Joint Strike Fighter can resume flight tests

Faulty maintenance procedures were found to have caused the March 9 in-flight failure of the engine generators aboard an F-35 Joint Strike Fighter aircraft, the program office said last week.

Those procedures have now been revised, and the entire fleet of F-35s has been cleared to resume flight operations.

The problem was revealed when an Air Force F-35A test aircraft, numbered AF-4, suffered a failure of the generators during a test flight from Edwards Air Force Base, Calif. The test pilot was able to use the backup electrical generator to return safely to base.

In flight, the generator provides the aircraft’s primary electrical power.

The configuration of the generator on AF-4 and other, newer F-35s was different than the original installation on the first test aircraft, and the problem was traced to the newer, or alternate, configuration. Test aircraft with the earlier configuration — three F-35As and four Marine Corps F-35Bs — were cleared on March 14 to resume flight operations.

Three other test aircraft — AF-4, BF-5 and CF-1, the first Navy F-35C — remained grounded, along with the first two low-rate initial production F-35As, while the investigation continued.

According to the program office, the investigation revealed that the maintenance procedure for the alternate engine starter/generator configuration allowed excess oil in the generator’s lubrication system.

Even though previous procedures allowed a small amount of extra oil in the generator following servicing, extra oil churning inside a narrow air gap within the unit could cause internal temperatures to increase, the program office said. The high temperatures led to the generator failures.

The loss of flying time caused “no significant impact” to the flight test program or to production operations, the program office said March 25, since the schedules are made up in anticipation of such delays. The Air Force, Marine Corps and Navy flight test programs “remain ahead of their monthly flight test schedules,” according to the program office.

Under production by prime contractor Lockheed Martin, the JSF is being produced in three versions. The F-35A land-based strike fighter for the Air Force is undergoing testing at Edwards, while the F-35B short-takeoff-or-vertical-landing version for the Marines and the F-35C carrier variant are being tested at Naval Air Station Patuxent River, Md.