WASHINGTON — Production mistakes and quality problems with Boeing’s F-15EX Eagle II program have caused the fighter’s delivery schedule to slip by at least six months, which could endanger its ability to meet key deadlines, the U.S. Government Accountability Office said in a new report.

Boeing originally expected to start delivering its latest batch of F-15EX Eagle II fighters to the Air Force in December 2022, the federal watchdog wrote in its annual assessment of weapon systems, released Thursday.

Several production problems delayed the delivery of those six fighters in lot 1B, GAO said. Those delays were mainly caused by what office referred to as “supplier quality problems related to a critical component in the forward fuselage assembly that ensures safety of flight.” The report did not provide further details on that component, but said quality problems were fixed by the time Boeing built the seventh and eighth F-15EXs.

Boeing also mis-drilled windscreen installation holes on four F-15EXs in this lot because the company used tooling with a design error, GAO said. Program officials told auditors the problem was caught before more planes were mis-drilled, and that Boeing will redrill the holes on affected aircraft before starting production on the second lot of fighters.

Boeing declined to comment to Defense News on the quality issues highlighted in GAO’s report, and referred questions to the Air Force. Defense News has reached out to the service for comment.

Boeing spokeswoman Deborah VanNierop confirmed Friday that the only F-15EXs so far delivered to the Air Force are the two test aircraft that were delivered in spring 2021, which were considered lot 1A.

More than two years later, the service is still waiting for the next batch of fighters.

The F-15EX is an upgraded version of the fourth-generation Eagle fighter, with advanced avionics such as fly-by-wire controls and improved electronic warfare capabilities.

But those problems are having ripple effects on the F-15EX program, GAO said. Each lot 2 fighter is now delayed by two months as a result of the earlier lot’s problems, the report read, and delivery schedules are in danger of slipping further.

The report noted that program officials thought Boeing could deliver the six F-15EXs in that lot between May and July, with two delivered per month. But Boeing and the federal Defense Contract Management Agency warned that more delays could occur, GAO said.

Boeing’s analysis predicted it would be unable to deliver the first fighter in this batch until July, and the second in August.

The Defense Contract Management Agency concluded the last deliveries in the lot would probably not happen until September because of the problems that cropped up so far.

GAO said that if delivery of these planes is delayed beyond July, it will be tough to meet planned deadlines in 2023, including the declaration of initial operational capability in July and full-rate production in October.

Boeing referred Defense News’ questions on the fighters’ schedule to the Air Force.

GAO also warned that cybersecurity vulnerabilities remain the F-15EX’s primary vulnerability. The fighter’s design was derived from versions of the F-15 that were sold to foreign militaries, GAO said, and weren’t designed to meet the Air Force’s own cybersecurity requirements.

Program officials told GAO that it is working through the Defense Department’s six-phase process for assessing cybersecurity vulnerabilities on the F-15EX, with the first four either already done or expected to be finished in early 2023. The program expected to finish the final two phases on the aircraft delivered in lot 1B.

The Air Force is now planning to buy 104 F-15EXs, and requested money to buy 24 of the fighters in the proposed fiscal 2024 budget.

The Air Force last year moved to scale back its F-15EX procurement in the FY23 budget, from the original 144 to 80. This was intended to free up funds for higher priority programs, GAO said.

The watchdog warned that a lack of enough procurement funding for the F-15EX could cause the program to be curtailed slightly below that already reduced 80. A June 2022 cost estimate showed the Air Force wouldn’t have enough money for 80 fighters, and only be able to buy 78, GAO noted.

Air Force Secretary Frank Kendall said in a March budget briefing that the service had decided to partially reverse its decision to cut the F-15EX procurement to 80, bringing it back up to 104.