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Boeing engineers and mechanics are scrambling to meet an already stretched-out schedule for the first flight of the 767 jet that will become the Air Force’s new KC-46 refueling tanker.

The first prototype plane, a modified commercial aircraft that lacks the refueling systems to make it a tanker, is now expected to fly just days before year end — six months later than projected last January.

And the cost of the effort is mounting steeply for Boeing, which is responsible for cost overruns above a contract ceiling of $4.9 billion in this initial development phase.

The government’s latest projection for the cost of the tanker’s development has ballooned to $1.5 billion above that contract ceiling, Brig. Gen. Duke Richardson, who heads the Air Force tanker program, said Monday.

The Air Force’s previous estimate had been for a $1 billion overrun.

Richardson said in an interview that the new estimate is based on Boeing’s performance on the work completed thus far, factors in manufacturing delays due to wiring issues this year, and potential risks ahead, including possible surprises once flight tests begin.

Boeing spokesman Chick Ramey declined to comment on the Air Force’s cost figure. He said the tanker team is “aggressively working our plans to mitigate risk and lower costs.”

He noted that because it’s a fixed-price contract, “there won’t be any additional costs to the U.S. government.”

Two Boeing insiders with knowledge of the program said the tanker team in Everett is working feverishly to resolve remaining systems problems and is under orders to prep the first plane “with the minimum capability to make it fly.”

Richardson indicated the Air Force’s priorities are much the same. He said Boeing’s target date for first flight is Dec. 27 or 28.

“At this point we need Boeing to get Number One in the air,” he said.

Wiring redesign

The upcoming flight from Paine Field is the first public milestone for the program. Boeing’s contract calls for it to receive $51 billion for delivering 179 tankers to the Air Force.

The plane’s takeoff will mark the beginning of flight tests that eventually will involve four test aircraft.

The first plane is not outfitted with the military systems that would make it a tanker, such as the air-to-air refueling boom.

It’s just the basic airplane platform, designated a 767-2C model. This is a modified commercial 767 with a 787-style cockpit, a strengthened airframe, four extra fuel tanks in the cargo bay, and the plumbing and wiring to support the tanker mission.

The run-up to first flight hasn’t gone smoothly.

After the airframes for all four test aircraft were completed this year, Boeing had to repeatedly remove and reinstall complex wiring systems in the first airplane.

Richardson said the wiring had to be redesigned because the various redundant wire bundles that independently control critical systems were not sufficiently separated.

Once that was corrected, he said, Boeing had to further adjust the design so that the wires would still physically fit into the various bends and crevices in the airframe.

The painstaking unwiring and then rewiring of the first airplane delayed this initial flight by monthsand added an extra $425 million in unplanned expenses to the cost overrun that Boeing must swallow.

Because of the delays, Boeing has committed to submit a detailed revised schedule to the Air Force in February.

Even if the cost overrun is as high as the government forecasts, Boeing could potentially make up for the loss in the later stages of the program.

Dennis Muilenburg, Boeing vice chairman and chief operating officer, said earlier this month that the company anticipates follow-on orders from both the Air Force and international customers such as South Korea.

He said Boeing expects to build in total “probably closer to 400 to 500 aircraft.”

Among the fairly routine issues being tackled this week is a problem in starting the engines using the Auxiliary Power Unit in the tail of the airplane.

A person with knowledge of the details said a fix is ready.

However, some lower-priority matters that don’t affect the safety of flight will be left until later.

Richardson said that some 23 less-critical system tests won’t be performed until after first flight.

That’s because crucial parts of the Federal Aviation Administration (FAA) certification process cannot proceed until flight tests begin.

“Reducing the risk on the airworthiness certification paperwork is more important than finishing some of the jobs that could have been done,” said Richardson.

Little margin left

Despite the delays to first flight, Boeing continues to insist that it remains on track to fulfill its contract requirement of having 18 operational tankers built and ready to deploy by late 2017.

Richardson declared himself “pretty confident” that the 767-2C will fly by month-end.

The next milestone after that, scheduled for April, is the first flight of the second airplane to roll off the line: the first to be fully equipped with air-to-air refueling systems and therefore the first real KC-46 tanker.

Richardson said that second plane has more wiring than the base airplane, and its wiring is now 78 percent complete.

He said he feels “very good” about the wiring fix on the first plane and will feel “really, really good” once the wiring on the second is completed.

Two of the four test planes will be baseline 767-2Cs to be used for achieving FAA certification.

The other two will be fully equipped KC-46 tankers and will be used to test the military systems and to certify that the tanker is ready to refuel a variety of receiver airplanes.

The KC-46 design includes an advanced refueling boom that can be hooked up to a jet fighter by an operator sitting at a station behind the tanker’s cockpit.

Using a 3D video display, the boom operator will navigate the telescopic tip of the refueling tube toward the receiving fighter’s fuel receptacle.

Integration of the complex software systems that control this military hardware is a major challenge.

One potential risk ahead, according to Richardson, is the possibility of finding problems in controlling the boom in flight once the KC-46 flight tests start.

Next summer, after the FAA completes initial flight-test inspections, Boeing will have to demonstrate the ability to pass fuel in flight to a variety of fighter aircraft.

Only if that’s successful will the Pentagon in September give the go-ahead for Boeing to begin building the first production tankers.

At this stage, any further delays or unexpected problems could push that target date out.

“Most of the margin … is gone” said Richardson.

To keep on track for 18 tankers ready to deploy in 2017, Boeing must hand over the first tanker for the Air Force to test and evaluate in the fall of 2016.

Yet despite the program glitches this year, Muilenburg said Boeing management is “feeling very good about where that program is at now.”

“We’ve got some of those technical issues behind us,” Muilenburg said Dec. 3 at an investor conference in New York. “We’ll now focus on executing the flight-test program under development, and then getting the program into production.”

Dominic Gates: 206-464-2963 or