A report from the Australian Transport Safety Bureau found the Lockheed Martin EC-130Q large air tanker crashed on January 23, 2020 after it aerodynamically stalled, having very likely been subjected to hazardous environmental conditions including windshear and an increased tailwind.

The crash killed crew members Ian H McBeth, 44, Paul Clyde Hudson, 42, Rick A DeMorgan Jr, 43, who were employees of Coulson Aviation, a Canadian aerial firefighting company that operated the air tanker under contract to the Rural Fire Service.

A report published on Monday found the RFS tasked the air tanker to fight a fire at Adaminaby without aerial supervision, despite hazardous conditions in the Snowy Mountains including strong gusting winds and mountain wave activity causing turbulence and likely exacerbated by the fire and local terrain.

Local aerial operations had ceased due to hazardous conditions, a birddog pilot had declined the tasking due to the forecast, and a Boeing 737 crew had reported that conditions stopped them from returning to the fire-ground.

The RFS relied on the large air tanker pilot in command to assess the safety of the tasking, but did not provide him all the available information to make an informed decision, the transport safety bureau's report said.

"The crew of N134CG were therefore very likely not aware that the birddog pilot had declined the tasking to the Adaminaby fire-ground, nor that the smaller fire-control aircraft had ceased operations in the area, due to the hazardous environmental conditions," the report said.

"While this was only one risk factor among others that would be considered by the crew when accepting a task, having this information would have allowed them to make a more informed decision about the weather conditions."

Australian Transport Safety Bureau chief commissioner Angus Mitchell, speaking at a press conference on Monday, said the report recommended that if such information is available, it should be made available to the tasking agency or aircraft to make an informed decision.

The report found the pilot accepted the tasking to fight the Adaminaby fire, however after assessing the conditions there as unsuitable, the crew accepted an alternative tasking to continue about 58km east to the Good Good fire-ground at Peak View, which had the same weather conditions.

The transport safety bureau said the decisions to accept the taskings were consistent with Coulson Aviation's practices to depart and assess conditions to find a workable solution, rather than rely solely on a weather forecast.

At the Good Good fire-ground, north of Cooma, the tanker performed a partial retardant drop and left turn, but was very likely subjected to low-level windshear and an increased tailwind.

The transport safety bureau said that, based on a combination of witness video, and real-time position and flight path data, it was established the aircraft's climb performance degraded before it crashed.

Its investigation found that while a safety management system was not required under Australian regulations at the time of the crash, Coulson Aviation's safety risk management processes did not adequately manage the risks associated with large air tanker operations.

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It had not conducted formal risk assessments of the hazards identified in its operations manual, and did not maintain tools to track risk acceptance levels, the report said.

Incident reports submitted through the safety reporting system were mainly related to maintenance issues, and therefore operational risks were less likely to be considered or monitored.

"This limited their ability to identify and implement control measures to manage the risks associated with their aerial firefighting operations," the transport safety bureau said.

The bureau also found that the RFS had limited large air tanker policies and procedures for aerial supervision requirements, and no procedures for deployment without aerial supervision.

"In addition, they did not have a policy or procedures in place to manage task rejections, nor to communicate this information internally or to other pilots working in the same area of operation," the report said.

"Such policies and associated procedures would provide frontline personnel with the required steps to effectively and safely manage taskings, and provide guidance for decision-making."

As raised in the bureau's interim report, it found the aircraft's cockpit voice recorder did not record the flight.

The bureau recommended Coulson Aviation consider fitting a windshear detection system to its C-130 aircraft to minimise the time taken for crews to recognise and respond to an encounter, and to incorporate foreseeable external factors into its pre-flight assessment tool to ensure crews can consistently assess the overall risk profile of a tasking.

It recommended the NSW RFS address the absence of policies and procedures for personnel to effectively manage and communicate task rejections on the basis of safety concerns, and regarding minimum aerial supervision requirements.

Mr Mitchell said responsibility for safe aerial firefighting had to be shared between the tasking agency and aircraft operator.

"This accident highlights the importance of having effective risk management processes, supported by robust procedures and training to support that shared responsibility," he said.

"As part of this investigation we have sought to understand the risk mitigations in place at the time of the accident, and have identified a number of safety issues that if resolved through actions will further mitigate risks for large air tanker aerial firefighting in the future."

Mr Mitchell welcomed the actions and commitments already made by the RFS and Coulson Aviation to improve safety.

Coulson Aviation has introduced a pre-flight risk assessment tool, a new three-tiered risk management approach, and windshear procedures and training.

The RFS has committed to a review of its aviation doctrine and to undertake detailed research to identify best practice relating to task rejection and aerial supervision policies and procedures, as well as initial attack training and certification.

In a written statement released with the report, Mr Mitchell also said the bureau recognised the critical importance of aerial firefighting where aircraft were flown at low altitudes and low airspeeds often in challenging conditions, in managing bushfires.