A rigging supervisor's decision to pull on a lifting line led to the spectacular helicopter crash at Auckland's Viaduct on November 23, 2011, the Civil Aviation Authority says in a preliminary report out today.
Saying that the helicopter suffered "massive out-of-balance forces", the CAA report essentially confirms what everybody saw happening as pilot Greg Gribble tried to lift the star on to the top of a Christmas tree and instead one of the three blades on his Aerospatiale helicopter hit a wire.
He crashed but there were only minor injuries to Gribble and the ground crew.
The report said Gribble made the first lift of gear on the artificial tree well, until he had to release the lifting line from the helicopter's hook.
The report does not name anybody, but the rigger beneath the helicopter, Scott Anderson, was identified at the time.
CAA said Gribble was talking with the rigger underneath the helicopter on the radio.
"When the helicopter hovered at approximately five metres above the ground, the rigging supervisor was seen to jump up and grab the lifting line which was sagging below the helicopter," the report said.
"The act of pulling downwards on the lifting line to release it from the helicopter's hook instantly tightened the lifting line, which was still attached to the top of the adjacent tower, and the lifting line came into contact with the main rotor blades.
"The force of the impact of the main rotor blades on the lifting line caused massive out-of-balance forces within the helicopter's rotating components, which resulted in the loss of the structural integrity of the helicopter whilst in flight."
The CAA said the helicopter "then fell to the ground."
Once the helicopter came to rest Gribble was extracted from the wreckage almost immediately, by ground personnel standing close by.
The investigation showed the main rotor had been sheared by the impact forces and the main rotor transmission and the tail boom had separated from the helicopter.
The pilot's seat had separated from the helicopter during the accident.
The CAA said it was continuing to investigate the accident and in its final report would consider if preventive measures could have been used.
It would also consider the pre-flight safety briefings, the pilot's experience and training, human factors in the crash and health and safety aspects of the helicopter lifting operation.