Key findings from the preliminary investigation into the mid-air helicopter collision that killed four people on the Gold Coast on January 2.

THE LEAD-UP

* Two helicopters were operating five-minute scenic flights from separate helipads about 220m apart near Sea World.

* At 1:52pm, helicopter XH9 pilot Michael James makes an inbound radio call to alert that he is landing.

* Ground crew do not see any other aircraft and signal for XKQ helicopter pilot Ashley Jenkinson to take off.

THE SECOND HELICOPTER

* Mr James sees passengers being loaded into XKQ and the doors closing, thinking it will pass behind his aircraft.

* He doesn't hear any outbound taxi radio call from Mr Jenkinson or see XKQ approaching.

* Mr James focuses on avoiding creating downwash for a boat passing below as he makes a final approach to the helipad.

THE COLLISION

* At 1:56pm, two passengers notice Mr Jenkinson's helicopter approaching and one tries to tell Mr James. After he doesn't respond, one passenger starts tapping his shoulder.

* Mr James later remembers being alerted by passengers but still doesn't see the other chopper.

* About 23 seconds after Mr Jenkinson takes off, the helicopters collide 40 metres off the ground.

IMPACT

*The main rotor blades of Mr Jenkinson's helicopter XKQ strike the windscreen and forward cabin of XH9.

* XKQ breaks apart in the air and crashes into shallow water next to a sandbar.

* Mr Jenkinson and three passengers are killed with three other passengers seriously injured. The helicopter is destroyed.

* The cabin, instrument console and main rotor blades of helicopter XH9 are significantly damaged, but Mr James is able to land on the sand bar.

* Mr James and two passengers are seriously injured, while other passengers sustain minor injuries.

OTHER FACTORS

* The helicopters were too low for air traffic control to guide them so they were relying on radio calls and sight.

* The surviving pilot didn't hear the other pilot make a standard outbound taxi call before he took off.

* A third pilot heard the surviving pilot make an inbound call, but did not hear the second pilot.

* The surviving pilot didn't see the other helicopter before the collision.

* Both helicopters were fitted with a traffic collision avoidance system, but they could only issue audio alerts. The surviving pilot said he did not hear one before the collision.

* XH9 was not transmitting secondary surveillance radar responses at the time of the flight.

DEATHS AND INJURIES:

* Pilot Ashley Jenkinson, 40, British couple Ron and Diane Hughes, and Sydney mother Vanessa Tadros, 36, were killed.

* Vanessa's son Nicholas, 10, is recovering in hospital after having his leg partially amputated, while Victorian mother Winnie de Silva, 33, and her nine-year-old son Leon were also seriously injured.

* Pilot Michael James, and Zealand women Marle Swart and Elmarie Steenberg suffer shrapnel injuries, while their husbands Edward Swart, Riaan Steenberg, and an unnamed 27-year-old West Australian woman were not physically injured.

(Source: Australian Transport Safety Bureau)