A long-awaited report on a Southern Ocean helicopter crash that forced survivors to swim to a remote island for safety in the dark has found the pilot probably mistook sea fog for cloud through night vision goggles when coming in to land.

Four years after the crew's extraordinary ordeal, the Transport Accident Investigation Commission has delivered the findings of its investigation into the crash near the subantarctic Auckland Islands, 465 kilometres south of Bluff.

The commission has made two safety recommendations to the Civil Aviation Authority to address a "regulatory void" with night vision imaging systems in helicopter air ambulances and training for non-pilot crew members.

On 22 April 2019, pilot Andrew Hefford, paramedic John Lambeth and winch operator Lester Stevens set out to rescue a fishing vessel patient but ended up being rescued themselves after their chopper plunged into the sea.

The report said the men had intended to arrive at Enderby Island during the day, but unexpected delays meant it was dark by the time they approached the coast.

Hefford was flying under visual flight rules and wearing night vision goggles, along with Lambeth, who was acting as a night vision imaging system crew member for additional safety.

Hefford believed the landing spot was covered in cloud, so planned an alternative approach to descend in a clear area, but 20m-high cliffs suddenly loomed ahead, the report said.

During a manoeuvre to avoid the cliffs, the helicopter crashed into the sea, temporarily knocking Stevens unconscious.

Lambeth dragged Stevens out of the partially submerged wreckage, before the men paddled 100 metres as a group in their immersion suits and climbed through a kelp bed onto rocks.

"This accident was not fatal and the crew were fortunate to have survived an unexpected impact in cold water at a remote location at night, but it could easily have been fatal," the report said.

After sheltering in the bush overnight, the men were spotted by a rescue helicopter the following day and flown to Invercargill Hospital with minor injuries.

Visual error

The commission's chief accident investigator Naveen Kozhuppakalam said the pilot had likely misinterpreted fog near the sea surface as cloud through his night vision goggles.

"The area believed to be clear air near cloud was in fact calm water near fog," he said.

"The pilot flew towards it while relying primarily on visual references, looking outside through the night vision goggles, without enough scanning of the instrument panel. At the time, a radio altimeter warning light was illuminated and would have alerted the pilot to being below their selected altitude."

The radio altimeter did not have an aural alert signal.

The report noted Lambeth went back into the wrecked cabin to try to retrieve the life raft, but it was difficult to identify items in the dark and a gear-bag was mistakenly taken instead.

None of the crew had an emergency beacon or survival equipment on them or in their immersion suit pockets.

The men had packed all their personal locator beacons and other survival gear into an emergency grab-bag that was stowed in the cabin, and lost with the helicopter when it sank.

The commission found Southern Lakes Helicopters' procedures for single-pilot visual flight rule operations in the Southern Ocean were inadequate at the time of the accident.

However investigators did not direct any safety recommendations to the company because it had made significant improvements, including hiring an external auditor and introducing a new standard operating procedure for subantarctic island flights.

The commission also found there were no formal risk controls in New Zealand for helicopter night vision imaging systems or air ambulance operations, unlike other jurisdictions that had set minimum standards in response to high accident rates.

New Zealand lacked comprehensive "crew resource management" training and guidelines for crew members helping pilots during night vision operations, the report said.

The report also noted the helicopter was loaded above its maximum certified weight limit for the rescue flight and life rafts must be able to be easily extracted from aircraft.

The TAIC recommended the Civil Aviation Authority address the growing gap between New Zealand's minimum performance requirements and technical standards for night vision imaging systems and helicopter air ambulances and current international best practice.

The commission also recommended the authority address safety issues with crew resource management competency.

By Jean Edwards, RNZ

Reproduced with permission