Suppliers of the Air Force tanker’s aerial-refueling boom and wing pods are substantially behind schedule, according to a GAO report that sheds light on the latest delay to the Boeing program.

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Suppliers to Boeing’s KC-46 Air Force tanker program have been running substantially behind schedule, in part due to design changes by Boeing, according to a newly released report by the Government Accountability Office (GAO).

Boeing also has experienced delays in integrating the military-systems software.

“The (aerial refueling) boom that was to be installed on the first KC-46 has been delayed by eight months due to design changes and late parts deliveries,” the GAO report states. “Another supplier has experienced significant delays in manufacturing wing aerial refueling pods for qualification testing and development aircraft due, in part, to challenges with parts delays and engineering design changes.”

The refueling boom is a rigid tube that extends from underneath the tanker and connects to a receptacle in the crown of a refueling aircraft.

The wing pods are an alternative system for offloading fuel, in which a flexible hose is extended backward from a pod on the underside of the wing to the receiving aircraft.

Moog, which makes the refueling boom’s actuators in Torrance, Calif., and Cobham, which makes the wing aerial-refueling pods in Davenport, Iowa, are the key suppliers of those parts.

Earlier this week, the Air Force confirmed that the first flight of a tanker equipped with its refueling systems has slipped again from April, which was already three months later than planned, to sometime before the end of June.

Although the GAO report was written in January before that latest schedule slip, the supply of those key military systems may be a large part of the latest holdup.

Boeing tanker spokesman Chick Ramey said Boeing has designed and built the boom and has been testing it at a facility in Everett.

“We’ve definitely had some delays with parts,” said Ramey.

“But we’ve made significant progress with suppliers,” he added. “We’re working with them to help them improve their processes. We’re sending Boeing people on site, where it makes sense, to help them.”

The GAO report also says the tanker program is struggling with integration of the software that controls the plane’s military systems.

“Although all software has been delivered, Boeing is encountering more than twice the number of software problems than originally estimated,” the GAO report states. “As a result, Boeing delayed the start of the laboratory verification testing needed to demonstrate the defensive and aerial refueling software by more than two months.”

Boeing attributed previous delays to serious wiring issues that required the four test planes to be almost completely rewired after they were built.

“This is a large-scale development program. It’s common to experience challenges,” Ramey said. “Issues come up. Finding and solving those issues before you fly reduces risk.”

The Air Force contract calls for Boeing to receive about $51 billion for delivering 179 tankers to the Air Force.

Brig. Gen. Duke Richardson, program director for the KC-46A Pegasus tanker, said in December that Boeing was already $1.5 billion over budget on the development phase. He also said Boeing had used up “most of the margin” built into the schedule, which requires delivery of the first 18 tankers in 2017.

Ramey said that delivery target has not changed.