Additional high-profile problems have cropped up with the Air Force's new tanker, the KC-46 Pegasus.
They add to other issues that have plagued the program, which is being developed by Boeing.
Boeing has already incurred billions in pretax costs because of delays and technical problems.
Boeing's KC-46 tanker program, long waylaid by technical issues, is now facing more "category one" deficiencies, adding to the problems the contractor has to fix and further endangering the tanker's delivery deadline.
On Thursday, the Air Force said that two additional issues had emerged, one involving the remote vision system, which is used by the boom operator to guide the boom into the fuel receptacle on the aircraft being refueled.
The remote vision system is not meeting Air Force requirements, according to Defense News. Problems with the RVS hinder "the boom operator's ability to guide the refueling boom and drogue into receptacles on the receiver aircraft," Aviation Week Pentagon editor Lara Seligman said on Twitter on Thursday.
Issues with the RVS have also contributed to the likelihood of the tanker's boom scraping against the exterior surface of the receiving aircraft — a previously disclosed category-one problem that can damage stealth aircraft. Boeing is working on a software update that it hoped would correct the RVS issue, Seligman added.
The other issue involves the centerline drogue system, the tanker's other refueling system, which in tests was unexpectedly disconnecting from the aircraft receiving fuel. The Air Force and Boeing "are performing the necessary systems engineering analysis to determine the root cause and path forward," Seligman said. The problem does not present any "immediate" safety risk, according to Defense News.
The Air Force does not have a set timeline for when these new problems will be fixed, Air Force spokeswoman Ann Stefanek told Defense News.
In addition to the previously reported issue with the boom scraping the receiving aircraft, the KC-46 program has also been dogged by problems with its high-frequency radio, which uses the skin of the tanker to broadcast and can cause sparks and fires, and with the boom itself extending on its own when disconnecting from a refueling aircraft.
Those two issues have recently been downgraded to category-two deficiencies, however.
Boeing missed a self-imposed deadline to deliver the first KC-46 to the Air Force by the end of 2017, and the Air Force said earlier this month that it expects the contractor to miss the expected delivery window in spring 2018, instead presenting the first tanker later this year.
Boeing is obligated to hand over 18 of the new tankers to the Air Force by October, but the service now expects the planes will arrive in spring 2019. However, Stefanek told Defense News, the recent deficiencies have not led the Air Force to change its schedule projection.
"The timeline for first delivery was modified based on known risks and predicted impacts associated with airworthiness certifications and slower-than-expected flight test execution," Stefanek told Defense News. "While these deficiencies weren't explicitly accounted for, the potential for additional technical findings were accounted for in the schedule risk assessment."
Missing the October deadline will likely subject Boeing to additional financial penalties.
Because of the way the KC-46 contract is structured, the Air Force is not responsible for costs beyond its $4.82 billion commitment. Boeing has already been hit with nearly $3 billion in pretax costs.
The Air Force currently plans to buy 179 of the KC-46. Once they start to arrive, the service will begin phasing out its older KC-10 tankers. It will hold on to 300 of its KC-135 — which average 55 years old.
Asked about the status of tanker program during a House hearing on Wednesday, Air Force acquisition chief Will Roper said the speed with which Boeing fixes problems will be "key metric" in the service's evaluation of the program in the future.
"The fact that there are issues on the program is less concerning to me. What will concern me during this year is if issues don't get retired quickly," Roper told lawmakers, according to Defense News.
During a March 14 visit to Boeing's KC-46 production and modification facility in Everett, Washington, Air Force Undersecretary Matthew Donovan lauded the planemaker but encouraged the firm to "double down" to "get this program over the goal line."
"I've been the guy that shows up behind a tanker in a single seat aircraft, low on fuel, 500 miles above the Arctic Circle, 800 miles out over the North Atlantic, or in hostile areas such as over Iraq," Donovan said. "Air refueling is a no-fail mission."