Three F-35As at Hill Air Force Base, Utah and one Norwegian jet at Luke AFB, AZ are flying again after faulty insulation in the aircraft’s avionics cooling lines temporarily paused flight operations.

A third jet returned to flight 11 November at Hill, after repairs wrapped on two jets 24 October. One of four Norwegian F-35As flew at Luke Friday morning, an Air Force spokesman tells FlightGlobal.

The insulation issue did not affect the F-35B and -C variants, but the USAF grounded 15 F-35As, including 10 at Hill AFB, four at Luke Air Force Base, Arizona, and one aircraft completing operational test and evaluation at Nellis Air Force, Nevada. The issue also affected 42 production line aircraft, which the USAF, Israel, Norway, Japan and Italy have not yet accepted.

In September, the USAF discovered insulation from an avionics cooling line within an F-35A fuel tank was found detached. Lockheed’s supplier used the wrong coating for the insulation, which deteriorated when touched by fuel. The crumbling insulation clogged the vent between the wing tank and fuselage tank, but would not affect the engine’s performance.

The two repaired F-35s at Hill have returned to their initial operational capability status, according to a spokesman at Hill. Although repairs involved cutting through the aircraft’s stealthy skin, the work did not affect the F-35’s stealth signature. After USAF maintainers remove the aircraft’s fuel and paneling, Lockheed cuts pre-engineered holes in the jet’s skin in order to access the fuel tank. Once the coating inside the coolant lines is cleaned, screens are installed to prevent clogging in the fuel siphon tubes, according to the service. The USAF runs a radar verification check to test the jet’s stealth signature after the aircraft skin and low-observable coating are restored.

With the two repaired jets, Hill has seven operational F-35As and plans to have three more by the end of this week. The base should fly all 10 of its F-35s by the end of the year, but in the meantime, the smaller fleet has affected the number of sorties and training opportunities available for pilots.

Meanwhile, Lockheed is working on repairs for two production jets for Israel and plans to maintain their original delivery schedule, a Lockheed spokesman tells FlightGlobal.

“Those are well underway to the point where we’re flying at the plant,” he says. “But they won’t be delivered until the ceremony 12 December in Israel,” Lockheed says.

The insulation problem will result Lockheed delivering fewer F-35s this year, Lockheed chief financial officer Bruce Tanner said during the company’s recent third quarter earnings call. Lockheed expects to be back on their contractual schedule by the end of 2017, he says.