WASHINGTON — Two of the F-35s that had been sidelined in September because of faulty insulation have been repaired and are now flying, and the remaining 13 planes are currently going through the modification process, Air Force and Lockheed Martin officials have confirmed.

The service in September suspended the flight operations of 15 F-35As after discovering that the insulation around the jet's avionics cooling lines had begun disintegrating into the fuel tank. Another 42 F-35As in various stages of production at Lockheed Martin's Fort Worth-based facility were also affected by the supply issue.

After undergoing repairs, two aircraft from Hill Air Force Base, Utah, returned to flight on Oct. 24, and another three planes are scheduled to finished being fixed on Nov. 4, said Air Force spokesman Micah Garbarino.

All 15 operational F-35As affected by the faulty insulation are in various stages of undergoing repairs, Greg Ulmer, Lockheed's vice president for F-35 production, told Defense News in a Nov. 2 interview. The company is also working through 42 production aircraft — including four aircraft at the final assembly and checkout facility in Italy and four in the Japanese FACO — and will complete modifications by next summer so that all aircraft can be delivered by the end of 2017.

Ten of the 15 F-35As at Hill were impacted by the insulation problem. The other five affected aircraft include two US and two Norwegian F-35As at Luke AFB, Arizona, and one plane at Nellis AFB, Nevada. All of the US Air Force jets are planned to be back in service by the end of the year.

Lockheed works with multiple suppliers that provide the insulation for the coolant tubes, and as such the F-35Bs and F-35Cs used by the Navy and Marine Corps were not affected by the faulty insulation. The company has not revealed which supplier is responsible for the nonconforming part, but Lockheed has said it will continue working with that company to meet the growing F-35 production demands.

"It's really important to note that this is not a design flaw with the F-35A," Air Force spokesman Mark Graff said in a statement. "This is a case of a supplier using improper material and improper sealing techniques for a part on the aircraft. The situation was addressed and the supplier will manufacture parts using correct materials and processes."

Graff said the service is seeing "rapid progress" on repairing the operational joint strike fighters. Repair work began on Oct. 7, and each aircraft takes about three weeks to complete.

Air Force maintainers from the 388th and 419th fighter wings begin the repair process by removing the aircraft’s fuel and paneling, Garbarino said. Then a Lockheed Martin team takes over, first cutting holes in the aircraft so that they can access the fuel tank and then removing the bad insulation. The team also installs screens to keep the fuel siphon tubes from becoming clogged. Finally, the contractors mend the aircraft skin and low-observable coating and deliver it back to Air Force maintainers, who check the plane before it returns to operations.

Because Hill had only five operational F-35s available before repairs on the first two jets wrapped up, pilots were sent to Luke AFB, Eglin AFB in Florida and Nellis AFB in Nevada for the flight training needed to keep up their proficiency, Col. Jason Rueschhoff, the 388th Operations Group commander, said in a news release.

Maintainers also turned around aircraft three times a day to allow for three sorties daily, said Col. Michael Miles, the 388th Maintenance Group commander.

"Normally, the most a fighter wing will turn their aircraft is twice a day," he stated in the release.

Production Aircraft

Ulmer said that Lockheed is making its "best effort" to fix the 13 impacted production F-35As — 10 US Air Force planes, one jet for Japan, and two for Israel — that were due to be delivered this year. Its foremost priority was modifying the first two Israeli aircraft, a task that is now complete.

"Those aircraft have actually flown out, post-mod," he said. "Right now, having flown those airplanes out, they are on track to support the December in-country delivery for Israel."

Lockheed is also on track to deliver the Japanese jet as promised by the end of 2016. Modifications are complete on that airplane, and it is going through normal aircraft acceptance procedures ahead of a first flight planned for this month, Ulmer said. One Air Force F-35A has gone through the repair process, and four other airplanes are in various stages of the modification.

Lockheed and its international industrial partners will compete modifications for eight planes being built by the Italian and Japanese FACO in the first quarter of next year, "significantly prior to their intended flying operations," he said.

The method of fixing the fighters is the same whether the planes are operational or still in production, Ulmer said. The only difference is that each plane at Lockheed’s Fort Worth facility is inspected by boroscope so that the company can tell exactly where the nonconforming insulation is on all of the planes, preventing any extraneous maintenance work from having to be done.

But unlike the fielded jets, the production planes are in differing stages of construction and assembly. This presents a challenge to company, which has to ascertain the most efficient way to fix each airplane without pulling it out of the normal production flow. For instance, it would be difficult to do the modifications just after wing construction is complete, so that work would be pushed off until after the wings are mated with the fuselage.

"We look at our processes and [see] where does it make sense to insert the mod over or on top of concurrent work to try and save as much time as you can. We are essentially starting a new airplane in the factory every four days for this modification in general," he said. "We are learning as we go, as we learn, we may find there are specific times to do this."