-5, CF-1 and all Low Rate Initial Production (LRIP) aircraft. Both generators provide the functions of engine start and primary electrical power generation in the overall architecture.

There was no significant impact to the System Development and Demonstration (SDD) program or production operations. Flight test schedules are built with a margin for precautionary safety stand-downs. After it was determined the dual generator failure was unique to aircraft with the AES/G configuration, seven SDD jets (AF-1/2/3, BF-1/2/3/4) were cleared to return to flight on March 14. The F-35 test programs at Edwards AFB, Calif. and Naval Air Station Patuxent River, MD remain ahead of their monthly flight test schedules.