First flight of the US Air Force’s KC-46 aerial refueling tanker has been delayed by several months to April 2015 amid issues relating to redundant wiring in the modified Boeing 767’s forward hold.

Maj Gen John Thompson says rewiring the first four test aircraft and reaching first flight within the next six months is critical to reaching a milestone C decision to move into low rate initial production (LRIP) on time. The contractual date for Boeing to deliver the first 18 KC-46s is set for August 2017.

Boeing earlier this year alerted the air force to “anomalies” in the aircraft’s wiring, which is required to be triple redundant to meet military and US Federal Aviation Administration specifications.

The company launched a wiring audit that found about 5% of the aircraft’s 98,000 wiring bundles were installed too close to redundant counterparts. For safety reasons, military specifications require that redundant systems are installed at a physical distance from one another inside the airframe.

Of 1,700 wiring bundles, about 350 have to be removed and reinstalled because redundant systems were installed in the same bundles, Thompson says. The aircraft is strung with about 120 miles of wiring. Because the K-46 is derived from a commercial aircraft, it was evaluated by FAA designated engineering representatives, as well as air force inspectors. For that reason, the wiring problems were identified earlier in the development process that they otherwise would have, he says.

“This is not a big performance issue,” Thompson says. “This is a compliance issue and it is something that I’d rather deal with earlier in the programme than later in the programme. It is a production issue that requires minor redesign. The United States government is not panicked about it. We’re disappointed about it because we are eager to get it into flight test.”

Having to remanufacture and install the bundles has “chewed into” some of the margin that Boeing and the air force have built into the engineering and manufacturing development (EMD) process, Thompson says.

A provisional test 767-2C, a freighter variant of the aircraft, and the first EMD KC-46 are scheduled to fly in the first quarter of calendar year 2015. That is essential for meeting milestone C on time, he says.

The first of four EMD aircraft is 96% complete and requires only that its wiring bundles be reinstalled. The second aircraft is about 75% complete, while the third and fourth are just over 60% finished, Thompson says.