The Pentagon’s fleet of F-35s remained grounded Monday, four days after grounding and two weeks since a temporary flight suspension was declared for the Air Force’s fleet.

The Defense Department said late Thursday night in a statement technical air worthiness authorities of the Air Force and Navy grounded their fleets based on initial findings from a runway fire incident that occurred June 23 at Eglin AFB, Fla. The root cause of the incident remains under investigation.

The Marine Corps' F-35B short takeoff and vertical landing (STOVL) variant. Photo: Lockheed Martin.
The Marine Corps’ F-35B short takeoff and vertical landing (STOVL) variant. Photo: Lockheed Martin.

Additional inspections of F-35 engines have been ordered and return to flight will be determined based on inspection results and analysis of engineering data, DoD said. The Air Force’s F-35As have been out of service since June 23. The Navy’s F-35Cs went out of service June 26. The Marine Corps’ F-35Bs also went out of service June 26, but briefly returned to flight June 27 before a Pentagon-wide grounding was declared Thursday.

A defense official said Monday the Navy, in coordination with the F-35 Joint Program Office (JPO), could make an announcement by Thursday with “some sort of update” as to the F-35Cs. The official said an investigation board tabled by Navy Vice Adm. David Dunaway would make the final call as to whether the F-35Cs should return to flight. Dunaway is the chief of Naval Air Systems Command (NAVAIR).

Air Force spokeswoman Lt. Natasha Waggoner said Monday the grounding declaration will require a more in-depth investigation into the matter as opposed what was required under the previous temporary suspension. Waggoner said each individual Air Force air worthiness authority–Air Force Materiel Command (AFMC), Air Education Training Command (AETC) and Air Combat Command (ACC)–will make their own individual decisions on whether to green-light a return to the skies.

The Marine Corps’ F-35B is supposed to make its international debut the weekend of July 11 at the Royal International Air Tattoo (RIAT) in England and be shown at the Farnborough International Air Show the week of July 14. Marine spokesman Capt. Richard Ulsh said Monday the Marines still plan to show the F-35B at both airshows.

Waggoner said the Air Force plans on providing the F-15E and MC-130J for static display, or display on the ground, at RIAT. A July 4 notice posted on the RIAT website said event representatives were told the Air Force intended for “one or two” aircraft to come to the tattoo for static display.

The F135 engine used in the F-35 is developed by Pratt & Whitney of United Technologies Corp. [UTX]. Pratt & Whitney spokesman Matthew Bates said Monday the company is working closely with the Air Force Safety Investigation Board to determine the root cause and inspect all engines in the fleet. Bates declined to comment further, citing an active investigation.

The F-35 is developed by Lockheed Martin [LMT] with subcontractors BAE Systems and Northrop Grumman [NOC].