This article, originally published at 11:09 A.M. April 1, 2016, has been updated to include a statement from Boeing. 

WASHINGTON — An issue that prevented Boeing's KC-46 tanker from transferring fuel to a US Air Force C-17 during a recent test could delay the start of production of the new aircraft, according to the Air Force.

During a recent test of the tanker's refueling boom, higher than expected axial loads prevented the transfer of fuel from the KC-46 to a C-17 transport plane, service spokesman Daryl Mayer said April 1. The boom, a rigid, telescoping tube that an operator on the tanker extends to and inserts into a receptacle on the receiving aircraft, is used to refuel most Air Force aircraft.

Boeing has a "good understanding" of the problem and is working to identify a fix, Mayer said. However, the Air Force does not yet know the schedule impact to a planned "Milestone C" decision to formally approve production of the new tanker in May, he said.

"We don't yet know the schedule impact to the planned May Milestone C decision, but the problem is well understood and we don't expect an extended delay," Mayer said.

The KC-46 successfully refueled an Air Force F-16, a much lighter aircraft than the C-17, in a previous test, Mayer noted. The tanker has also successfully transferred fuel to a Navy F/A-18 and Marine Corps Harrier, which use the aircraft's hose-and-drogue system for refueling, he said.

Boeing spokesman William Barksdale declined to say how much the fix will cost, saying the company is "aggressively working the problem now."

"We expected to find items like this in development test and we are evaluating system changes to improve boom response," Barksdale said April 1. "Over the coming weeks, we will have a better understanding of program impacts, if any. We continue to make steady progress in flight test and aircraft production, and believe we are taking the right steps to fulfill our commitments to the Air Force."

The Air Force is planning to buy 179 KC-46 tankers to recapitalize its aging tanker fleet. Boeing's KC-46 is unique in its ability to switch between the boom and hose-and-drogue during the same mission, allowing the Air Force to refuel more aircraft more quickly.

Boeing is on tap to deliver 18 full-up KC-46s by August 2017.

Email: lseligman@defensenews.com

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