The U.S. Defense Department should give “serious reconsideration” to the rate at which it plans to buy the Lockheed Martin Corp. F-35 fighter because the aircraft’s design has proven more unstable during testing than anticipated, according to a special internal report. A team of Pentagon test, systems engineering and structural experts concluded in an Nov. 29 report there were “no fundamental design risks sufficient to preclude further production” of the jet for the Air Force, Navy and Marine Corps.

Still, it identified 13 current or likely test issues of varying severity, the combined impact of which “results in a lack of confidence” in the aircraft’s “design stability.” The issues include the Navy version’s tailhook for carrier landings, the system for dumping extra fuel on landing approach and excessive aircraft shaking during flight.