HUNTSVILLE, Alabama Feb 21 (Reuters) - Lockheed Martin Corp must redesign bulkheads on its F-35 fighter jet that developed cracks after extended stress testing, but the issue is not expected to delay the U.S. Marine Corps' plans to start using the plane in combat beginning in 2015, Pentagon and Marine Corps officials said on Friday.
"We consider this significant, but by no means catastrophic," Frank Kendall, the Pentagon's chief weapons buyer, said in a statement. "Root cause analysis is still ongoing, however based on preliminary analysis a redesign of the affected F-35B structural members will be required."
Kendall said the F-35 remained the Pentagon's highest priority conventional warfare program. "Nothing has changed," he said. "There isn't any question about the department's commitment to the F-35."
The latest technical issue involving the $392 billion F-53 fighter program, the Pentagon's most expensive arms development project, arose during on-ground durability testing of the short takeoff, vertical landing (STOVL) version of the plane in September, according to the Pentagon's F-35 program office.
The issue was first flagged in a report by the Pentagon's chief weapons tester, Michael Gilmore, that also raised concerns about the F-35 software development effort.
The news comes at a time when Turkey and Canada are weighing orders for the plane. Both countries helped fund development of the plane, but have not yet ordered aircraft.
Britain, which is buying the Marine Corps version of the plane, had been expected to finalize orders for 14 more aircraft early this year, but the announcement has been delayed. UK officials took a "hard look" at the bulkhead issue but did not alter their procurement plans, according to a source familiar with the program.
Joe DellaVedova, spokesman for the Pentagon's F-35 program office, said officials were keeping all the international partners on the program informed about the bulkhead issue.
DellaVedova said cracks were initially found in the airplane's four primary bulkheads in September, but additional cracks appeared during follow-on inspections in January.
The additional cracks were first reported earlier Friday by Bloomberg.
In a joint statement, Lockheed and the F-35 office said the issue would not affect flight testing of the planes because the cracks developed at over 9,400 flight hours - well beyond the planned 8,000-hour lifespan that the plane is designed for.
"This discovery does not affect current F-35B flying operations, nor is it expected to impact the U.S. Marine Corps' ability to meet its Initial Operating Capability (IOC) in 2015," the statement said.
Marine Corps spokesman Captain Rich Ulsh said the Marines still expect to start using the F-35B for military operations in July 2015, as planned.
"This finding only affects the future modification schedules to our aircraft so they can achieve the intended service life of the aircraft, which is 8,000 hours," he said.
It was not immediately clear how much the redesign of the bulkhead or any needed retrofits would cost.
In his statement, Kendall said the program hoped to have modified parts available in time for the ninth production batch of F-35s, and was assessing the impact on the eighth batch. Lockheed is in negotiations now with the Pentagon about a contract for the eighth batch of jets.
"STOVL aircraft that have been produced already will also need some strengthening in the affected areas," he said.