EVERETT — Boeing is adding a sixth test aircraft to avoid further delays to its new aerial refueling tanker program. A sixth test airplane is slated to start flying by late April, a company official said.

The company’s Everett-based KC-46 program is already a year behind schedule and is at risk of slipping further, according to a recent federal report.

Boeing’s current plan to start delivering KC-46 tankers in September requires the company to complete remaining tests twice as fast as the current pace and deliver airplanes faster than previously planned, according to a Government Accountability Office, or GAO, report from late March.

Also, the Air Force and Boeing are negotiating changes to their contract. Boeing could have to cover the Air Force’s additional costs caused by the late deliveries, according to the report.

The company already has covered about $1.5 billion in cost overruns on the development program so far, including $243 million before taxes announced in April 2016.

While the aerospace giant has spent more than anticipated to develop a military tanker based on its popular 767 commercial jet, the overall cost to taxpayers has steadily declined since Boeing won the Air Force contract in 2011. The deal to develop and deliver 179 tankers was expected to cost $51.7 billion in 2011. That has dropped 14 percent to $44.4 billion this year, according to the report.

The contract includes a fixed-price deal worth $4.9 billion for developing a KC-46 tanker with the ability to handle multiple combat and support missions, and options for the U.S. military to buy production versions of the tanker. The Air Force ordered the KC-46 to upgrade its aging tanker fleet, which largely consists of 1950s-era KC-135 tankers.

Boeing agreed to deliver the first batch of 18 combat-ready airplanes by August 2017. Production and design problems have delayed development. The company plans to start delivering three tankers a month in September 2017. Complete delivery of the first batch would be by February 2018, the company says.

“We’re continuing to build aircraft on pace in the factory and currently have more than 20 aircraft in the production flow,” which Boeing streamlined last year to increase output, company spokesman Chick Ramey said.

However, it will take several more months to deliver refueling pods hung on the tankers’ wings. The pods can extend a hose for an aircraft needing to take on fuel in flight. With a pod attached to each wing, the KC-46 can refuel two aircraft at a time. It also has a hose and boom — essentially a giant straw — that can extend from its fuselage.

The wing pods will not be delivered until October 2018, according to the GAO report.

The test airplanes are meeting or beating the Air Force’s performance requirements so far, according to the report and a Boeing spokesman.

The KC-46s are consuming less fuel than expected, and so far have needed less maintenance work than anticipated. Boeing expects that even after the test planes add flight hours, the aircraft will still need less replacement parts and other maintenance than as required by the military, according to the report.

“Regarding flight test, we continue to make steady progress,” Ramey said.

Boeing believes the GAO is overestimating how long finishing tests will take, he said.

The company and Air Force are looking to eliminate unnecessary tests or rearrange test schedules to move things along more quickly.

“The test team expects to be much more efficient during the second half of testing now that the design has stabilized,” Ramey said. “We’re also shortly going to be adding a sixth aircraft to the flight test program, which will help complete additional ground and flight” tests.

Dan Catchpole: 425-339-3454; dcatchpole @heraldnet.com; Twitter: @dcatchpole.