A Robinson R44 helicopter broke up in flight as a result of extreme teetering, either due to turbulence or inappropriate control inputs, an Australian Transport Safety Bureau investigation has found.
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The subsequent crash near Tallong on the afternoon of December 2, 2020 killed Goulburn based instructor pilot 36-year-old Nicholaas Brink and his student, Moama Shire councillor and businessman Andrew Goldman, 54, from Victoria.
Mr Brink was described as a great husband and father of two young children who had a "zest for life," while friends said Mr Goldman was "a fantastic bloke" who was highly respected in his community.
A search was launched after the helicopter, flown by both men, failed to return from a training flight from Goulburn Airport. It had departed at 3.58pm in what the Bureau of Meteorology found were likely "fine and clear conditions."
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Mr Brink, who was described as an experienced pilot with 690 flying hours under his belt, was taking Mr Goldman on the training flight as part of an "intensive 12-day" course. Mr Goldman had almost 38 hours dual flying experience and had accrued three hours of solo flying.
Wreckage of the helicopter was found later that evening in the Bungonia State Conservation Area, some 31km east of Goulburn. The accident site was also 4km northwest of the helicopter's last recorded ADS-B transmission, which indicated it was descending into a valley.
The helicopter's engine and fuselage had been exposed to fire, which had self-extinguished before emergency services arrived. Both pilots were fatally injured.
It could not be determined who was flying at the time.
ATSB investigators surveyed the wreckage trail and main wreckage site and, despite fire damage, recovered and examined several components of the helicopter's main rotor.
Examinations confirmed a mast bump had occurred, and that the helicopter had subsequently broken up in flight.
"Mast bumping is where the inboard end of the main rotor blade contacts the helicopter's main rotor shaft," ATSB Director Transport Safety Stuart Macleod explained.
In R44 helicopters, this can generally be identified by extensive damage to the teeter stops and varying degrees of damage to the main rotor shaft.
"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," Mr Macleod said.
"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."
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.
"The ATSB investigation found that, while flying in the vicinity of the valley, the helicopter entered a low-G condition due to turbulence, inappropriate control inputs, or a combination of both," Mr Macleod said.
"Low-G conditions can be catastrophic for helicopters with semi-rigid rotor heads. 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."
Mr Macleod noted 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," Mr Macleod said.
The Bureau of Meteorology found that a weather change had occurred at the accident site between 3.40pm nd 4pm, which then possibly moved across the 1km wide valley, likely creating "turbulence and rotor winds induced by the local terrain."
The investigation report also urges operators to consider the benefits of installing recording devices on their helicopters.
"While the fire would likely have prevented data recovery in this case, the inclusion of readily-available cockpit video recorders on helicopters with semi-rigid rotor heads would provide valuable insights into low-G mast bumping events, which could help prevent future occurrences," Mr Macleod concluded.
Since the crash, Robinson has introduced cockpit video/audio recorders as standard equipment on new R44s, having previously been available on the larger R66 models. The recorders are currently optional on the smaller R22s, but will also become standard on these models in 2023, with retrofit kits made available.
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