Washington: Lockheed Martin on March 6 morning resumed conducting acceptance flights of newly built F-35 Joint Strike Fighters, nearly three months after an engine problem grounded new jets and halted deliveries.

Lockheed announced that the first flight took off from Fort Worth, Texas, where it builds the majority of F-35 fighters.

“We resumed F-35 production flight operations today following an F135 engine mitigation action,” Lockheed Martin said in its statement.

Lockheed stopped acceptance flights after a December 15 mishap involving an F-35B fighter that was undergoing a quality check flight. A video showed the fighter bouncing on the ground, tipping forward and spinning before the pilot safely ejected.

The investigation that ensued found signs of a problem with the fighter’s Pratt & Whitney-made F135 engine, and engine deliveries were halted in late December. Further investigation found the engine had vibration issues.

Pratt & Whitney as well as engineers from the Pentagon and Lockheed developed a solution to fix the vibration issue in the engine, the F-35 Joint Program Office announced in February. That solution takes about four to eight hours to install in the engines of F-35s.

Pratt & Whitney, which is owned by Raytheon Technologies, and the JPO said in February that the vibration problem was rare.

But the office issued an order March 1 that said all F-35s should have this vibration fix installed within 90 days because the solution is “inexpensive [and] non-intrusive,” and would mean all F-35 engines have the same configuration.

Acceptance flights are required before the government can take delivery of new F-35s. The resumption of acceptance flights will also pave the way for deliveries to resume, though Lockheed did not say when deliveries might resume.

“Safety remains our top priority,” Lockheed said. “We will deliver the aircraft as quickly as possible after undertaking the multiple checks and test flights needed.”