Just weeks before the planned first flight of a fully outfitted KC-46 Air Force tanker, the plane’s fueling system has been damaged by a chemical mix-up, temporarily grounding the jet. Meanwhile, Boeing put veteran executive Scott Fancher in charge of the troubled tanker program.

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In a new setback just weeks before the planned first flight of a fully outfitted KC-46 Air Force tanker, the Boeing plane’s fueling system has been damaged by a chemical mix-up, temporarily grounding the jet.

The jet — the first test plane outfitted with working air-refueling systems and designated as a tanker — was at the fuel dock on Paine Field last week when mechanics used the wrong chemical during a test of the fuel system, according to people familiar with the details.

The chemical, supplied by a vendor and mislabeled, caused corrosion and damaged the fuel system, including the advanced new fuel boom designed to offload gas to fighter aircraft, the sources said.

The fuel boom as well as the auxiliary fuel tanks in the fuselage of the airplane have been removed from the aircraft for inspection and any necessary repairs. The Air Force has been informed of the incident.

The auxiliary tanks appear to be undamaged, one source said.

Boeing spokesman Chick Ramey would not provide details but confirmed Boeing is “determining a plan of action” for “an emergent KC-46 issue.”

“We are currently assessing the potential impact of the issue on scheduled program activities,” Ramey said.

It’s unclear how much this incident will delay the plane’s first flight, which was expected in late August or early September.

Boeing remains committed to delivering the first 18 air-refueling tankers to the Air Force by August 2017. Before this latest setback, the company acknowledged that schedule was extremely tight with no room for further delays.

The chemical damage to the fuel system is the latest in a line of mishaps that began when the four test tanker planes were incorrectly wired last year.

Then, within the past two months, the fuel system ruptured during a pressure test; that same plane was in the latest incident.

The previous events, in addition to delays caused by the slow supply of parts, have resulted in enormous rework for which Boeing took large accounting charges: $425 million last year and $835 million earlier this month, for a total $1.26 billion write-off.

In a message to employees sent Tuesday, Boeing Commercial Airplanes chief Ray Conner and his defense side counterpart Chris Chadwick reiterated the 2017 initial delivery timeline and emphasized that “we must keep our promise to deliver — on schedule.”

That message followed a memo the previous day from new Boeing CEO Dennis Muilenburg, announcing the appointment of Senior Vice President Scott Fancher to take charge of the tanker program.

This new incident, coming fast on the heels of the latest write-off, may have been the final straw that prompted that decision.

Muilenburg said he’s given Fancher “a special interim assignment” to “return this program to successful execution.”

Fancher is senior vice president for airplane development, responsible for overseeing all Boeing’s new airplane programs — the 737 MAX, 777X and 787-10 in addition to the tanker.

Muilenburg clearly has decided the troubled tanker program needs more of his focus.

Fancher is credited with finally getting the 787 Dreamliner program on track in an earlier assignment.

After taking over that jet program in December 2008, when it was nearly two years late, he had to work through major new problems and further delays. A design flaw was discovered in the wing-to-body join; quality issues surfaced in the horizontal tails built in Italy; and a flight-test airplane suffered a major electrical fire in the air.

But after just over three years heading the program, Fancher delivered the first Dreamliners into service with airlines.

Muilenburg’s memo said Fancher’s new appointment will “add to his role,” so he’s still nominally in charge of all Boeing’s new planes.

But with Fancher spending more time on the tanker effort, Muilenburg also appointed Bob Feldmann, head of the 777X program, to serve as Fancher’s interim deputy to help oversee all the other jet-development programs.

Tim Peters remains as head of the tanker program, now overseen by Fancher.

According to the memo, Fancher will also work closely with Jim O’Neill, who heads new program development on Boeing’s defense side, and with Pat Dolan, who oversees the company’s lean-manufacturing efforts.