Lockheed Martin F-35s resumed flight testing over the weekend, but two production aircraft remained grounded while US Air Force safety investigators continue scrutinising their power and thermal management system.

Flight tests included at least three catapult launches of an F-35C carrier-based variant from a runway at a naval base in Lakehurst, New Jersey, according to the joint programme office.

The navy has also completed jet-blast deflector testing, moving the F-35B short take-off and vertical landing variant one step closer to shipboard trials.

The catapult launches were among the first flight operations for the F-35 test fleet since a grounding order was lifted on 18 August, after a 16-day hiatus.

Lockheed Martin F-35 fleet
 © Lockheed Martin

The joint programme office suspended flight operations on 2 August and the Air Force safety oversight board launched an investigation after conventional take-off and landing flight test aircraft AF-4 experienced an "explosive event" during a routine ground test.

A control valve malfunction caused the Honeywell integrated power package (IPP) to fail after starting up the F-35's engine. The IPP is used to start the engine, and then powers the system that cools the F-35's power supply.

Flight operations are limited to the 12 aircraft flight test fleet, which are heavily instrumented and can be monitored in real-time.

As many as eight completed production aircraft, including the AF-8 and AF-9 models delivered to Eglin air force base, Florida, remain grounded while the safety investigation continues.

The production aircraft are expected to return to flight after a series of hardware and software fixes are installed, expected in September and October.

Source: Flight International