All 20 Lockheed Martin F-35 Joint Strike Fighters now flying were grounded August 3, following the failure of the integrated power pack (IPP) on an F-35A during a ground run at Edwards AFB.

The Joint Program Office in the Pentagon said that the move was precautionary, until the root causes of the problem were understood. Lockheed Martin said it is working hard to resolve the issue. Honeywell supplies the IPP.

This is the second time this year that an electrical system problem has brought F-35 flight tests to a halt. Last March, an in-flight failure of an engine starter/generator (ES/G)–on the same Edwards F-35A–caused a one- or two-week grounding of some aircraft.

Hamilton Standard supplies the ES/G, which is dual-redundant. The IPP provides the aircraft’s environmental control system, as well as another backup to the ES/G.