Some Lockheed Martin F-35s remain grounded a week after an oil leak forced the pilot of the single-engined fighter to make an emergency landing on 10 June.

The onboard fault detection system alerted the pilot of the US Marine Corps F-35B of a fuel loss, prompting the pilot to return to a base in Yuma, Arizona, safely.

The pilot shut down the engine on the ground, according to a statement by the F-35 joint programme office.

A root cause analysis is continuing, but the source of the oil leak in the Pratt & Whitney F135 engine “appears” to be a supply line to bearings and a special fitting within a device called the oil flow managing valve, the programme’s statement says.

The bearings and the fitting “separated from the body” of the valve, the programme says.

Three days later, the programme ordered all other F-35s grounded until inspectors could examine the same valve. The 90min inspection revealed three “suspect findings” on other F-35Bs at Yuma, but cleared the rest of the fleet to return to flight status.

“We are currently in the data collection and analysis phase to determine root cause,” says P&W in a statement.

The company notes that a metric for F135 engine availability has been steadily at or above 98%.

The Yuma incident follows two other malfunctions within the last 18 months involving the F135 engine, including a cracked low-pressture turbine blade discovered early last year and a cracked fan blade on a test engine last December.