The air safety watchdog has released its preliminary report into a fatal helicopter crash that killed four people near Sea World on the Gold Coast Broadwater in January.
Here's what we know from the Australian Transport Safety Bureau (ATSB) about how the crash happened.
What were the helicopters doing?
Both aircraft were operated by Sea World Helicopters, a separate entity to the theme park.
They were conducting scenic flights from the company's base at Sea World, from two distinct helipads about 220 metres apart.
At the time of the collision both helicopters were on scheduled five-minute flights — one returning and the other taking off.
ATSB chief commissioner Angus Mitchell said the helicopters were operating outside controlled airspace, flying below 1,500 feet and both following the same counter-clockwise flight route.
How did they collide?
As the returning helicopter began to fly north along the Gold Coast Broadwater, its pilot made a radio call stating the aircraft's position and intended track.
At the same time passengers on the departing helicopter were being loaded into the aircraft by ground crews.
The pilot of the returning helicopter told the ATSB investigation he had seen passengers being loaded into the departing helicopter on his return track and had seen its doors closing.
Mr Mitchell said the pilot testified that he had not heard a taxi call from the departing pilot announcing his intention to take off.
"The pilot of the returning helicopter recalled that their assessment was that the departing helicopter would pass behind them," he said.
"And that they did not recall the pilot of that helicopter making a standard taxiing call.
"This does not necessarily mean that a taxiing call was not made."
The ATSB report says the helicopters collided at 1.56:05pm without the pilot of the returning helicopter ever seeing the departing helicopter taking off.
What about the voice recordings and black boxes?
Because the flights were being operated outside controlled airspace neither helicopter was required to report to any external flight controller.
Instead, the pilots were communicating with each other on a Common Traffic Advisory Frequency (CTAF).
The frequency the pilots were using was also used by the Southport Aerodrome which was recording communications but not all taxi calls from helicopters at Sea World were recorded.
The ATSB report says this is consistent with the helicopters being at low altitude.
Mr Mitchell said the helicopters were not fitted with the same kind of black boxes often used in investigations into aeroplane crashes.
"We have no recording devices on the actual helicopters of the calls," he said.
"Some of those calls have been recorded at Southport at the tower but that tower sits behind some of those tall buildings and only picks up some of those calls at a certain altitude.
"We're yet to determine exactly what calls were made … but that'll be part of our investigation."
What about radar?
Sea World Helicopters was not required to equip its helicopters with what is known as an Airborne Collision Avoidance System (ACAS), but both were fitted with one known as a traffic collision avoidance system (TCAS).
Pilots reported, though, that the TCAS between the two helicopters was not fully integrated and had no visual information.
Neither pilot could see on a screen, for example, a digital representation of the other aircraft on a map.
The TCAS in use on the helicopters was giving an audio alert, but the surviving pilot testified he did not hear an alarm prior to the collision.
The ATSB investigation's review of the helicopter's movements showed the departing helicopter's transponder was correctly transmitting its altitude and location but the returning helicopter's transponder was not working correctly on the day of the crash.
Mr Mitchell said because the helicopter was operating outside controlled airspace its transponder was not required to be working to be air-worthy.
"There are different requirements when you go into controlled airspace and we're yet to determine all the functionality of particularly systems like TCAS that were onboard those helicopters," he said.
"That's part of what we will now be looking at — the full functionality and how they were being used."
What information is the ATSB using in its investigation?
As well as examining the wreckage of both helicopters, the ATSB has already conducted interviews with many people who were involved in or witnessed the accident.
They include the surviving pilot and adult passengers, eyewitnesses and other pilots flying in the area around the time.
Ground crew, maintenance personnel and pilots and operators conducting similar helicopter operations have also been interviewed to assist the ATSB's investigation.
Video footage and images taken by passengers on the aircraft, onlookers and captured on CCTV is also forming part of the investigation.
Mr Mitchell said the policies, procedures and records of Sea World helicopters were being reviewed as part of the investigation, as was the regulatory surveillance of the operator.
"One of the avenues that we'll be exploring is the procedures that were in place here and the regulatory oversight," he said.
"They were operating under a certificate and that was reviewed in July of 2022 for five years, so that will be part of what we look at."
When will we know more?
The ATSB's final report into what happened is not anticipated until the second half of 2024.
Mr Mitchell said the preliminary report only detailed the facts that the ATSB had established in the first two months of its investigation.
"We are yet to analyse all the factors that potentially contributed to this tragedy," he said
"Nor are we yet to determine any safety issues that may need to be addressed to lessen the likelihood of a re-occurrence in the future."
Sea World Helicopters remains closed.
The company said they had seen the report and would "take time and due care to review the findings".
