A report into the helicopter crash that killed former SAS soldier and pilot Steve Askin has revealed he was not following best practice.
Mr Askin, 38, was killed while fighting the Port Hills fires on Valentine's Day last year - a day after they broke out.
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The investigation by the Transport Accident Investigation Commission (TAIC) found Mr Askin was on his way to refill the Squirrel chopper's monsoon bucket, when he hit turbulence and wind shear - dislodging a side window.
The doors on the pilots side had been removed to make it easier for Mr Askin to see the monsoon bucket. However the doors on the left side were closed.
That door configuration he used was one prohibited in the flight manual for the model of Squirrel he was flying, the TAIC says.
The Transport Accident Investigation Commission's Captain Tim Burfoot told RadioLIVE Drive that aircraft manufacturers test all variants and configurations before approving.
"The configuration the pilot had on that day had not been tested and therefore wasn't approved by the aircraft manufacturer.
Mr Askin reduced the speed of the helicopter, causing the tail rotor to dip and the monsoon bucket to swing into it - resulting in the helicopter losing control.
A video recording taken from a camera mounted underneath the helicopter showed the monsoon bucket rising towards the tail rotor.
The recording also showed that an object fell from the helicopter shortly beforehand.
"Having the doors open in that configuration would have made the pressure within the cabin fluctuate a lot more and make the cabin flex which gives more opportunity for windows to pop out.
"It was a very unfortunate combination of events that all just came together at the time," Captain Burfoot said.
Mr Askin had experienced a similar loss of the left rear window while flying the same helicopter on a firefighting mission in 2015.
Listen to the full interview with Tim Burfoot above.