FARNBOROUGH—A KC-46 equipped with a reworked boom has successfully refueled a C-17 heavy cargo aircraft, a sign that Boeing’s tanker troubles may finally be coming to an end.                                                                       

“While it took some time, this week’s results confirm my confidence the Boeing team will get this figured out. It’s reassuring to see the program take this important step toward the production decision in August,” U.S. Air Force Chief Gen. Dave Goldfein said in an email statement.

The successful C-17 test took place July 12, a few days after the KC-46 with the fixed boom refueled an F-16 on July 8. Boeing’s tanker will attempt to transfer fuel to an A-10 later this month, the final flight test required before a Milestone C decision to formally approve production.

This most recent flight test shows Boeing has successfully fixed the boom instability problem seen during refueling of large aircraft that has plagued the next-generation tanker program for months.

“I’m encouraged by these results. The KC-46 program continues to move forward, making important progress that will get this vital capability into the hands of the warfighter,” U.S. Air Force Secretary Deborah James said.

The problem came down to excess pressure on the boom telescope control loop, which controls the extension and retraction of the boom during refueling. The control loop must maintain enough contact pressure to keep the refueling nozzle connected, but not so much pressure as to place excessive force on either the tanker or the receiver aircraft, U.S. Air Force spokesman Daryl Meyer told Aviation Week. The problem was discovered during recent flight tests with the C-17 heavy cargo aircraft.

Boeing’s hardware solution to the problem involves using a hydraulic relief valve on the KC-46 to ensure it will properly alleviate axial loads in flight, Meyer said.

The program’s schedule has not changed beyond what Boeing discussed in late May, officials say. Boeing will begin delivering ready-to-go tankers in August 2017 and the last of 18 required aircraft in January 2018—a five-month slip.

To accelerate the flight-test program, Boeing recently added a fifth engineering and manufacturing development (EMD) aircraft to the fleet, Meyer told Aviation Week. Once the hardware fix is verified, a KC-46 with the updated boom will complete regression testing on the F-16, followed by aerial refueling demonstrations with the C-17 and A-10.

The Pentagon is in the process of determining whether Boeing will incur any penalty for the most recent schedule delay.