The Lockheed Martin F-35 fleet is grounded while the joint programme office (JPO) investigates the cause of a failure in the aircraft's electrical system during ground tests.

The failure occurred on an F-35A conventional take-off and landing test aircraft, numbered AF-4, but the precautionary grounding applies to all 20 F-35s that had reached flying status, according to a JPO statement.

"Once the facts are understood, a determination will be made when to lift the suspension and begin ground and flight operations," the JPO said.

The incident marks the third grounding order for the F-35 fleet since last October, and the second in five months involving the AF-4 test aircraft.

In this case, the Honeywell-built integrated power package (IPP) failed during a standard engine test following a maintenance check at 08:30 on 2 August, the JPO said.

The IPP is primarily used as both a starter for the engine and a back-up electrical system, supporting the two main generators. In March, the IPP proved its worth by activating after both generators shut-down with AF-4 still in flight. The power generated by the IPP allowed the flight control system to keep operating until the pilot landed.

That incident also triggered a fleet-wide grounding in March until Lockheed fixed a flaw with a new generator system design introduced on AF-4. Some aircraft remained grounded for several weeks until the problem was fixed.

Last October, the fleet was also grounded after Lockheed discovered a software glitch that could have allowed the fuel pump to shut down above 10,000ft (3,050m).

The electrical system has been the cause of three of four major incidents so far during the development phase. The first incident was recorded in May 2007, when the test aircraft numbered AA-1 experienced an electrical short that disabled flight controls on the horizontal stabliser.

Source: Flight International