A coroner's inquest into a Robinson helicopter crash that killed two men has been told a more thorough, collaborative investigation is needed into the aircraft's rotor design.
The crash occurred in Queenstown in 2014 after what's called a mast bump, where the blade can end up striking the cockpit.
Robinson's are lightweight and versatile - but our Transport Accident Investigation Commission (TAIC) has put them on a safety watch list.
Engineer and Independent Air Crash Investigator Andrew McGregor backs the move.
"TAIC is evidently concerned about the design and associated training requirements collectively provide the degree of airworthiness needed for the New Zealand conditions and in our view, their concerns are justified."
At the centre of the inquest are questions about the Robinson's main rotor blade.
James Patterson, 18, and his highly experienced instructor Stephen Combe were killed when their Robinson 44 crashed in the Lochy Valley in 2014.
It happened after the helicopter experienced a mast bump, where there's contact between the rotor and driveshaft, and the helicopter broke apart mid-air.
In the US, the Robinson Company has also been doing modelling and simulation work on the rotor design, but the preliminary results have not been released publicly.
McGregor says that raises concerns.
"Without a proper qualified and independent review the results can not be considered to be totally objective and independent."
In an effort to learn more about mast bumping, Robinson also started work on cockpit video recorders in 2018.
McGregor says it "is regrettable that nothing has emerged yet".
More than 20 years ago, international safety agencies started investigating the Robinson, but the work was never completed.
McGregor told the Court the Robinson's main rotor design needed more investigation and suggested our Transport Accident Investigation Commission should be part of it
He said it would be expensive but worth it.
Eighteen people have died in Robinson's in New Zealand, including nine in mast bumping events.
There were also calls for cameras to be mandated in helicopter cockpits - like black boxes are in commercial passenger planes.
TAIC recommended their use five years ago - but the Transport Ministry is yet to act.