The Lockheed Martin F-35 has been cleared to resume ground operations, but no flight tests are allowed until safety investigators determine what caused a control valve to malfunction on 2 August.
The decision extended an eight-day-old grounding of the 20 flyable F-35s, but allows for some development tests to continue on the ground.
The joint programme office halted all F-35 testing on 2 August, after a control valve in the Honeywell integrated power package (IPP) malfunctioned during a routine ground test involving conventional take-off and landing test aircraft AF-4.
The valve allows the IPP to switch its power source from combusted air to bleed-air after it has started up the F-35's propulsion system.
Highly compressed air built up after the valve failed to open, producing what was described as an "explosive event" by one F-35 programme official, who was quoted in Australian Aviation magazine.
© Lockheed Martin
The Lockheed Martin F-35 has been cleared to resume ground operations following its grounding on 2 August
The US Air Force convened a safety board on 4 August to determine the root cause for the valve malfunction. It subsequently determined that closer monitoring of the valve operations would allow the programme to resume ground test operations, starting on 10 August.
Resuming flight operations will depend on the results of the root cause analysis for the control valve failure.
The IPP is relatively unheralded, but plays a major role in the F-35's performance. It combines the functions of an environment control unit, engine starter and back-up power generator into a single system.
Its importance also means safety investigators have to be cautious about clearing the F-35 for flight operations.
The same test aircraft, AF-4, could have crashed last March after both main generators failed, but the IPP kicked in as the back-up power supply.
Although it has never failed before in more than 1,500 flight hours, the IPP has been a target for concern during the development phase.
Last March, the Government Accountability Office reported exhaust from the IPP could damage amphibious landing decks. In 2007, an IPP failed on a test stand, which required Honeywell to redesign the system.
The IPP failure marked the third grounding for the F-35 programme in 10 months, and is nearing the longest to date.
The joint programme office is still assessing the impact of the grounding on the schedule for the system development and demonstration phase, but the latest version of the schedule included a margin for unexpected problems.