A but of bad luck has struck the F-35 Joint Strike Fighter program just as the first production jets are hitting the fleet. The fleet of 20 test and training jets has been grounded following a failure of an F-35A test jet’s Intergrated Power Package during an Aug. 2 ground test.

The IPP combines the auxiliary power unit, emergency power system and environmental controls. No one knows what caused the system to fail on test jet AF-4, (shown above).

Here’s the text of a JSF program office announcement on the grounding:

Yesterday, at approximately 8:30 a.m. PDT during a standard ground maintenance engine run, aircraft AF-4’s Integrated Power Package (IPP) experienced a failure. Following standard operating procedures, the engine was immediately shut down and the jet was secured. No injuries to the pilot or ground crew occurred. The F-35’s IPP is a turbo-machine that provides power to start the engine and generates cooling for the aircraft. The government and contractor engineering teams are reviewing the data from the incident to determine the root cause of the failure. Implementing a precautionary suspension of operations is the prudent action to take at this time until the F-35 engineering, technical and system safety teams fully understand the cause of the incident. Once the facts are understood, a determination will be made when to lift the suspension and begin ground and flight operations of the 20 F-35s currently in flying status. These aircraft are part of the System Development and Demonstration (SDD) and Low Rate Initial Production (LRIP) fleet. Determinations of root cause and potential mitigating actions have the highest priority of the F-35 Team. Impact to SDD execution and production operations is being assessed. The program has built schedule margin into the test schedule to accommodate these kinds of incidents that occur in a development effort. Periodic updates concerning this situation will be released as warranted.

Sister site DoDBuzz makes the interesting point that this incident combined with the grounding of the F-22 Raptors means that the U.S.’ entire fleet of fifth generation fighters is out of action.

This is the second time that the F-35 fleet has been grounded due to a problem with AF-4’s power system. The jets were benched in March after the conventional take off variant experienced a generator failure in flight.