Boeing will not meet its contractual deadline to deliver 18 KC-46 tankers to the Air Force by August 2017 due to design problems with the plane’s refueling boom.

The delay, announced on Friday afternoon before the Memorial Day holiday weekend in the United States, comes after countless warnings that there was no more margin for error in the project’s schedule due to numerous technical problems that have cropped up over the past four years.

“Technical challenges with [the] boom design and issues with certification of the centerline drogue system and wing air refueling pods have driven delays to low rate production approval and initial aircraft deliveries,” Brig. Gen. Duke Richardson, the program executive officer for tankers, said in a statement.

The new tanker, a modified version of the 767 jetliner, has trouble refueling the massive C-17 cargo plane due to “higher than expected boom axial loads” on the refueling boom, a pipe that lowers from the rear of the aircraft, the Air Force said.

The tanker uses a boom to refuel Air Force planes and hoses that extend from the wings and center body to refuel Navy, Marine Corps and allied aircraft. The plane has successfully refueled smaller F-16 fighters in flight tests.

Four test tankers and one production plane have flown more than 500 hours. Another seven aircraft are in final production and another eight are being built, Boeing said in a statement.

The Chicago-based firm, which builds the tankers in Washington State, was supposed to deliver 18 planes to the Air Force by August 2017. Under the new schedule announced on Friday, it will not deliver all 18 planes until January 2018.

The Air Force was supposed to decide in April whether to buy more planes beyond the first 18. That decision has now been pushed to August. The Air Force plans to buy a total of 179 KC-46 tankers from Boeing.

Despite the delays, the Air Force has structured its contract with Boeing so taxpayers do not have to pay for cost increases. To date, the company has had to eat $1.3 billion, the latest $156 million “charge” being announced in April.

The latest charge “primarily reflects the cost [of] incorporating engineering changes identified during testing into aircraft already built and in production along with the certification of those changes,” Dennis Muilenburg, the firm’s chairman, president and CEO, said during the company’s quarterly earnings call in April.

At the same time, the Airbus KC-30, the plane that the KC-46 beat in the Air Force tanker competition in 2012, has deployed to the skies over Iraq to refuel coalition aircraft bombing Islamic State militants. The Royal Australian Air Force deployed a KC-30 tanker, a plane based on the A330 jetliner, in 2014.

The KC-46 is replacing the KC-135, a plane that has been around since the 1950s.