As predicted, the service's latest tanker will miss its projected September deadline, said Air Force spokeswoman Capt. Emily Grabowski.
"The top issues slowing progress are achieving the [Federal Aviation Administration] airworthiness certifications and completing the flight test program," she said in an email. "Once Boeing receives the remaining design approvals from the FAA, they expect testing to proceed on a faster pace."
Grabowski said Boeing believes it can deliver an aircraft by December and establish a set of equipment and procedures for airmen by October 2018, as planned.
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The delivery push comes as no surprise after the Government Accountability Office, the investigative arm of Congress, issued a report in March disclosing that testing on the tanker might further delay the program.
"GAO's analysis shows there is risk to the current delivery schedule due to potential delays in Federal Aviation Administration certifications and key test events," the report's analysis reads.
"Boeing must also complete over 1,700 test points on average for each month from February to September 2017, a level that is more than double what it completed in the last 11 months," it said.
Boeing was supposed to deliver the initial 18 aerial-refueling planes by August 2017. With ongoing snafus throughout the program -- which began in 2013 -- such as initial design problems with its refueling boom, Boeing and Air Force officials earlier said the first delivery could be delayed to February 2018.
The GAO noted it was more likely to be October 2018 -- if that.
Lt. Gen. Arnold Bunch, the Air Force's military deputy for the Office of the Assistant Secretary for Acquisition at the Pentagon, told reporters Wednesday that he isn't frustrated with the setback.
"Well, I don't like delays," he said after a House Armed Services Committee hearing on Capitol Hill. "Boeing is throwing resources and doing everything they can to pull it back; they're doing everything they can to try to execute. That's the part I'm very upbeat about.
"We're all for that. We would welcome that, and we will provide them the resources we can to make that happen," Bunch said.
The Air Force awarded Boeing a fixed-price $4.9 billion contract in 2011. The company is responsible for any cost overruns.