The US Air Force’s Boeing KC-46 tanker is facing three outstanding issues as it moves through testing, including a boom scraping problem that could pose serious risk to the tanker’s aircrew.

Earlier this week, the USAF’s chief of air mobility command revealed the air force has discovered three major deficiencies during testing on Boeing’s next-generation tanker. Video and data gathered during developmental testing showed the tanker scraped receiver aircraft outside the receptacle, according to the USAF’s programme executive for tankers, Brig Gen Donna Shipton.

The USAF is also working to understand a high-frequency transmit and “uncommanded boom extension issue,” which the air force plans to solve this October. The service will collect data on the scraping problem throughout October and November, and, until that data is analysed, Shipton is not sure when the issue will be solved.

Based on a schedule risk assessment, the KC-46 programme office does not believe Boeing will be able to complete first delivery in December and instead, expects a spring 2018 delivery. Those delays, which the Government Accountability Office predicted in a report last spring, are not related to the deficiencies but to test points Boeing must complete to acquire US Federal Aviation Administration and military aircraft certifications.

During developmental testing last October, the KC-46 boom’s tip struck receiver aircraft outside their refuelling slipways.The USAF did not discover the issue until testing completed and the service analysed data and completed a deficiency report in May.

“When the boom isn’t being carried into the receptacle, there’s instances where there’s contact outside the receptacle by the boom and in some instances, it goes undetected by the boom operator,” Shipton says. “We have aerial refueling procedures that require... the boom operator [to] notify pilots, make them aware that the boom contacted outside the receptacle.”

The air force believes KC-46 is potentially scraping aircraft at a higher rate than legacy tankers, but Boeing and the KC-46 programme office are analysing historical data to compare how often the issue occurs in the current fleet, Shipton says. While the two other category one issues are not severe, scraping could pose a significant risk to aircrew, she adds.

The USAF is concerned about KC-46 scraping low observable aircraft, but the tanker has not yet refueled stealth aircraft in testing. KC-46 has refueled the F-16, F/A-18, AV-8B, C-17 and A-10.

Less severe but still unknown is a high frequency (HF) transmitting issue during aerial refueling. HF transmitting must be turned off during refueling to avoid electrical sparking between the boom and receiver. The USAF first identified the issue in 2016, but does not have sufficient test data to confirm that when transmitting is turned off, it stays off, says Col John Newberry, KC-46 system programme manager.

“If for some reason it’s off but somehow failed, we needed the test data to prove it wouldn't inadvertently come back on,” he says.

The service will conduct testing in October and, assuming the results are positive, will be able to close out the deficiency report.

The service is also grappling with what it calls an “uncommanded boom extension” on KC-46. During ground testing, fuel flowed through boom, exerting pressure which pushed the boom forward and extended the boom into a test stand acting as a receptacle. The issue also occurs on the legacy fleet, where if a pilot somehow disconnects unexpectedly then the boom operator retracts the boom from the aircraft. That phenomenon is known as a commanded scenario, Shipton says. With the KC-46 ground testing, the test stand was not rated to withstand the same impact as an aircraft receptacle.

“Initially there was some concern,” Shipton says. “After looking at the data, we believe this is not going to be an issue, however we won’t make a decision on closing this deficiency report until October.”