R22 Excessive Teeter And Mast Bumping

The in-flight break-up of a Robinson R22 near Koorda, WA highlights the potentially catastrophic effects of low-g and/or low rotor RPM/rotor stall conditions in helicopters with semi-rigid rotor heads.

The two-seat Robinson R22 Beta II was conducting a private flight from Koorda to Jandakot on 2 October 2022 and broke-up in flight shortly after reaching cruise altitude, an ATSB investigation report details.

Recorded flight data showed that, during cruise, the helicopter’s altitude increased by about 100 ft and then rapidly descended, almost vertically. The helicopter collided with terrain inverted, on a dry salt flat, and the pilot and passenger were fatally injured.

ATSB examination of the accident site identified signatures consistent with the main rotor assembly being subject to excessive teeter and mast bumping – where the main rotor spindle impacts the mast.

“It was likely the helicopter entered either a low-g and/or a low rotor RPM/rotor stall condition, which, along with delayed and/or inappropriate control inputs, resulted in extreme teetering of the main rotor assembly, and the in-flight break-up,” ATSB Director Transport Safety Kerri Hughes said.

The ATSB explored multiple scenarios to determine the reason for the extreme teetering and mast bumping, and the investigation would have been considerably aided had the helicopter been fitted with an in-cockpit video recorder.

“Better understanding of the circumstances leading up to extreme teeter and in-flight break-up events such as in this accident will assist investigators in determining appropriate steps for ongoing safety improvement,” Ms Hughes said.

Robinson introduced cockpit video cameras as standard on new R66 and R44 helicopters from 2021 and 2022 respectively, and they are now also optional for new R22 helicopters. The manufacturer also offers a camera retrofit for in-service R44s and R66s and most R22s.

“While not required by regulations, we urge owners and operators to consider the benefits of installing cockpit video cameras, which not only aid safety investigations, but can also be used for flight instruction debriefing and as a maintenance tool,” Ms Hughes said.

The investigation report also noted that the helicopter was fitted with dual flight controls, and that Robinson recommends removing them if the passenger is not helicopter rated or under flight training instruction.

“When carrying passengers, the helicopter manufacturer recommends removing the passenger-side controls to avoid inadvertent bumping or interference,” Ms Hughes said.

Ms Hughes stressed there was insufficient evidence to determine if the passenger made an inadvertent control input or if they were operating any of the controls during the flight.

“Where dual controls cannot be removed, the passenger should be fully briefed to keep their hands and feet clear.”

Ms Hughes noted the accident demonstrates the catastrophic potential of low-g and low rotor RPM/rotor stall conditions in helicopters with semi-rigid rotor heads, such as the R22.

“A pilot’s ability to identify the developing condition and promptly apply the correct flight control inputs is vital to effective recovery and continued safe operation,” she concluded.

You can find here the report: In-flight break-up involving Robinson R22 Beta II, VH-RAS, 13 km south-west of Koorda, Western Australia, on 2 October 2022

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