WASHINGTON — The US Air Force has ordered the grounding of 13 F-35A models, as well as a pair of Norwegian F-35As, following the discovery of "peeling and crumbling" coolant tube insulation.

The issue appears to have been with a supplier of coolant lines, which are installed in the wings of the jet. During a routine maintenance check, it was discovered that the insulation on the lines were in some cases decomposing, which left residue in the fuel itself, according to a release from the Norwegian government on the grounding.

The issue has been traced back to the insulated coolant tubes manufactured by one particular provider that have only been installed in the wing fuel tanks of the 15 aircraft — 10 from Hill Air Force Base, Utah, two US and two Norwegian F-35As at Luke AFB, Ariz., and one plane at Nellis AFB, Nev. 

The problem was first discovered this summer during depot maintenance of an F-35A being prepared for initial operational capability, Lockheed Martin spokesman Mike Rein said. 

After maintainers found three aircraft with crumbling coolant tubes, Lockheed conducted subsequent tests that "indicated it was possible for this crumbling insulation to become lodged in the siphon lines connecting wing and fuselage fuel tanks," said US Air Force spokeswoman Ann Stefanek. "This could result in excessive negative pressures in the fuel tanks during flying operations or excessive positive pressures during air or ground refueling. In either case, the under- or over-pressure could cause structural damage to the fuel tanks."

Lockheed works with several suppliers that are responsible for manufacturing the coolant lines, but Rein declined to disclose which of its subcontractors had been responsible for the nonconforming part. The company plans to continue to work with that supplier in future F-35 lots, he said. 

"There has been no discussion about changing doing business with them," he said.  

Stefanek said the Air Force ordered the temporary suspension of flight operations for those jets out of an "abundance of caution" regarding potential effects from the degraded insulation.

"Although testing and simulation are ongoing, initial indications are that impacts are either minimal or can be mitigated. However, it is too early to outline specific issues that might arise," she said. "Again, our primary concern is the safety of our pilots. This is a prudent precaution. Identifying and addressing issues is a standard part of the lifecycle of any of our aircraft."

The impact expands further than the operational F-35 inventory, as there are 42 aircraft currently on the production line that have received parts from the same provider. That includes three three Norwegian aircraft scheduled for delivery early next year. It is unclear if those parts will need to be replaced or if other nations planes will be impacted.

According to an Air Force press release, engineers from the F-35 Joint Program Office, Lockheed AFB and have inspected eight aircraft and are working on a plan for mitigate issues connected with the pause in flight operations. The service expects a proposed mitigation strategy as early as next week, but even after a fix is identified, it could take "days to weeks" to repair each airplane, Stefanek said. 

Lockheed is developing potential fixes for the impacted jets that would allow them to return to flight as soon and as cheaply as possible, but Rein declined to comment on when the company would have a plan finalized. It is also working on a root cause analysis.

The F-35 JPO stressed that the problem was caused by a manufacturing defect rather than a technical problem that would affect the aircraft's performance.

"The root cause of the problem was determined to be use of nonconforming material for the tubing insulation and improper manufacturing processes during fabrication of the cooling lines," it said in a statement. "The non conforming material that was used is not compatible with fuel, causing degradation of the insulation and resulting in it falling off the tubing." 

In a statement, Maj. Gen. Morten Klever, director of the Norwegian F-35 Program Office, emphasized that the issue was not a design flaw but instead was caused by a supplier using improper materials and techniques for the parts.

"I expect Lockheed Martin to identify the appropriate measures to correct this issue, and that they implement these as quickly as possible," Klever said.

This story is developing and will be updated.