F-35 manufacturer Lockheed Martin will retrofit early production lots of F-35Bs delivered to the U.S. Marine Corps with modified bulkheads to address cracking issues that came to light during durability testing of ground articles last fall. It will build redesigned bulkheads into the fighter beginning with low-rate production (LRIP) Lot 9, said Rear Adm. Randy Mahr, deputy program manager with the Pentagon’s F-35 joint program office (JPO).
“The repair for that bulkhead for the [F-35Bs] that are out there in the fleet is known, it’s understood, and we’ll be retrofitting LRIP Lots 1 through 7,” Mahr told the Navy League Sea-Air-Space conference on April 7. “On LRIP Lot 9 and forward, we have the redesign finished. We’re working on some of the final detailed design. The forgings for that design are already done and are sitting waiting for permission to start machining,” which the JPO expects this summer. Mahr said the aluminum structures for LRIP 8 have already been manufactured, requiring the program to “find a way to retrofit those bulkheads before the aircraft are delivered—that’s the goal.”
The modification addresses cracking in an aluminum wing carry-through bulkhead specific to the F-35B short takeoff, vertical landing (STOVL) variant. In an annual report the Pentagon delivered to Congress in January, the Office of the Director, Operational Test and Evaluation described cracking issues with all three F-35 variants, identified during stress testing of ground articles. Cracks discovered in F-35B bulkheads in early 2013 had grown by September, when the testing reached 9,000 estimated flight hours, exceeding the fighter’s planned 8,000-hour airframe life. “The cracks continued to grow during subsequent testing until at 9,056 (estimated flight hours), at the end of September, the bulkhead severed and transferred loads, which caused cracking in the adjacent FS518 bulkhead,” the office said. The JPO subsequently suspended durability testing. It has said, and Mahr reiterated, that the retrofits will not delay the Marine Corps from declaring initial operational capability of the F-35B next year.
Mahr said engine manufacturer Pratt & Whitney is redesigning a blisk fan blade in the F135 engine to address cracking that was revealed during engine ground testing in December. “We’re not concerned about the fleet,” he added. The fan blade retrofit “can be managed comfortably” when the fighter returns to the depot for maintenance.
The initially deficient tail hook of the F-35C carrier variant has been redesigned and proven at the Navy’s carrier suitability test site in Lakehurst, N.J., without requiring structural changes to the airframe, Mahr said. The redesigned tail hook catches an arresting wire “comparable to that of legacy airplanes, including the F-18,” he said. “Nobody catches the wire every time, but we’re in the high 90-percent [range]. The hook works.” The Navy plans to fly an F-35C for the first time to an aircraft carrier, the USS Nimitz, this fall.
The F-35 still falls short of reliability targets for mean flight hours between critical failure of parts or systems. Later production Lots 5 and 6 have shown improvement over earlier lots. “The F-35 is not yet a reliable airplane. It’s getting there,” Mahr said. “We’re learning the nuances of maintaining the F-35, all three variants…We’ve stood up a reliability team [that] is focused every day, looking at what parts are failing, why they’re failing and what we can do about it. It’s not just reliability of individual parts; we’re looking at the entire supply chain posture, when a part fails how quickly can we get [a replacement] to the flight line.” He added: “These [fighters] are being flown by operational units. They’re not being babied.”