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The Pentagon this week relaxed some flight restrictions on the test fleet of F-35 strike fighters, allowing the 20 test aircraft to fly six hours between mandatory engine inspections, instead of the three-hour limit still imposed on the 79 other Air Force, Navy and Marine Corps F-35s. Program officials said the older F-35s are considered less likely to have the engine problem because they are already “broken in,” and engineers think that if the problem were to occur on those jets, it would have happened already. The restrictions date to July 15 after the fleet was grounded following the June 23 engine fire that damaged an Air Force F-35A at Eglin AFB, Fla. The test aircraft already had been cleared for higher airspeeds and G loads than those in the training and nascent operational units. The F-35 program office said investigators are still working to pinpoint the exact cause of the fire at Eglin, although they have tentatively said a fan blade rotor was excessively rubbing against the engine casing. Program officials said fixes under consideration range from an engine redesign to establishing a “break-in” period for each motor that would allow for the gradual contraction, expansion, and strain on the engine to prevent premature structural cracking. However, a final solution awaits a definitive “root cause” for the fire.