The US Air Force expects Boeing to deliver the first of its KC-46A tankers in the "late spring" of 2018, several months after a previously planned date during September 2017.

Following an annual KC-46A schedule risk assessment earlier this week, the USAF determined that slower-than-expected progress in the flight test programme and Boeing’s ability to achieve US Federal Aviation Administration certifications would delay the first delivery by several months. Boeing plans to secure the remaining design approvals in July, the service said on 8 June.

Although Boeing has argued that the USAF never set September as a concrete timeline, the service's military deputy for the assistant secretary of the air force for acquisition says the date was outlined in the programme's schedule after its last re-baselining.

"I’m not frustrated," Lt Gen Arnold Bunch told reporters on 7 June. "Well, I don’t like delays. Boeing is throwing resources and doing everything they can to pull it back, they’re doing everything they can to try to execute, that’s the part I’m very upbeat about."

Boeing believes it could speed up testing after receiving the remaining design approvals from the FAA, and achieve first delivery by December 2017, with required assets available delivered by 2018.

The air force’s official assessment pushes the programme’s schedule further to the right, after also announcing delivery delays last year. Citing complexities associated with qualification and certification of the aircraft’s centreline drogue-and-wing aerial refuelling pod systems, the service projected first delivery would move from March 2017 to August 2017, with the last of an initial 18 aircraft to be handed over in January 2018.

In a May roundtable with reporters outside Seattle, Washington, Boeing officials said it would deliver the first tanker by the end of 2017. The USAF had announced a KC-46A delivery ceremony set for 15 September in a Federal Business Opportunities notice earlier this year, but Boeing's programme manager described this as a planning date.

"There’s uncertainty to when that date is," Mike Gibbons said. "The air force was planning on a potential date that could have been as early as September. It’s likely that we will deliver later than that, probably late this year."

Boeing officials agree with the USAF that risks still exist, but say they are working closely with the service to deliver aircraft.

"Regarding test points, we expect to be much more efficient during the second half of testing now that the design has stabilised," Boeing says. "The majority of developmental testing is complete, which reduces risk to the certification and verification testing. Additionally, we recently added a sixth aircraft [a second low-rate initial production example] to the flight test programme, which will help with test efficiency and improve our ability to complete ground and flight test points going forward."