The F-35 Joint Strike Fighter is an “immature aircraft” that’s not ready yet for pilots to train on, a U.S. study found. Michael Gilmore, the Pentagon’s chief weapons tester wrote in a memo dated Oct. 21 that training programs on the $100-million-apiece jet should be delayed by 10 months, Wired magazine reports. In the memo, Gilmore cites a verity of problems that could result in fatal accidents, such as incomplete flight manuals, an untested ejection seat and a faulty generator. The rate at which F-35s currently being tested have been experiencing in-flight problems is also abnormally high, Gilmore writes. Ten months of additional testing, he noted, should be enough to fix the technical flaws and make the jets safe for pilot training. The Air Force, the Navy and Lockheed Martin, which manufacture F-35s, disagreed with Gilmore’s findings.