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.

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