Lockheed Martin is designing a fix to address cracks found in one of four primary wing carry-through bulkheads on an F-35B ground-test article that was undergoing durability tests for a second life of service beyond 8,000 flying hours.

The suspect bulkhead is the same structure found in 2010 to have had a crack at the 1,500 hr. mark, temporarily halting testing until a fix was implemented.

Durability testing, which subjects the test article to conditions simulating actual flight to validate parts longevity and identify any design flaws before fleet use, was stopped on the B-model ground test article Sept. 29, according to Joe Dellavedova, a spokesman for the F-35 Joint Program Office (JPO).

This does not affect flight operations for F-35Bs already delivered to the fleet for testing, operations or training as the flaw was found at a point that far exceeds the flying hours on fielded jets. The part is specific to the B, which is designed for short-takeoff-and-vertical landing, so the discovery has no effect on the conventional F-35A or the F-35C optimized for carrier use, program officials say.

Lockheed Martin, the prime contractor for the airframe, and the JPO have been conducting a root cause analysis since testing stopped, and a fix is not expected to add more than 2 lb., to the aircraft, Dellavedova says. The F-35B is weight sensitive, constraining the options for a fix.

The program office estimates that roughly 50 F-35Bs already delivered to the fleet will have to undergo modification for the fix, Dellavedova adds.

These cracks are not to be confused with those found in bulkhead 496 earlier in 2010. Those were discovered after 1,500 hr. of durability testing and were due to stress mounting where the main landing gear attaches to the fuselage. A fix was incorporated into production at F-35B number 24.

These new cracks developed in the second “life” of durability testing; the F-35 is expected to provide 8,000 hr. of flight service. “The F-35B full-scale durability test article had more than 9,400 hours of equivalent flight usage when testing was stopped to conduct root cause analysis on discovered bulkhead cracks,” says Laura Siebert, a Lockheed Martin spokeswoman. These “9,400 durability test hours equate to more than 17 years of operational flying.”

She says that the “anticipated solution, in comparison, is simple to implement for both production and retrofit” of these current cracks, relative to the problems with the 496 bulkhead.

The exact cost of the fix is not yet known, but the F-35 program “has budgeted for findings during the durability test program,” Dellavedova says. The cost to each aircraft receiving the retrofit will depend on which lot it was manufactured in. Beginning with the recently negotiated LRIP 6 contract and LRIP 5 before it, Lockheed Martin equally shares financial responsibility for the cost. The government pays for a larger share of the cost for aircraft in earlier lots.