The Air Force's top military acquisition official says Boeing is being challenged by integration of its KC-46A Pegasus tanker, and warned that further slips may imperil key target dates.

"Boeing has struggled, from my past experience, on probably the most challenging part of any program, which is the integration and test piece," Lt. Gen. Ellen Pawlikowski, military deputy for acquisition, told reporters Wednesday morning. "They had the issue with the wiring and that has proven to be more challenging for them to overcome than I think they originally anticipated.

"So we are continuing to see them being challenged by their near-term schedules."

Pawlikowski, speaking at a Defense Writer's Group breakfast event, said the KC-46 program office has been working step by step with Boeing to limit any unexpected roadblocks but noted that much of the margin for delays built into the program's schedule is being eroded.

"The more I'm slipping this out, the tougher it is to meet that date," Pawlikowski noted. "If there are additional challenges going forward, it will be harder to overcome and still hold that [timetable] because you already used the margin that is out there."

The closest milestone for Boeing to hit is getting the EMD test aircraft, a 767-2C without the refueling boom and other tanker equipment, up for its first flight. Pawlikowski said that is still expected to come before the end of the calendar year.

"Boeing remains committed to delivering 18 combat-ready KC-46A tankers to the U.S. Air Force by 2017," spokeswoman Caroline Hutcheson said. "The challenges that Boeing is addressing on its initial engineering and manufacturing development aircraft are leading the company to re-plan elements of its tanker work flow, and its internal schedules, to remain on track to meet our commitment to the Air Force."

The test aircraft was supposed to fly in June, a date that quietly slipped over the summer. Speaking in August, Gen. Mark Welsh, service chief of staff, warned that slipping the EMD plane's first flight into 2015 would be a sign it's time to get "concerned" about the program.

That should be followed by first flight for an actual KC-46A, currently planned for April, all building up to the biggest milestone for the program — the Required Assets Available (RAA) date of August 2017, at which point Boeing must have 18 KC-46 aircraft and all necessary support ready on the ramp.

"As it looks right now, we still believe the RAA date is achievable, but we are continuing to watch as these near-term schedule dates slip, just how much flexibility you have in the long-term date to accommodate that," Pawlikowski said. "And of course, every slip makes it more and more challenging to make the adjustments you need to make."

Based on Boeing's 767-200ER commercial plane, the KC-46 program calls for 179 new planes to replace the aging KC-135 tanker fleet, with production ending in 2027. It is the first step in a planned three-part program to recapitalize the entire tanker fleet, and has been identified as one of the three top priorities for the service.

The general also told reporters that the long-range strike bomber program remains on track to downselect sometime in the spring. Boeing and Lockheed Martin have teamed up to compete against Northrop Grumman on that program.