WASHINGTON — Meetings between Boeing and the U.S. Air Force on the KC-46’s schedule appear to have stagnated, with both parties still at odds about when the first tanker will be delivered.

“We have had meetings with Boeing, and additional meetings last week to get an agreement on a schedule,” Air Force Secretary Heather Wilson told the Senate Armed Services Committee on Tuesday.

“We believe — the Air Force believes — that the schedule that Boeing has is still overly ambitious, and we’d like to get agreement on a delivery date and drive to that delivery date.”

Last month, the Air Force projected that the first KC-46 delivery would be delayed yet again — an assessment with which Boeing has vehemently disagreed.

Based on the service’s estimates, Boeing may be able to deliver the first KC-46 aircraft by the end of the year, with a contractually required delivery of 18 tankers slipping from October to spring 2019. But Boeing asserts it will be able to deliver the first aircraft this summer, with a total of 18 tankers delivered by the end of the year.

“We are working with the Air Force to complete all the required testing and are committed to delivering the first tanker as soon as possible,” a Boeing spokesman said in a statement. “Discussions are ongoing as to a specific delivery date.”

Missing the October deadline for required assets available, or RAA — in this case, 18 certified KC-46As and nine refueling pods — could trigger another round of penalties for Boeing, which is locked into a fixed-price contract that leaves the company financially responsible for schedule delays and cost overruns. Boeing has already had to pay about $2 billion post-tax out of its own pocket.

The Government Accountability Office estimates that RAA “could slip to May 2019, 21 months from the original schedule, if risks are not mitigated.” The agency’s findings were in an report on the program released this month.

Those risk factors include ensuring all aircraft are in the correct configuration and fixing a number of deficiencies, including one that involves the boom scratching the receiver aircraft. Boeing will also have to complete 6,550 test points in a window of February 2018 to June 2018 to meet its schedule projection, which GAO says amounts to a rate of double its current pace.

Addressing the boom-scraping issue, a Boeing spokesman said the company had begun flight testing a software enhancement that is expected to improve visibility.

Despite the schedule issues, lawmakers have generally been supportive of the program.

Oklahoma’s Jim Inhofe, currently the top Republican on SASC, told Wilson that the committee was “anxious for the first KC-46 to get delivered” to the service.

The fiscal 2019 budget requested an additional 15 tankers, he added. “Do you agree that is an adequate number, an achievable number and a desirable number?”

Wilson said that it was.

Air Force Chief of Staff Gen. Dave Goldfein added that the 15-aircraft-per-year rate was the best way to sustain production as the weapon system comes online.

“We are a global power because of our global reach, and it’s all of the services that rely on that tanker force, and our allies and partners, to be able to project power globally. So it’s a critical capability that we need to bring on as fast we can bring it on,” he said.