Lockheed Martin has discovered a potentially significant problem with one model of its F-35 joint strike fighter, the company reported Wednesday.
The news comes just days before an important Pentagon meeting on the program's continuing delays and rising costs.
Lockheed issued a statement saying its Fort Worth engineering staff had found cracks in the rear bulkhead -- a major structural part weighing about 300 pounds -- of an F-35B ground test plane undergoing fatigue testing.
The cracks were found after the plane had been subjected to the equivalent of about 1,500 hours of flight time. The airplane's structural components are designed to last at least 8,000 hours.
Lockheed said the cracks were found in a special inspection after engineers discovered unusual data from test instruments.
The latest problem comes at a key juncture for the troubled F-35, which, at an estimated $382 billion, is the costliest weapons program ever.
Pentagon officials have been reviewing the program for weeks to determine how to expedite testing and fix failing components, particularly of the short-takeoff-vertical-landing F-35B designed for the Marines.
A Defense Acquisition Board plans to meet Monday to review cost and test data and approve any changes in budgets and schedules.
There have been unconfirmed reports that senior Pentagon officials were at least considering the possibility of canceling the F-35B model to focus on getting the other two versions tested and into service.
Lockheed did not disclose when the inspection took place but said it had inspected all of the other flight test aircraft and the ground test F-35A model.
"No additional cracks were found, and flight testing has not been impacted," the company said.
Eight F-35s are now in flight testing, including four F-35Bs.
Four more are built and being prepared for flight testing, and six have been built for ground testing.
At least 14 production planes are in the final assembly stages on the Lockheed production line, with most of the major structural components installed around the bulkheads.
An investigation into "the root cause" of the cracks is under way, Lockheed said.
The test equipment, an improperly manufactured bulkhead or a design error could have caused the problem.
"Sometimes it's a machining issue. Sometimes it's a design issue. They'll figure it out pretty quickly," said Hans Weber, an aerospace engineering executive and consultant in San Diego.
A design issue is probably the worst-case scenario, since it would mean redesigning and then remanufacturing the bulkheads, then installing them on planes already built.
The F-35B has bulkheads made of a lighter aluminum alloy than those on the F-35A and C models.
An Arlington company, Progressive Inc., mills the bulkheads to exact measurements and extremely tight tolerances from large aluminum forgings made by Alcoa.
The bulkheads, which the engine sits inside of, provide the core strength of the airplane and are designed to withstand massive pressures and loads.
The wings, the tail, the nose section and other components all attach to the bulkheads.
Joe DellaVedova, public affairs director for the Pentagon's joint strike fighter program office, said in an e-mail that it's "too early in the process to talk about the significance" of the problem.
The F-35B is the most complex of the three models because of the unusual stresses involved in making short takeoffs and vertical landings.
The Navy has long resisted operating F-35Bs aboard its aircraft carriers, but the Marines want the plane to replace their aging Harrier jump jets.
The British military initially planned to buy F-35Bs for its Air Force and Navy but recently altered its plans and now expects to buy the same version as the U.S. Navy for its carriers.