The Air Force said Friday that Boeing’s KC-46 tanker program, already late and over budget, faces a further serious delay of five months for initial tanker deliveries — and an additional nine months beyond that to achieve fully operational tankers.

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The Air Force said Friday that Boeing’s initial KC-46 air-to-air refueling tankers will be delivered five months later than the contractual deadline — and they’ll not be fully operational until nine months after that.

Brigadier General Duke Richardson, who heads the Air Force tanker program, attributed the delays to “technical challenges.”

The 767-based tanker program, in total worth more than $40 billion to Boeing, had already suffered multiple setbacks that delayed flight testing, ate up all buffer in the schedule and sent costs soaring.

The latest delay finally knocks out Boeing’s contract target of delivering the first 18 tankers to the Air Force by August next year. The delays also point to the likelihood of further cost overruns that would add to Boeing’s previous write-offs on the program, already totaling almost $1.6 billion.

Boeing and the Air Force now say the first tanker delivery will move from March to August next year, and the Air Force now anticipates receiving its 18th tanker in January 2018.

However, those first tankers will be incomplete. They won’t have the capability to dispense fuel to two aircraft at once using a hose-and-drogue system attached to pods on the wings.

The Air Force said Boeing anticipates delivering the wing aerial refueling pod capability by October 2018.

The tankers refuel some aircraft, such as the C-17 cargo plane or the F-16 fighter, using a rigid boom extending from the underside of the aft fuselage.

Other aircraft, such as the F/A-18 fighter, are refueled through flexible hoses that trail either from the fuselage or from pods on the wings of the tanker.

At the end of these hoses, a basketlike device called a drogue helps stabilize the hose in flight and acts as a funnel through which the receiving aircraft connects a probe to the hose.

If the wing pods are used, two aircraft can be refueled at once; otherwise, it’s one at a time.

The initial 18 aircraft delivered will have the capability to dispense fuel through the boom and through the fuselage drogue system, but not through the wing pods.

Boeing spokesman Chick Ramey said the Air Force doesn’t require all of its tankers to have wing pods and that only half of the initial 18 aircraft are intended to have that capability.

So in October 2018, Boeing will deliver nine wing-pod sets, he said.

In an Air Force statement, Richardson acknowledged the latest delays but added, “We understand that no major procurement program is without challenges … The Air Force considers the KC-46 a critical capability and it’s important to take the time necessary to get it right.”

Defense analyst Rebecca Grant of IRIS Independent Research said she’s “surprised at the magnitude of the slip.”

“It makes you wonder what are their plans that give them the confidence for January 2018,” she said.

While Boeing recently publicly admitted a problem with high stress loads on the refueling boom when the tanker connects with a C-17, it had not shared the fact that development of the wing refueling pods is also problematic.

Ramey said the hose-and-drogue refueling systems, both on the wings and on the fuselage, have performed as expected in flight tests, but “It’s taken longer than we anticipated to get through qualification and certification” of those systems.

In addition to the delay in initial deliveries, the Air Force said it will also push out its formal go-ahead to Boeing to build production tankers, from June to August.

That’s to give Boeing more time to develop a software fix for the refueling-boom stress problem.

Last month, Boeing took a $313 million pretax write-off on the tanker program due to cost overruns from incorporating and certifying engineering changes necessary as a result of problems that have surfaced during flight tests — including the boom stress issue.

Earlier delays in the tanker program resulted in previous write-offs in 2014 and 2015 totaling $1.3 billion.

The U.S. Government Accountability Office (GAO) estimates that the Air Force will ultimately pay Boeing $43.7 billion to deliver 179 aircraft.

Boeing projects that with additional international sales, it could sell as many as 400 tankers in all.

The jet maker has four tankers currently in flight test and eight more production tankers under final assembly in Everett.

The delay news upends recent pronouncements from Boeing Chairman and Chief Executive Dennis Muilenburg. A month ago, on an earnings call discussing the latest tanker write-off, he insisted Boeing was sticking to its original schedule commitment.

And on May 11, at Boeing’s annual investor conference in Bellevue, Muilenburg reiterated the point: “Delivering 18 airplanes by next August is clearly on target.”

The new delay almost certainly means further cost overruns beyond what was projected last quarter and likely another write-off ahead. On that question, Boeing was noncommittal Friday.

“Any potential financial impact on the program from these changes will be assessed as part of our second-quarter closing process,” Ramey said.

The Pentagon structured the tanker contract so that Boeing is responsible for cost overruns above $4.9 billion in this initial development phase. As demonstrated by the write-offs to date, Boeing has long since blown through that ceiling.

Gen. Richardson offered the reassurance to taxpayers that “There is no increased cost to the government as a result of these changes.”

Information in this article, originally published May 27, 2016, was corrected May 28, 2016. A previous version of this story understated the previous write-offs on the program. Boeing last month wrote off $313 million due to cost overruns, not $243 million, bringing the total written off since 2014 to almost $1.6 billion.