WASHINGTON — The 13 US Air Force F-35s that had been grounded since September because of a supply problem have been repaired and are back in action, the service confirmed on Friday.

"Today Lockheed Martin and Air Force aircraft maintenance teams completed repairs on the last of 13 F-35As requiring modifications to avionics coolant tubes inside aircraft fuel tanks, and all affected F-35As have returned to normal flying operations," Capt. Mark Graff, a service spokesman, said in a Nov. 18 statement. 

The Air Force announced on Sept. 16 that it was suspending flight operations for 15 operational F-35A conventional takeoff and landing variants because coolant lines inside the wing fuel tanks had incorporated faulty insulation that did not meet requirements and had begun disintegrating. A further 42 aircraft in various stages of production were also found to have used the non-conforming insulation.

The affected operational aircraft included 10 Air Force aircraft at Hill Air Force Base, Utah, two at Luke Air Force Base, Arizona, one at Nellis Air Force Base, Nevada, and two Norwegian jets at Luke. The Norwegian aircraft have also gone through repairs and returned to flight status, Graff said.

Air Force maintainers and Lockheed Martin contractors began repairing the airplanes on Oct. 7. After service maintainers removed the impacted aircraft’s fuel and paneling, Lockheed workers cut a hole into the wings to access the coolant lines and remove any deficient insulation. After mending the aircraft skin and low-observable coating, the aircraft were returned to the service.

F-35 Joint Program Officer Lt. Gen. Christopher Bogdan had promised that all 15 operational planes would return to service by the end of the year. The company will be held responsible for the cost of the repairs.

Fixing the remaining 42 production planes will take a little longer, but all will be repaired and delivered by the end of calendar year 2017, Greg Ulmer, Lockheed’s vice president for F-35 production, told Defense News earlier this month. Thirteen aircraft — 10 US Air Force planes, one jet for Japan, and two Israeli joint strike fighters — were planned to be delivered by the end of this year.

Lockheed is prioritizing the delivery of the three foreign planes and is on track to be able to meet that commitment, Ulmer said, but how many US jets it will be able to deliver in 2016 is an open question.