AUSA 2019: SB>1 Defiant returns to flight after PTSB gearbox bearing creep delay

The Sikorsky-Boeing SB>1 Defiant co-axial compound helicopter returned to flight in late September after a bearing creep issue in the Power System Test Bed (PTSB) gearbox grounded the aircraft for six to eight weeks.

Ken Eland, Boeing Future Vertical Lift (FVL) programme manager, told reporters on 15 October at the Association of the United States Army (AUSA) convention that the team in the second quarter of this year found an issue in the guts PSTB during a scheduled disassembly required to create a flight release for the aircraft. Sikorsky and Boeing, he said, realised that the Defiant was not going to reach its speed point until this issue was fixed.

The Sikorsky-Boeing team discovered a bearing creep issue in the SB>1 Defiant during a planned teardown of the aircraft. The aircraft was grounded for six to eight weeks as a result, but the team said it could have been out twice as long. (Sikorsky-Boeing)

Eland said that although the bearing creep issue sidelined the Defiant for six to eight weeks, it could have been longer. He said if the team had waited until early 2020 to fix this issue, it would have grounded the aircraft for twice as long. Boeing developed the Defiant’s gearbox.

Bearing creep is a phenomenon in bearings in which relative slippage occurs between fitting surfaces. Possible causes include insufficient interference or loose fit or insufficient sleeve tightening. Remedies include checking the interference and preventing rotation, correcting the sleeve tightening, or preloading in the axial direction, among others.