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Boeing was expected to deliver 18, fully functional KC-46 tankers by August 2017

The last of those 18 planes will not be handed over until January 2018

Atlanta CNN  — 

The U.S. Air Force is going to have to wait six months longer than expected to receive its first batch of KC-46 aerial refueling tankers, another delay for the $40 billion program that is already behind schedule and more than $1 billion over budget.

As part of its contract with the USAF, Boeing was expected to deliver 18, fully functional KC-46 tankers by August 2017.

But now, the last of those 18 planes will not be handed over until January 2018 due to ongoing issues with the aircraft’s refueling systems, the aircraft and satellite manufacturer announced Friday.

The KC-46 Pegasus aircraft are intended to replace Boeing’s KC-135, which first entered service in 1956.

The KC-135s have been flying regular missions as part of the air campaign against ISIS in the Middle East and the 12 KC-135s based in Incirlik, Turkey, have enabled A-10 ground attack planes to regularly conduct operations against ISIS, according to 447th Air Expeditionary Group Commander Air Force Col. Sean McCarthy.

Boeing insists that it will have “more than 18 aircraft through the factory line and in various stages of final change incorporation and certification” by August 2017 in order to meet its new January 2018 delivery deadline.

But the contractor warns that the planes will only be equipped with two refueling capabilities until the wing-aerial refueling pod (WARP) systems are certified and delivered to the Air Force separately in October 2018.

Implementation of the WARP systems is required for the aircraft to complete the contractual milestone.

Each 165-foot KC-46 is estimated to cost approximately $241 million, according to the Government Accountability Office.

The Air Force is seeking to buy 179 total planes and, despite the delays, Air Force officials say the KC-46 program remains a top priority.

The Air Force was supposed to formally decide whether or not it wanted to buy more planes beyond the first 18 in April but has pushed that decision back to August.

“Throughout KC-46 development, the Air Force remained cautiously optimistic that Boeing would quickly address these issues and meet the original goal,” Brig. Gen. Duke Richardson said in a statement. “However, we understand that no major procurement program is without challenges and the Air Force remains committed to ensuring all aircraft are delivered as technically required.”

Under its contract with the Air Force, Boeing, not the U.S. government or taxpayers, is responsible for any additional costs resulting from delays.

CNN’s Ryan Browne contributed to this report.