The U.S. Air Force will test the KC-46 aerial refueling tanker next month to better understand a boom-scraping problem that it considers a potential safety issue. Based on the test results and its assessment of the risk involved, the service will decide whether to accept the still-developmental tanker as planned or require a fix, senior officers said on September 22.

Following the completion of aerial refueling trials earlier this year, testers discovered that the KC-46’s telescoping, fly-by-wire refueling boom had made contact with receiver aircraft fuselages outside the area of their fuel receptacles, a problem that also occurs with current tankers. Air Force procedures call for the boom operator to notify the flight crew when that happens; at issue are contacts that occur without the boom operator knowing—a situation the service calls undetected contacts outside receptacle (UCOR).

Flight-testing of the KC-46, a modified Boeing 767-2C freighter, is 65 percent complete, the Air Force reports. Thus far, the service has conducted KC-46 aerial refueling exercises with the C-17 transport, the A-10 ground attack jet and the F-16 fighter. It has not yet tested the tanker with the F-35 Lightning II or other aircraft with specialized skin coatings that help make them low observable (LO), or nearly invisible to enemy radars.

Next month, the Air Force will begin testing the KC-46 to gather additional data on UCOR occurrences to “better characterize the issue,” and it will also review historical flight-test data from other tankers as a comparison. It is too early to say if the service will require a fix for what is labeled as a “Category I” technical issue, senior officers said during a teleconference with defense reporters.

“That’s a decision for the Air Force that we can’t make at this time,” said Brig. Gen. Donna Shipton, program executive officer for tankers. “We have to get a full understanding of the issue and we won’t know that until after the testing is complete this October-November, and we better characterize the issue.

“We will have discussions with Air Mobility Command and potentially the program offices for the receiver aircraft to make sure we understand the risk going forward once we have the data,” Shipton added.

“Undetected (boom scraping) could have significant risk to the aircrew and that’s why it’s a Category 1,” said Col. John Newberry, Air Force KC-46 program manager.

The topic also came up during the Air Force Association Air, Space & Cyber conference on September 19. Asked if the KC-46 boom had scraped any LO aircraft, and if the service could accept that happening, Lt. Gen. Arnold Bunch, the Air Force’s top acquisition officer, said: “We’re not deeply into a lot of the specific platforms with the refueling we’ve done, and it is one of the things that we will have to consider as we go forward, whether we can live with it.”

The UCOR issue apparently was the most serious of three KC-46 “deficiency reports” (DRs) officers explained during the teleconference. The two other deficiencies were: “high-frequency transmit inhibit during aerial refueling non-compliance,” referring to a requirement that HF radios remain off during aerial refueling to prevent electrical arcing between the boom and receiver aircraft; and “uncommanded boom extension during flowing disconnect,” a deficiency discovered during a ground test in May. In the latter case, the boom pushed forward into the test stand on disconnect.

The Boeing-built refueling boom, a derivative of that used on the KC-10 Extender, has had other problems. After testers detected “higher than expected” axial boom loads during aerial refueling exercises in 2016, the Air Force postponed a planned low-rate initial production decision from June that year to August. Boeing resolved the problem by installing two bypass valves to relieve pressure on the boom from fuel loading.

The $4.83 billion, fixed-price engineering and manufacturing development contract the Air Force awarded Boeing in 2011 called for the manufacturer to deliver 18 tankers, nine sets of wing aerial refueling pods (WARPs) and two spare engines by August this year. The parties later amended the schedule to allow Boeing to deliver the first 18 aircraft and WARPs separately by October 2018, the Government Accountability Office reported in March.

Boeing is working toward obtaining an amended type certificate from the Federal Aviation Administration for a 767-2C baseline freighter and a supplemental type certificate for the full KC-46 tanker. The manufacturer has pledged to deliver the first tanker to the Air Force by this year; the service anticipates the first delivery next spring.

The Air Force has now contracted with Boeing for 34 KC-46 tankers; six development tankers are flying. Plans call for awarding a production Lot 4 contract in January, Shipton said.

Despite the technical issues encountered during the development phase, Newberry said he still believes the KC-46 program, which is based on converting a commercial airliner into a military refueling tanker, is a low-risk effort.

“One of the risks that we probably underestimated collectively was the concurrency of doing an amended type certificate on a brand-new aircraft—the 767-2C is a brand new 767 (that) obviously takes advantage of the heritage of the 767 family,” Newberry said. “Doing that all at the same time was underestimated (as to) the complexity and scope of effort. I think Boeing is now paying for those underestimates.

Newberry added: “Overall these are minor DRs compared to other programs. In this fixed-price (contract) structure, we’re going to have a combat-ready aircraft Day 1.”