WASHINGTON — Despite the latest schedule slip and yet another pretax charge for cost overruns, Boeing is charging ahead with testing on its KC-46 tanker program.

During a January test of one of the tanker's refueling systems, the KC-46 was unable to transfer fuel to a C-17 transport plane, Defense News reported in April. The problem was caused by load issues with the refueling "boom," a rigid, telescoping tube that an operator on the tanker extends to and inserts into a receptacle on the receiving aircraft. The boom system is used to refuel most Air Force aircraft.

"This development set the need for Boeing to make changes to how quickly the boom responds to changing loads between the tanker and receiver aircraft during flight," Air Force spokesman Daryl Mayer said Wednesday.  "We expect to find items like this in development test."

Boeing began flight testing a software fix for the issue in mid-April, Mayer said. That testing is ongoing, he added.

A "Milestone C" review to formally approve production of the new tanker is expected in June, once the test team verifies that the software fix works as planned, Mayer said. This is a delay of about a month; a production decision was originally planned for April or May.

Meanwhile, Boeing rang up a $243 million pretax charge on the KC-46, bringing the total amount Boeing has paid for tanker-related cost overruns to $1.5 billion, the company reported in April.

The Air Force is planning to buy 179 KC-46 tankers to recapitalize its aging tanker fleet. Boeing is on tap to deliver 18 full-up KC-46s by August 2017.

Twitter: @laraseligman