WASHINGTON — The U.S. Defense Department will not clear the F-35 fighter jet for full-rate production this year, and it may even have to push that milestone as far as January 2021, the Pentagon’s acquisition executive said Friday.

The Pentagon had intended to make a full-rate production decision — also known as Milestone C — by the end of 2019. But because the Joint Simulation Environment continues to face delays in its own development, the Defense Department will have to defer that milestone by as many as 13 months, Under Secretary of Defense for Acquisition and Sustainment Ellen Lord told reporters during a news conference.

The Joint Simulation Environment, or JSE, is needed to conduct simulated evaluations of the F-35 in a range of high-threat scenarios.

“We actually had signed out of the JPO [F-35 Joint Program Office] earlier this week a program deviation report that documented expected schedule threshold breach in the Milestone C full-rate production decision of up to 13 months,” Lord said.

It is unclear whether the delay will cause an increase in program costs.

Although the Defense Department already buys the F-35 in large numbers, the full-rate production decision is viewed as a major show of confidence in the program’s maturity. During this time, the yearly production rate is set to skyrocket from the 91 jets manufactured by Lockheed Martin in 2018 to upward of 160 by 2023.

But before Lord signs off on the production decision, the F-35 must complete operational testing, the results of which will be validated by Robert Behler, the Pentagon’s director of operational test and evaluation.

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The F-35’s testing community intended to complete initial operational test and evaluation, or IOT&E, by this summer; however, the JSE is not yet complete.

“We are not making as quick progress on the Joint Simulation Environment, integrating the F-35 into it. It is a critical portion of IOT&E. We work closely with Dr. Behler and DOT&E [[the office of the director of operational test and evaluation]. They are making excellent progress out on the range with the F-35, but we need to do the work in the Joint Simulation Environment,” Lord said.

“We have collectively decided that we need the JSE [to be] absolutely correct before we proceed, so I will make some decisions about when that full-rate production decision will be made shortly," she added.

Specifically, the Defense Department and F-35 prime contractor Lockheed Martin are lagging behind in integrating the "'F-35 In-A-Box” — the simulated model of the F-35 and its sensors and weapons — into the JSE, reported Military.com, which broke the news of the testing delay in September.

Even before IO&TE formally started, the F-35 test community had noted the challenge of maintaining the planned schedule.

The F-35 began operational tests in December 2018, three months after the originally scheduled start date in September. The program office maintained that its goal was to see the test phase finished by the summer of 2019. However, F-35 test director Air Force Col. Varun Puri documented concerns in a September 2018 presentation that the test phase could slip until September 2019, which could add budget pressure to the program.

In a statement, Lockheed Martin expressed confidence in its ability to ramp up production over the next few years.