WASHINGTON — Boeing won’t be able to deliver the first KC-46 refueling tanker to the U.S. Air Force until the end of 2018, the service found during its latest schedule review.

That could make it very difficult for the company to meet a contractual obligation to deliver 18 certified KC-46s and nine refueling pods by October 2018 — which the Air Force now estimates will occur in late spring 2019, service spokeswoman Ann Stefanek told Defense News on Wednesday.

Should Boeing miss that “required assets available” deadline, also known as RAA, it would be subject to additional financial penalties.

The Air Force based its new estimate of the first KC-46 delivery date on a recently completed schedule risk assessment completed by it and Boeing, the service said in a statement Tuesday. While the program will continue to execute to Boeing’s plans, which calls for first delivery sometime this summer, the service believes that schedule is overly optimistic.

“The Air Force assesses first aircraft delivery is more likely to occur in late calendar year 2018. This assessment is based on known risks and predicted impacts associated with airworthiness certifications and slower than expected flight test execution. The Air Force will continue to work with Boeing to develop schedule mitigations, where appropriate, to expedite the program,” the service said.

Boeing spokesman Chick Ramey told Defense News that it is still the company’s intention to deliver 18 KC-46 tankers by the end of 2018.

“We worked closely with the U.S. Air Force on the updated KC-46 schedule risk assessment and discussed a range of delivery dates,” he said in a statement. “While there is always risk on any development program, we’re relying on our partnership with the Air Force to help mitigate those risks, complete KC-46 testing and deliver 18 game-changing tankers to them as quickly as possible.”

The Air Force noted that “these potential delays will not result in additional program cost to the taxpayer” because Boeing agreed to a firm, fixed-price contract where the company is responsible for any costs over $4.9 billion.

Boeing has already had to pay $2.9 billion pretax, or about $1.9 billion after tax, due to schedule delays and cost overruns on the program — including missing its original August 2017 RAA date.

The Air Force’s statement marks the second time the service has publicly rebuked Boeing’s proposed KC-46 delivery schedule for being overly rosy.

Last spring, the service announced that it no longer believed the company would be able to deliver the first KC-46 tanker by the end of the year. Boeing maintained that it would be able to do so until December, when Leanne Caret, the top executive for the company’s defense sector, told Defense News that it would miss the 2017 goal.

Late last year, there were signs the delivery schedule was slipping even further. In December, Lt .Gen. Arnold Bunch, the service’s top uniformed acquisition official, told Defense News that the Air Force had expected to accept the first KC-46 in March 2018, but now “we think it may be a little later than that.”

The Air Force plans to buy 179 KC-46s over its program of record. Boeing finalized its first international KC-46 sale to Japan last year.

Updated 3/7/2018 at 4:53 EST with additional comment from the Air Force.