Only the two men who died will know why helicopter crashed in Northland forest

The only people who will ever know what happened in a fatal helicopter crash in Northland bush were the two men who were killed, says the wife of the pilot.

Kirsty Jessop has spoken about the deaths of her husband and pilot Allan Jessop, 42, and forestry worker and Derek Hammond, 49, who both died when the Robinson R44 helicopter they were in crashed in the Glenbervie Forest, north of Whangarei on October 31, 2016.

Helicopter pilot Allan Jessop died in a 2016 crash at Glenbervie Forest. Photo/ Supplied

Derek Hammond was hard working, full of life and laughter, loved his hunting, music and had a passion for V8's. Photo/ Supplied

A report released on Thursday by the Transport Accident Investigation Commission concluded the damage done to the helicopter in the crash and subsequent fire was so extensive the cause of the crash could not be determined.

"Clearly the only two people that know what happened that day are Allan and Derek," Jessop said.

"No one should have to face such a traumatic moment in their lives like we all have."

The family had waited nearly two and a half years for answers and closure and the investigation process had not been easy and at times had been demoralising, she said.

"Answers and factual evidence were what we were focused on getting. All we wanted was a positive outcome and something to keep us looking forward so these two great men can rest easy.

"With an accident of this nature there is always going to be speculation and everyone seems to have negative opinions. Allan was an amazing pilot who had professional ethics so it was important to me his name wasn't tainted in any way within this industry."

The Robinson helicopter was extensively damaged after crashing into Northland bush. Photo/ Supplied

Investigators said a bolt that attached the pitch link to the pitch horn on one of the main rotors was found to be missing.

But said it was virtually certain that the missing bolt came out of position during the impact sequence, meaning it did not contribute to the accident.

The report said it appeared the chopper had struck the ground at a "high rate of descent and a low forward speed" while the men were surveying an area of the forest ahead of spraying.

"The damage to the helicopter caused by the impact and post-accident fire masked much of the evidence, and as there were no witnesses to the accident and there were no on-board flight data recorders installed, the Commission has been unable to determine conclusively the factors that contributed to the crash," the report said.

Yesterdaywould have been Derek Hammond's 52nd birthday.

"It's quite an emotional day to finally get the TAIC report on what would be Derek's birthday," his younger brother Les Hammond said.

"It's very disappointing that the report couldn't identify the cause of the helicopter crash as there was no flight data recorder on board. Hopefully flight data recorders will become compulsory so that the causes of these incidents could be determined sooner."

Hammond said his brother was his best friend and was sadly missed by family and friends.

"He was hard working, full of life and laughter, loved his hunting, music and had a passion like me for V8s."

He said his family's thoughts were with Kirsty Jessop and her two children.

The crash site was about 1km to the north of the planned spraying area and why the helicopter flew beyond the planned area could not be determined.

The fire that followed the crash was intense enough to melt aluminium components, including the main rotor gearbox casing.

The fire also melted other aluminium components in the fuselage and consumed the fibreglass cabin structure.

However, the tail boom, tail rotor, main rotor blades and mast head were unburnt.

The report said pilot Allan Jessop had started helicopter flight training in July 2006 and had been issued with a private pilot licence in April 2007.

Jessop had qualified on the Robinson R44 in September 2007 and been issued with a commercial pilot licence in February 2011.

His last flight crew and agricultural competency checks had been completed on September 19, 2016, and he held a current Class 1 medical certificate which had no conditions, restrictions or endorsements.

The helicopter had been imported to New Zealand in October 2013 and issued with a certificate of airworthiness.

According to the operator's documentation, the helicopter had been maintained in accordance with the Robinson maintenance schedule.

The Transport Accident Investigation Commission has previously made a recommendation that on-board recorders be fitted to certain classes of helicopter to aid accident investigation.

Investigators said if such a recorder had been fitted and recovered, it would very likely have helped to identify the cause or causes of this accident.

The investigators said there were no new recommendations identified, no new safety issues or key lessons.