FARNBOROUGH - F-35 flights have resumed since the U.S. Air Force and Navy issued a limited flight clearance July 15.

Lockheed Martin has conducted a flight of the F-35C, a Navy variant designed for aircraft carrier arrested landings and catapult takeoffs, at its Fort Worth final assembly facility, according to spokesman Mike Rein. U.K. F-35Bs also are flying, according to a Ministry of Defense spokesman.

As of last night, the U.S. Marine Corps had not resumed flights of its F-35Bs, three of which were unable to make their international flying debut at the Royal International Air Tattoo (RIAT) scheduled for July 11 or the Farnborough air show this week. A fourth F-35B, the U.K.’s BK-3, also was a no-show.

U.S. officials could not be reached this morning for further details on the U.S. F-35 flights.

The test fleet was down for just over three weeks due to a catastrophic fire in a Pratt & Whitney F135 engine on June 23 as the pilot was preparing to take off; he egressed safely. The fire was caused by excessive friction between the third-stage integrally bladed rotor and an abradable strip lining the engine casing.

Pentagon procurement chief Frank Kendall says engine inspections indicate this problem is not endemic to the fleet. However, the limited flight clearance requires borescope inspections of engines after three hours of flight. Pilots are limited to fly at 0.9 Mach, 18 deg. angle of attack and -1 to 3 Gs. They also are limited to half a stick of deflection for rolls.

Though the aircraft have returned to flight, these limits will affect the kinds of test points the team can address. Last week Program Executive Officer Lt. Gen. Christopher Bogdan said the lag thus far did not jeopardize the planned operational debut of the F-35B for the Marine Corps on  July 1, 2015. However, the constraints could affect the F-35C's ability to address test points needed to conduct its first round of ship trials on the aircraft carrier Nimitz this fall, he said.

Program officials are now planning for a debut at RIAT next year.