Only One of Six Air Force F-35s Could Actually Take Off During Testing by Clay Dillow @FortuneMagazine April 28, 2016, 1:46 PM EDT E-mail Tweet Facebook Linkedin Share icons Five of six Air Force F-35 fighter jets were unable to take off during a recent exercise due to software bugs that continue to hamstring the world’s most sophisticated—and most expensive—warplane. During a mock deployment at Mountain Home Air Force Base in Idaho, just one of the $100 million Lockheed Martin lmt F-35s was able to boot its software successfully and get itself airborne during an exercise designed to test the readiness of the F-35, FlightGlobal reports. Nonetheless, the Air Force plans to declare its F-35s combat-ready later this year. Details surrounding the failed exercise were disclosed earlier this week in written testimony presented to Congress by J. Michael Gilmore, the Pentagon’s chief weapons tester. Get Data Sheet, Fortune’s technology newsletter. “The Air Force attempted two alert launch procedures during the Mountain Home deployment, where multiple F-35A aircraft were preflighted and prepared for a rapid launch, but only one of the six aircraft was able to complete the alert launch sequence and successfully takeoff,” Gilmore wrote. “Problems during startup that required system or aircraft shutdowns and restarts – a symptom of immature systems and software–prevented the other alert launches from being completed.” It’s not the only recent example of “immature systems and software” stalling progress on the $400 billion F-35 program. Aside from reports of glitches affecting both the onboard and ground-based software that drive the F-35—including bugs in the F-35’s radar software that requires periodic in-air radar reboots and maintenance software problems that could potentially ground the entire fleet—Gilmore detailed another recent example in which F-35s had to abort their test mission due to software stability issues. In that incident, two of four F-35s loaded with an earlier version of the combat jet’s software were forced to abort a test of the aircraft’s radar jamming and threat detection capabilities due to software stability problems encountered at startup. The aircraft that were able to fly didn’t do so well in the evaluation either, Gilmore added. Perhaps more troublesome for the F-35 program, overall, is the fact that software stability seems to be getting worse. U.S. Marine Corps F-35Bs loaded with an earlier version of the software are reportedly the most stable, enjoying up to eight hours between “software stability events,” military lingo for glitches in one of the aircraft’s computer programs. The Marine Corps has already declared its F-35s combat ready, though Gilmore acknowledged that in real-world combat the F-35B would require assistance acquiring targets and avoiding threats. The Air Force runs a newer version of the software known as “Block 3i” on its F-35s, and gets roughly half the time between significant software glitches—though F-35 program chief Lt. Gen. Christopher Bogdan recently told reporters that a new version of Block 3i software appears to have tripled in stability during tests, going up to 15 hours without a serious software issue. Earlier this week Bogdan told reporters that despite the software issues, the Air Force still plans to declare its F-35s combat-ready sometime later this year. That could happen as soon as August, he said, though problems with the F-35s ground-based maintenance software will likely push that declaration back 60 days to October.