The final crash investigator's report into the helicopter crash that killed war hero Steve Askin in the 2017 Port Hills fires found that the chopper's pilot-side doors had been removed, contributing to fluctuating pressure inside the cabin.
But the Transport Accident Investigation Commission (TAIC) report concluded that it was "very unlikely" recent cannabis use by the former decorated SAS soldier impaired his performance that day.
Wildfires broke out on the Port Hills above Christchurch on February 13 last year.
Askin, a war hero wounded in a five-hour shootout in 2011 after the Taliban stormed the InterContinental Hotel in Kabul, joined the fight to put out the rampant fires.
Captain Tim Burfoot, the TAIC's chief investigator of accidents, says Askin's AS350 'Squirrel' helicopter was on its way to refill its monsoon bucket when it encountered turbulence and wind shear, and a side window dislodged into the cabin and flew out of an open door.
"All the doors on the pilot's side had been removed to make it easier for the pilot to see the monsoon bucket. The doors on the left side were closed. This door configuration was prohibited in the flight manual," Burfoot said.
The TAIC found that it was "about as likely as not" that the door configuration contributed to the fluctuating pressure caused by the forward airspeed and turbulence, and that this "likely forced the side window panel into the cabin interior and ejecting out through the open side door".
"The pilot reduced speed, causing the tail rotor to dip," Burfoot said.
"The monsoon bucket swung up and backward, and the bucket line damaged the tail rotor assembly. This made the helicopter uncontrollable and it crashed."
The 38-year-old father-of-two was fatally injured when it crashed near the Sugarloaf carpark on the blustery nor'west afternoon.
His tragic death fighting the fires prompted a mass display of grief. A Givealittle page initiated to help support his young family topped $400,000.
Askin was named as the winner of the Herald's New Zealander of the Year People's Choice Award for 2017.
At the time, Elizabeth Askin said her late husband would be mortified by the public recognition.
"With Steve, he'd have been like, 'Just doing my job. All the other pilots were out there doing their job.' He wouldn't want to be the face of this or that."
"I grieve so much for my children because I know what an awesome father they have lost," she said. "I was so pleased knowing what a man of great character and fun the children had for a father."
The TAIC probe found that Askin was an occasional user of cannabis and had smoked cannabis when he was off duty two days before the accident.
An expert forensic toxicologist reviewed test samples and concluded that the pilot was "unlikely to have been under the influence of cannabis at the time of the accident".
However, the TAIC said substance use can have "significant implications for transport safety".
The Civil Aviation Authority (CAA) has advised New Zealand air operators that they are expected to include drug and alcohol policies in their safety management systems.
His employers, Way To Go Heliservices, had yet to establish an approved safety management system but did have a drug and alcohol policy and a policy for random or post-accident testing in its safety manual.
"The chief pilot was not aware of the pilot's occasional cannabis use and said that the pilot had never presented under the influence at work," the report said.
The TAIC report raises two safety issues for the industry following the crash, including a concern there "may not be a good awareness within the helicopter industry of the additional risks involved with underslung load operations, particularly with the use of monsoon buckets during firefighting operations".
It added: "The operator did not have adequate systems available for the pilot to determine the actual all-up weight and balance of the helicopter for the firefighting operation, or to ensure that incidents such as the previous loss of a window were recorded, notified to the CAA and investigated."
The TAIC said the key lessons from the accident are that:
· Vigilance during turbulence is vital.
· Pilots should understand the reasons for, and observe, the limitations of the aircraft they are flying.
· To help prevent future accidents, operators should record and investigate all operational incidents.
· Performance-impairing substances such as recreational drugs pose a serious risk to aviation safety. Their short- and long-term effects may be unpredictable and result in pilots being impaired when flying their aircraft.