Port Hills helicopter crash caused by a cable striking the tail rotor

A cable hitting the tail rotor caused the helicopter crash that killed Steve Askin as he fought February's Port Hills fires, an interim report has found.

Wildfires broke out on the Port Hills between Lyttelton Harbour and the southeastern suburbs of Christchurch on February 13.

That evening Askin was among those who helped survey the extent of the fires. The next day he flew from Rangiora to join the firefighting operation and was appointed the lead pilot for one group of helicopters because of his experience.

His total helicopter flying experience was about 2350 hours, of which about 80 hours was firefighting experience.

According to the Transport Accident Investigation Commission report, released today, several pilots heard an abbreviated mayday call about 2pm that day, but it was not clear on which radio frequency the call was made.

The supervisor asked for a roll call of all the aircraft involved, but Askin did not respond.

After a brief search, another pilot in the group found the helicopter wreckage on a steep slope near the head of a gully east of the Sugarloaf. Askin had died in the crash as he returned to the dipping pond to refill the monsoon bucket.

Evidence showed that the monsoon bucket suspension cables struck the tail rotor, damaging it and causing the loss of the entire vertical stabiliser from the tail boom, the report said.

The helicopter then rolled to the right and crashed into the ground.

The damage to the tail rotor blades was consistent with them striking the wire suspension cables of the monsoon bucket where the cables joined the lifting strop. There was corresponding damage on the cable cluster.

A digital video camera mounted under the forward fuselage, pointing down and back towards the monsoon bucket, recorded the entire flight. The video showed the bucket had flown up towards the tail while the helicopter was returning to the dipping pond.

The video also showed the bucket had not touched the ground before the tail rotor strike. After the vertical stabiliser detached, the helicopter had entered a gradual descending right roll until it struck the ground.

The report said underslung loads flying back into the tail rotor was a known risk to helicopter sling operations.

In 41 per cent of the crashes reported in the past six years, the cause was the sling or cargo attached the helicopter's hook striking the helicopter, according to Eurocopter.

The commission recommended the Civil Aviation Authority use the interim report, a service letter from Eurocopter on the dangers of carrying items below the aircraft on a sling and any other pertinent material to remind the aviation industry of the lessons learned from accidents involving sling loads, in particular the use of monsoon buckets. Askin was a member of the New Zealand Special Air Service (NZSAS) and was an active member of the SAS reserves until his death.

He was wounded in a five-hour shootout on June 29, 2011 after the Taliban stormed the InterContinental Hotel in Kabul.

In 2014, he received the NZ Gallantry Star for his fighting efforts in Afghanistan.