FILE PHOTO - Boeing's KC-46 aerial refueling tanker conducts receiver compatibility tests with a U.S. Air Force C-17 Globemaster III from Joint Base Lewis-McChord, in Seattle, Washington, U.S., July 12, 2016. Christopher Okula/ U.S. Air Force/Handout via REUTERS/File Photo

(Reuters) - The U.S. Air Force said the first delivery of KC-46 aerial refueling tankers from Boeing Co BA.N, scheduled for the second quarter of 2018, would more likely occur late in the year.

“This assessment is based on known risks and predicted impacts associated with airworthiness certifications and slower than expected flight test execution,” the Air Force said in a statement on Tuesday.

The potential delays will not result in additional program cost to the taxpayer, the statement said.

Boeing previously missed a forecast that the aircraft would be delivered last year.

In a statement on Wednesday, Boeing said: “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 world’s biggest maker of jetliners said it had discussed a range of delivery dates with the Air Force.

In the third quarter of 2017, the company took a $329 million charge for its KC-46 aerial refueling tanker program. Charges related to the program have reduced profit by $1.9 billion after tax.

Boeing expects to build a substantial business from the tanker, which is based on its twin-aisle 767 commercial jet. Boeing plans to deliver 179 tankers to the U.S. Air Force and expects to sell about 400 in total worldwide.