Mast bumping caused an in-flight breakup of a helicopter over Lake Wanaka, says the Transport Accident Investigation Commission (TAIC).
The Commission today released its Final Report into the 21 July 2018 accident, which fatally injured the pilot, the only person on board.
The Robinson R44 helicopter was cruising in mountainous terrain in conditions that were conducive for strong turbulence at a speed that increased the risk of an adverse outcome.
Mast bumping occurs when a two-bladed teetering rotor head see-saws (teeters) so much that its inside edge strikes its own main drive shaft (the mast). It can happen in a split second, usually results in the helicopter breaking up in flight, and is fatal for those on board.
TAIC Chief Commissioner Jane Meares says it is likely strong unexpected turbulence caused the main rotor disk to teeter excessively and the mast to bump.
“In this case, mast bumping caused a main rotor blade to bend down so far that it struck and entered the helicopter cabin, the helicopter broke up in flight, descended, impacted the lake, and sank,” said Ms Meares.
“Operators, owners and pilots of helicopters with this type of main rotor need to know that the risk of mast bumping increases with the likelihood of turbulence, mountainous operating environments, high power settings, higher speed and light weight.”
There is no need for TAIC to make a new recommendation in this this Final Report. Three recommendations from 2016 already address the relevant safety issues. We need more recorded flight data, and recorders are being developed for installation in Robinson helicopters. We also need to know more about the dynamic behaviour of two-bladed, teetering, underslung rotor systems, and this is the subject of a University of Maryland study.
In another move, Robinson Helicopter Company updated its R44 Pilot Operating Handbook to define the non-standard term, ‘significant turbulence’, after TAIC noted the potential for misinterpretation.
The TAIC Watch List item on mast bumping already recommends concerted action to promote safe operation of Robinson helicopters
NOTES FOR EDITORS
The Commission’s Final Report speaks for itself; it includes a wealth of evidence and explanation, and I recommend a thorough reading: www.taic.org.nz/inquiry/ao-2018-006.
The TAIC Watchlist draws attention of regulators, operators, and the Government to transport-related concerns of high social, economic or environmental risk, and systemic transport safety risks. More on the TAIC Watchlist item on mast bumping here: www.taic.org.nz/watchlist/robinson-helicopters-mast-bumping-accidents-nz.
The three recommendations made in 2016:
- Recorders: AO-2015-002: Mast bump and in-flight break-up, R44, Lochy River. www.taic.org.nz/inquiry/ao-2015-002 (two recommendations)
- Mast head design: AO-2013-003: R66, Mast bump and in-flight break-up, Kaweka Ranges. www.taic.org.nz/inquiry/ao-2013-003 (one recommendation)
The Transport Accident Investigation Commission opens an inquiry when it believes the circumstances of an accident or incident have - or are likely to have - significant implications for transport safety, or when the inquiry may allow the Commission to make findings or recommendations to improve transport safety.