An investigation into a helicopter crash that killed two Northland men has revealed the damage to the aircraft was so extensive the cause of the crash could not be determined.

Pilot Allan Jessop, 42, from Tangiteroria, and forestry worker and Derek Hammond, 49, died when the Robinson R44 helicopter they were in crashed in the Glenbervie Forest, north of Whangarei on October 31, 2016.

An report released by the Transport Accident Investigation Commission today said the damage to the helicopter and subsequent fire, and the lack of any other incontrovertible evidence, meant that the cause of the accident could not be determined.

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

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Pilot Allan Jessop was from Tangiteroria. Photo / File
Pilot Allan Jessop was from Tangiteroria. Photo / File

Investigators 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.

An intense fire followed, destroying the cabin and fuselage.

There are about 300 Robinson R22, R44 and R66 helicopters flying in New Zealand.

The helicopter was being used to spray recently-planted seedling blocks in two exotic forests in Northland. Prior to commencing each spraying job, the pilot flew a short survey flight with a forestry contractor to ensure that the boundaries of the target blockswere correctly identified and to check for any hazards and obstacles.

The crash happened when the pilot and contractor were undertaking the survey flight prior to sprayingthe Glenbervie blocks.

Emergency services near the scene of a helicopter crash in Glenbervie Forest.
Emergency services near the scene of a helicopter crash in Glenbervie Forest.

The call for help to the Glenbervie Forest crash came after the Wellington-based Rescue Co-ordination Centre picked up a solitary "ping" from the chopper's on-board locator beacon.

The single beep gave an accurate location, and the Northland emergncy services were alerted.

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The forest at the crash site was so dense no wreckage could be seen from the air - but a co-pilot in the Northland rescue helicopter spotted "a gentle wisp of smoke" at the right co-ordinates.

The helicopter caught fire when it crashed after it appeared the chopper had struck the ground at a "high rate of descent and a low forward speed".