Lockheed Martin and the F-35 Joint Program Office (JPO) say that two structural cracks have been found during durability trials on a short take-off/vertical landing (STOVL) variant ground test article.

“During a recent inspection of the F-35B short takeoff/vertical landing ground article used for durability testing, two cracks were identified in one of the ground article’s four primary wing carry-through bulkheads,” Lockheed and the F-35 JPO say. “Because of the high hours accumulated on this test article, 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 cracks are described as “minor” by JPO and Lockheed officials and were discovered in late August. According to Lockheed and the JPO, the F-35B full-scale durability test article had accumulated more than 9,400h of equivalent flight usage—which equates to about 17 years of operations--when the cracks were discovered. Testing was halted on 29 September “to conduct root cause analysis on discovered bulkhead cracks,” the company and the JPO say.

According to the JPO, there is a combined effort by government and Lockheed engineering teams that is currently underway to address modifications to the bulkhead that will be incorporated into the production and the fielded fleet. “While the team is still working on a final solution, initial estimates indicate any modification would add less than 2lb to the aircraft,” JPO and Lockheed officials say.

While Lockheed and the JPO say that the required modification is minor, the cost to repair aircraft that have either been fielded or are currently in production has not yet been determined. “The costs and timeframe associated with fixes to the current F-35B fleet are not known at this time,” Lockheed and the JPO say. “Depending on when the appropriate engineering solution is incorporated into the production line, about 50 F-35Bs will require bulkhead repairs.”

The Pentagon requires the F-35 to have a projected service life of 8,000 flight hours, which has to be verified through durability testing equivalent to two lifetimes or 16,000h. “The purpose of durability testing is to intentionally stress the aircraft to its structural limits so we can identify any issues and corrective actions needed to fix them,” the joint statement says. “These discoveries are expected and planned for in a developmental programme.”

The F-35 programme says that the discovery of the cracks will not affect the US Air Force’s F-35A or the US Navy’s carrier-borne F-35C model aircraft. “This test discovery has no impact on the F-35A or F-35C flight operations,” Lockheed and the JPO say.