The Air Force still expects Boeing [BA] to deliver the first 18 combat-ready KC-46A aerial refueling tankers by August 2017 despite concerns about higher than expected loads on the boom during refueling, the service’s interim head of acquisition said Tuesday.

Sen. Tom Cotton (R-Ark.) asked Darlene Costello, the acting assistant secretary of the Air Force for acquisition, how the issue would affect the KC-46’s delivery and testing schedule.

“We are currently working with the contractor in order to assess that,” she replied during a Senate Armed Services airland subcommittee hearing. Boeing is slated to provide fixes and analysis during an upcoming review, but “it will be a couple of weeks before we know what the recommended solution is.”

Immediately preceding the question about the boom issue, Costello said Boeing will be ready to meet the August 2017 delivery deadline in spite of schedule delays to the aircraft’s first flight and Milestone C decision, the latter of which is currently scheduled for May.

“Boeing stands by their position that they can deliver that, and the Air Force is cautiously optimistic that we can in fact achieve that,” she said.

After the hearing, a company spokesman confirmed that Boeing is looking into the cause of the issue.

“During testing, loads placed on the boom by receiver aircraft after contact were higher than expected,” Boeing spokesman Chick Ramey said in a statement. “We expected to find items like this in development test, and we are evaluating system changes to improve boom response. Over the coming weeks, we will have a better understanding of program impacts, if any.”

Boeing is under a fixed-price contract stipulating that it is responsible for any cost during the engineering and manufacturing development phase that exceeds the $4.9 billion contract value. The company already racked up a $536 million charge in 2015 due to problems with the tanker’s integrated fuel system. Further issues could affect the program’s schedule as well as Boeing’s bottom line.

During discussions with reporters traveling with the defense secretary on March 3, Mike Hafer, the company’s lead for KC-46 global sales and marketing, reiterated that Boeing was “on track” to deliver the first 18 aircraft on schedule. Defense Secretary Ashton Carter checked out the Air Force’s second KC-46A aerial refueling tanker during a March 3 trip to Boeing Field in Seattle.

The aircraft, called EMD-4, made its inaugural flight the previous day, taking off from Paine Field and landing at Boeing Field on March 2. Boeing will use the EMD-4 as well as the first KC-46A plane, EMD-2, to complete testing needed for the Air Force to reach a Milestone C decision.

Since the inaugural flight of the KC-46 in September, Boeing has used EMD-2 to conduct more than 180 hours of flight tests, including the refueling of three of the five aircraft needed to meet Milestone C requirements: the F-16, F/A-18 and AV-8B Harrier. It has also demonstrated that the tanker can receive fuel from the KC-10, Hafer said.

The refueling tests for the final Milestone C planes, the A-10 and C-17, have not been scheduled yet, Hafer said. Ultimately, the KC-46 will refuel 18 different aircraft as part of the test program.

The company will initially conduct mission system avionics and exterior lighting on EMD-4 before using it for aerial refueling tests and receiver certification, Col. John Newberry, Air Force KC-46 program manager, said in a statement.

The new tanker can refuel Air Force airplanes with its 56-foot boom, a rigid tube that can deliver 1,200 gallons of fuel per minute, Hafer said. The boom has been remodeled from the KC-10 version with new fly-by-wire technology.

For Marine Corps and Navy aircraft, the KC-46 employs a “probe and drogue” system that uses a flexible hose that dangles from the tanker and is outfitted with a small basket at the end that allows it to funnel gas into the receiving aircraft’s probe. The KC-46 has a 75-foot centerline drogue that can pump 400 gallons of gas a minute into a receiving aircraft, he said. Alternatively, the tanker can use two removable drogue on its wing tips to refuel up to two aircraft simultaneously.

The service plans to procure a total 179 aircraft by 2027.