Mast bumping caused an in-flight breakup of a helicopter over Lake Wānaka, said the Transport Accident Investigation Commission (TAIC).

Matthew Wallis died when the Alpine Helicopters Robinson R44 he was flying crashed on July 21, 2018.

Only months later his brother Nick Wallis, along with passengers Paul Hondelink and Scott Theobold, died in a second helicopter crash near Wānaka Airport on October 18, 2018.

 On Friday the Commission 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, and usually results in the helicopter breaking up in flight, and is fatal for those on board.

 TAIC Chief Commissioner Jane Meares said it was 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 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 Final Report, she said. Three recommendations from 2016 already addressed 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,” Meares said

 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

In 2018 the TAIC suggested Wallis was flying too fast for the conditions.

"While the pilot would have likely had an indication of the weather conditions as the helicopter flew up Stevensons Arm, there were various pieces of information available indicating that flying close to the maximum permitted speed would have likely increased the risk of an adverse outcome."

Wallis was flying at 178km/h airspeed in the moments before the crash when the pilot handbook suggested pilots reduce airspeed to 110km/h to 130km/h during "significant turbulence".

TAIC said a passing cold front, wind, forecast turbulence and reduced visibility meant flying conditions at the time were not good, making the decision whether to fly "ambiguous".

Read edition 1003 of the Wānaka Sun here.

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