In a letter sent in February to the Pentagon chief, Sens. Carl Levin, the committee chairman, and John McCain, the panel's top Republican, questioned whether the F-35B had met the criteria to warrant an end to its probation. The F-35B had been on a two-year probation because of "significant testing problems."
Levin, D-Mich., and McCain, R-Ariz., wrote that the program "has enjoyed some success over the last few months, after several years of having fallen short." But they said "more problems with the F-35B's structure and propulsion, potentially as serious as those that were originally identified a year ago, have been found. This is salient where the F-35B has completed only 20 percent of its developmental test plan to date. Your decision, therefore, appears at least premature."
Neither McCain nor Levin could be reached for comment on the squadron.
The developer of the aircraft, Lockheed Martin Aeronautics Co., is building three versions of the F-35 — one each for the Navy, Air Force and Marine Corps.
Schedule delays and cost overruns have dogged the F-35's development, making it the Pentagon's most expensive weapons program ever. Ten years in, the total F-35 program cost has jumped from $233 billion to an estimated $385 billion. Recent estimates suggest the entire program could exceed $1 trillion over 50 years.
Aviation safety consultant and retired Marine Corps Col. Pete Field, who served as the former director of the Naval Test Pilot School, said the Marine Corps' F-35B is the most complicated of the three versions because it can take off from shorter runways and land vertically. One of the problems earlier on in its development was it was heavier than predicted, Field said.