The USAF may be flying around in 500 F-35 fifth-generation fighter jets that are untested for combat missions for years to come, as assessment trials are frequently postponed, a Department of Defense weapons expert warns in a report.

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The F-35 Lightning II, Lockheed Martin’s costliest and most modern flagship project, is a family of single-seat, single-engine, all-weather, multirole stealth fighters intended to provide the bulk of the US Air Force’s manned airpower for decades to come.

The Pentagon plans to procure a fleet of 2,443 F-35s for the US military, and hundreds more are to be ordered by Washington’s allies, including the UK, Italy, Australia and Japan.

However, the widely anticipated program seems to be in trouble again. Testing of the “3F” software that will give the F-35 full combat capability is unlikely to be completed until at least January of 2018, J. Michael Gilmore, the director of the Pentagon’s Operational Test and Evaluation Office – the Department of Defense’s main weapons tester – said in a report.

'Better than US-made 5G': PAK-FA fighter to engage sea, air & ground targets http://t.co/a6QCosMhBppic.twitter.com/A0YC665sW2 — RT (@RT_com) May 28, 2015

“These aircraft will require a still-to-be-determined list of modifications” to be fully capable, Gilmore wrote as cited by Bloomberg. “However, these modifications may be unaffordable for the services as they consider the cost of upgrading these early lots of aircraft while the program continues to increase production rates in a fiscally constrained environment.”

Despite the plane’s many problems, “F-35 production rates have been allowed to steadily increase to large rates,” the report added.

The Pentagon wants to increase the number of F-35s it purchases from 38 last year, to 92 annually by 2020. Despite being the US’ costliest weapons program, at a projected $391 billion, the F-35 is being produced even as it’s still being developed.

The F-35’s $55 billion development phase involves assessing how well the aircraft and its systems meet contract specifications and military requirements.

Combat testing, which takes at least a year, evaluates the warplane’s performance in simulated fighting scenarios, such as the ability of F-35s to share data while in a four-jet formation.

The Pentagon needs the results of the software tests in order to launch full-rate production, which is currently scheduled for April 2019.

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Last month, the Air Force told a weapons review group for the Joint Chiefs of Staff that it considered the date unrealistic because of “inherited deficiencies” with the F-35’s “Block 3i” software and “new avionics stability problems,” Gilmore wrote.

In addition to the dysfunctional software system, the report cites several others critical deficiencies. The F-35s also have fuel system defects that limit their maneuverability when carrying a full load of fuel. In addition, the pilot escape system could kill ejecting pilots weighing 136 pounds or less by breaking their necks, Bloomberg reports.

The F-35’s main competitor, the Russian-made T-50 fifth-generation fighter jet developed by Sukhoi, is likely to outperform US aircraft of the same type, military experts believe. The final primary batch of PAK-FA fighter jets is almost complete, and tests will be finished in 2016, top military officials told the media.

The Russian Air Force hopes to start regular production of the new 5G jets soon, with the first planes coming in 2017.