NSW helicopter crash due to ‘mast bump’, says ATSB

The Australian Transport Safety Bureau (ATSB) investigation has concluded that the aircraft, which was being flown as part of a training flight by an instructor and student pilot, had experienced a ‘mast bump’, caused either due to turbulence or inappropriate control inputs.

“Mast bumping is where the inboard end of the main rotor blade contacts the helicopter’s main rotor shaft,” explained ATSB Director for Transport Safety, Stuart Macleod.

“The semi-rigid, or teetering, main rotor design of the R44 and several other popular helicopters relies on rotational force to lift the blades off the droop stops on the rotor hub during flight. During normal operation, the rotor is free to teeter and flap around its designed flight axis via the teeter hinge, while polyurethane teeter stops limit the degree of teetering,” he added.

A mast bump can occur when extreme teetering causes the rotor blades to exceed this allowance and contact the rotor hub at a high speed. Mast bumps are typically indicated in R44 helicopters through extensive damage to teeter stops and varying degrees of damage to the main rotor shaft.

The helicopter’s wreckage indicated that the engine and fuselage had both been exposed to fire, which had burned out before emergency services could reach the site. Both pilots were fatally injured during the crash.

The aircraft either crashed due to turbulence or inappropriate control inputs

The aircraft, which crashed in the Bungonia State Conservation Area, approximately 31km east of Goulburn, was found approximately 4km northwest of the aircraft’s last recorded ADS-B transmission, which had indicated that it was entering a valley.

Macleod added that the ATSB investigation had found that while flying in the vicinity of the valley, the aircraft had ‘entered a low-G condition due to turbulence, inappropriate control inputs, or a combination of both’.

“Low-G conditions can be catastrophic for helicopters with semi-rigid rotor heads,” he said. “A pilot’s ability to recover from low-G remains uncertain, and would be dependent on airspeed and time available. Pilots must therefore avoid low-G situations.”

Macleod warned that pilots should ‘make a careful study of terrain, forecasts and observations applicable to a proposed flight, to identify in advance any significant weather or mechanical turbulence, and avoid flying in these conditions’.

“Pilots should avoid flying downwind of hills, ridges or other potential sources of turbulence, particularly during changing or unpredictable weather conditions, and should use slow and small control inputs when encountering turbulence,” said Mr Macleod.

The ATSB also urged operators to consider installing recording devices in their helicopters, adding that while the fire would likely have prevented data recovery in the case of this crash, readily available video data on helicopters with semi-rigid rotor heads would provide valuable insights into low-G mast bumping events which could prevent future occurrences. It notes that since the occurrence, Robinson has introduced cockpit video/audio recorders as standard on the R44, with plans to make them standard on the smaller R22 from 2023, with retrofit kits made available.

Robinson Helicopter also issued a service bulletin in January 2022 warning of potential cracks in the tail rotor blades of on some of its R44 and R66 series helicopters.