The Australian Transport Safety Bureau (ATSB) this week urged owners and operators to install a symmetrical stabiliser to all Robinson helicopters after the existing asymmetrical design was found to contribute to the in-flight break-up of an R66 north of Sydney.
VH-KFT was crossing Yacaaba Headland near Hawks Nest in October 2023 when turbulent conditions forced the helicopter into a low-G condition, which resulted in an uncommanded roll to the right and a mid-air break-up. The sole person on board did not survive the crash.
ATSB investigators found that the aircraft was traveling at 115 KIAS, which is 45 knots above Robinson's recommended turbulence penetration speed. The ATSB issued a speed warning related to the accident in April 2024.
“This speed significantly increased the uncommanded right roll rate and reduced the time available for the pilot to respond by applying a gentle aft cyclic input to re‑load the main rotor disc," said ATSB Director Transport Safety Stuart Macleod this week.
“In addition, the pilot had been eating and had food in their right hand, and so they used their left hand to manipulate the cyclic, reducing their ability to slow the helicopter in a timely manner using coordinated flight control inputs.”
A video camera operating in the cockpit showed that instead the pilot applied increasing left cyclic during the right roll, which increased the risk of mast bumping due to the Robinson's teetering rotor head.
The aircraft rolled inverted before it broke-up, a condition that the ATSB found was exacerbated by the asymmetric design of Robinson's horizontal stabiliser.
“In this case the pilot’s response to encountering the low‑G contributed to the development of the in‑flight break‑up," Macleod explained, "but we would caution all Robinson helicopter pilots that a low‑G condition can result from turbulence directly, and pilots must be ready to respond appropriately.
“If the main rotor disc is not immediately reloaded, right roll can develop rapidly, particularly when an asymmetrical stabiliser is fitted."
Robinson's standard stabiliser fitted to R22, R44 and R66 helicopters extends from the right side of the tail cone, creating a right rolling moment, which is normally counteracted by a left tilt of the main rotor disc.
The ATSB discovered that in a low‑G condition where the main rotor disc is unloaded, the stabiliser can't counteract the right rolling moment.
Robinson has created a symmetrical stabiliser that will be fitted to all new helicopters and can be retro-fitted to existing aircraft.
Macleod said the ATSB strongly encourages fitment of the symmetrical stabiliser.
“The modification significantly reduces the right roll if a low‑G condition is encountered, allowing pilots more time to recognise and respond to the situation.”
Investigators also found that Robinson's POH sections for operation in high winds or turbulence did not provide warning of the potential for low‑G and rapid right roll at high airspeed, nor did it provide guidance for appropriate control inputs in response to a turbulence‑induced low‑G situation.
The full investigation report and incident video is on the ATSB website.