The Department of Defense has admitted that it is not able to maintain its current F-35 fleet, which is still failing to meet service sustainment requirements.For all variants, aircraft are breaking down more often than planned and taking longer to fix, Robert Behler, the director of the Pentagon's Operational Test and Evaluation Office, told lawmakers last Wednesday.News of the F-35's continued problems comes on the heels of an announcement that the Pentagon and Lockheed Martin have negotiated a $34 billion deal for 378 new F-35s over the next few years.They are facing significant problems now on all these fronts - spare parts, ALIS, depots - that's only going to get worse as they continue to field more aircraft, Diana Maurer, the director of Defense Capabilities & Management at the Government Accountability Office, told Business Insider.The Department of Defense has admitted that it's struggling to manage and maintain its current fleet of fifth-generation F-35 stealth fighters, even as it plans to spend at least $34 billion on hundreds more aircraft.Operational suitability of the F-35 fleet remains below service expectations, Robert Behler, the director of the Pentagon's Operational Test and Evaluation Office, told the House Subcommittee on Readiness and Tactical Air and Land Forces during a hearing last Wednesday.No F-35 variant meets the specified reliability or maintainability metrics, he added, explaining, In short, for all variants, aircraft are breaking down more often than planned and taking longer to fix.Lockheed disputed some of Behler's characterizations of the program, Bloomberg News reported.The Government Accountability Office, however, concurred with Behler's assessment. The GAO reports that the F-35 Joint Strike Fighter program has largely failed to meet four of eight of its reliability and maintainability metrics.Between May and November of last year, fully mission capable rates for the F-35 fleet were roughly 27 percent, well below the minimum requirement of 60 percent. Mission capable rates were a little higher at 52 percent, but still below the 75 percent requirement and even further below the 80 percent demanded by former Secretary of Defense James Mattis, the GAO revealed in a report this past April.Fully mission capable means that an aircraft can accomplish all of its missions while mission capable means that it can perform at least one successful mission.This year, across the fleet, about one-third of the aircraft were fully mission capable at any given time, and two-thirds were mission capable. The mission capable rates for deployed F-35s, as was explained during last week's hearing, tended to be significantly higher, in many cases well above requirement.There was an improvement, but they are still far short from what the services want to see and need to see, Diana Maurer, the director of Defense Capabilities & Management at GAO, told Business Insider.Lt. Gen. Eric Fick, program executive officer for the F-35 Joint Program Office, attributed the fighter's problems to a lack of spare parts, among other issues. If we're missing the parts, we can't get our jets airborne. The ability to deliver combat effects on this aircraft are significantly diminished, Fick told lawmakers during the hearing.The Pentagon was supposed to have completed work on a depot infrastructure for spare parts in 2016, but it will not be finished until 2024, Maurer, who also testified at the hearing, told BI that the while Lockheed Martin is on the hook for spare parts, the way the F-35 Joint Program Office contracts with Lockheed guarantees that the jets will not be able to fly at least 20 percent of the time.In fact, the jets cannot fly at least 30 percent of the time due to a lack of spare parts.Another complication is that ALIS (Autonomic Logistics Information System), which was designed to facilitate an improved operations and maintenance posture, is not working as promised or as advertised. ALIS is a critical deficiency in the F-35 that hinders sustainment, GAO officials told BI.The major underlying issue for all of this is that for far too many years, the Joint Program Office and the DoD more broadly failed to pay sufficient attention to sustainment issues. This is a program that's been around for two decades now, and for really the vast majority of those years, sustainment was largely an afterthought, Maurer explained.That has changed for the better in the last year or two. That's encouraging. But, what it means is there is a big hole. There's a lot of work that they need to do, a lot of catch-up that they need to do.Jon Ludwigson, a director on GAO's Contracting and National Security Acquisitions team, further explained to BI that many of the F-35s problems stem from its unusual development track.Typically, what you would do is build a relatively small number of aircraft early in the life cycle, that sort of limited production where you are ironing out the wrinkles, he said. The Department of Defense decided to deviate from that and start to build a significant number of aircraft before they were done with the development work, creating a lot of unanticipated challenges.These concurrency challenges are not necessarily the path that GAO likes to see from a weapon system acquisitions perspective, Ludwigson told BI.The F-35 is the most expensive weapons program in history, with expected acquisitions costs over the aircraft's six-decade life cycle reaching $406 billion and projected sustainment costs topping $1 trillion.News that the F-35 continues to face problems comes just after the Pentagon announced that it has reached an agreement with Lockheed Martin, finalizing a $34 billion deal for 478 aircraft, although some watchdog groups argue the Pentagon might end up paying a lot more for these aircraft.The F-35 program is currently set to hit full-rate production as early as next year, and the size of the fleet is on track to more than triple with the latest deal with Lockheed. And, right now, there are serious questions about whether the Pentagon will be ready to tackle the sustainment challenges.The Pentagon is much more focused on those kinds issues now than they were a couple of years ago, but it still remains to be seen whether they are going to make sufficient traction, Maurer told Business Insider.They are facing significant problems now on all these fronts - spare parts, ALIS, depots - that's only going to get worse as they continue to field more aircraft.And, she added, it doesn't necessarily advance national security interests if you are fielding a bunch of aircraft that you cannot fly and cannot use because there aren't a sufficient number of spare parts, you haven't worked out the details of the global supply network, and you still have not fully developed the necessary depot infrastructure capability.