F-35 Joint Strike Fighters grounded by an engine problem discovered in December will undergo an immediate retrofitting procedure to return to flight, the Joint Program Office said Thursday, and all other F-35s should be retrofitted within 90 days. The JPO issued the Time Compliance Technical Directive, spokesman Russ Goemaere said in an email, to mitigate a “ harmonic resonance ” phenomenon in the F135 engine, made by Pratt & Whitney. “While only a small number of aircraft were impacted by...