The report raises serious questions about whether the Pentagon should initiate a three-year “block buy” of up to 450 fighter jets beginning in 2018, something that was floated last year in the Defense Department as a way to save money. Doing so would drive down the cost of each single-seat, single engine aircraft and increase fielding of the jet to both the U.S. military and international partners like Australia and Britain, defense officials said.

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“Depending on the timing, it is possible a commitment to the ‘block buy’ would be made before operational testing is complete,” the report said. It added that there are still “significant discoveries requiring correction before F-35s are used in combat,” and questioned whether buying a large number of aircraft so soon would motivate the prime contractor, Lockheed Martin, to “correct an already substantial list of deficiencies in performance, a list that will only lengthen as… testing continues.”

The top officer leading the F-35 program, Air Force Lt. Gen. Chris Bogdan, said in two-page response that everything in Gilmore’s report is accurate, but that it “does not fully address efforts to resolve known technical challenges and schedule risks.” It’s the F-35 joint program office’s job to do so, Bogdan said.

“As a reminder, the F-35 program is still in its developmental phase. This is the time when issues are expected to be discovered and solutions are implemented to maximize the F-35’s capability for the warfighter,” the general said. “While the developmental program is 80 percent complete, we recognize there are known deficiencies that must be corrected and there remains the potential for future findings. Our commitment to overcoming challenges is unwavering.”

The F-35 program currently calls for the fielding of 1,763 aircraft to the Air Force, 680 to the Navy and Marine Corps, 138 to Great Britain, 100 each to Australia and Turkey, 60 to Italy, 37 to the Netherlands, 52 to Norway and 30 to Denmark, according to figures released by Lockheed Martin. The majority of those are conventional F-35A models, with the Marine Corps buying a short-takeoff version known as the F-35B and the Navy buying a version for aircraft carriers known as the F-35C.

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Gilmore’s report found that analysis of the fleet of F-35s that have been fielded between August 2012 and October 2015 “showed a weak rate of improvement of approximately 5 percent growth per year,” but noted that it was not consistent on a month-to-month basis. Each of the services also must send the current fleet of F-35s to receive modifications due to a variety of flaws in the initial aircraft design.

“Some of these modifications are driven by faults in the original design that were not discovered until after production had started, such as major structural components that break due to fatigue before their intended lifespan, and others are driven by the continuing improvement of the design of combat capabilities that were known to be lacking when the aircraft were first built,” the report said. “This ‘concurrency tax’ causes the program to expend resources to send aircraft for major re-work, often multiple times, to keep up with aircraft design as it progresses.”

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The report also questioned “significant deficiencies” in a laboratory that was established to compile information known as mission data files so that the F-35 can operate in combat. The facility, known as the U.S. Reprogramming Laboratory, was established at Eglin Air Force Base with $300 million promised, but “has still not designed, contracted for, and ordered the required equipment — a process that will take at least two years, not counting installation and check-out,” Gilmore’s report said.

“Unless remedied, these deficiencies in the USRL will translate into significant limitations for the F-35 in combat against existing threats,” the report added.

Another concern raised is the F-35 program’s inability to develop a simulator that will allow the military to complete initial operational testing. Known as Verification Simulation, or VSim, it is supposed to test the jet for a wide range of combat missions. In August, the mission was moved with a “sudden decision” to a similar simulator proposed by Naval Air Systems Command. Doing so, Gilmore’s report said, again complicates future F-35 testing.

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Pierre Sprey, a defense analyst who was involved with designing the A-10 attack jet and F-16 fighter jet, said Wednesday that he is surprised at the amount of candor in Gilmore’s report. He noted that it was released in what is likely Gilmore’s last year as the Pentagon’s top weapons tester, considering his position is a political appointment and President Obama leaves office early next year.

“This document is probably the most extraordinary review of any weapon that has come out of the DOT&E office,” Sprey said. “The fact that this escaped from the Pentagon is extraordinary… You really need to get a sense for what it takes to put out something as critical as this in the face of a 1.3- trillion dollar monster.”

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Sprey said the real measure of an aircraft’s reliability isn’t the amount of flight hours it has flown in total, but the amount of sorties it can complete in a day. The U.S. military typically expects modern aircraft to complete one flight per day, and some foreign militaries fly two or three. Gilmore’s report notes that F-35s so far have typically flown about one mission every five days.

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Sprey and a few other defense analysts with the Project on Government Oversight’s Center for Defense Information, a Washington think tank, met with a handful of reporters Wednesday, and released an analysis of Gilmore’s report that said it is likely the F-35’s final suitability for combat likely will not be known until at least 2022.