Story highlights The Air Force found problems with cooling lines

The F-35 program is the most expensive ever at $400 billion

Washington (CNN) The US Air Force said Friday it has grounded 10 of its F-35 Joint Strike Fighters, just over a month after they were declared "combat ready."

The decision affecting the most expensive weapons system ever was made "due to the discovery of peeling and crumbling insulation in avionics cooling lines inside the fuel tanks," the Air Force said in a statement, describing the action as a temporary pause in flight operations."

The faulty cooling lines affected a total of 57 aircraft, the statement said. Only 15 of those planes had been fielded with the remainder still on the production line and will be fixed there.

The plane's manufacturer, Lockheed Martin, has delivered 108 F-35As. The Air Force plans to buy 1,763 of the jets.

Of the 15 grounded aircraft already in the field, 10 had been declared combat ready, one was being used in testing, and the final four were for training, with two of those four training aircraft belonging to the Royal Norwegian Air Force.

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