(Bloomberg) -- The Pentagon office managing the military’s costliest program says it may be several months before it can even say when the last and most critical stage of combat testing for Lockheed Martin Corp.’s F-35 jet can begin.

The F-35 program office is evaluating results of an assessment by university software experts as to what’s needed for the final -- and repeatedly delayed -- combat simulation phase to get underway. The rigorous testing was supposed to have occurred in December, the latest missed starting date for the Joint Simulation Environment exercise once planned for 2017.

The review team assessed all the elements necessary to start the combat testing in a highly sophisticated simulator to evaluate how the F-35 -- and future aircraft and electronic warfare systems -- would perform against the most advanced Russian and Chinese aircraft and air defenses.

“The program office continues to work on a revised acquisition program” schedule and “will deliver it when complete,” spokeswoman Laura Seal said in a statement. “We expect this to be in the coming months. Our focus is on building an executable plan that addresses previous schedule overruns.”

Ellen Lord, who was undersecretary for acquisition and sustainment in the Trump administration, issued a memo on Dec. 15 directing the program office to complete a new schedule by Feb. 28.

Required Report

But Seal’s statement raises the likelihood that the testing process won’t be completed during debate on the fiscal 2022 defense budget.

Even after the one-month test occurs, it will take an additional two to three months to transfer and analyze the data and then draft a final report for delivery to Pentagon leaders and Congress. The report is mandated by law before a decision on whether to move into full-rate production -- the most lucrative phase of the contract for Bethesda, Maryland-based Lockheed -- can be made.

The Pentagon’s latest five-year plan from the Trump administration calls for requesting 85 F-35s in the year that begins Oct. 1, up from 79 this year. But the delay in the crucial combat testing may not sit well with lawmakers questioning reliance on the F-35, including Senate Armed Services Chairman Jack Reed.

“We’ve been building it” for years “and it’s still in operational testing and evaluation, and once that’s finished -- and we hope it’s finished promptly -- then we can make a much more thorough assessment of the system,” the Rhode Island Democrat said in an interview last month. “We hope that the answer is delivered soon about the effectiveness of the F-35 and the justification for its billing as the ‘premier’ fighter aircraft of the world.”

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