An investigation has found a combination of factors, including an unnecessary crewman and a steep, high-speed descent contributed to a $30 million Skycrane helicopter crashing into a dam during firefighting operations in country Victoria last year.

Key Points: The report found the chopper lost lift capacity while re-filling from a narrow and steeply sided dam

The report found the chopper lost lift capacity while re-filling from a narrow and steeply sided dam Three crew swam to safety through dark, fuel-contaminated water as the $30 million helicopter sank

Three crew swam to safety through dark, fuel-contaminated water as the $30 million helicopter sank Underwater escape training played a critical role in saving their lives, the report found

Two pilots and an engineer from Canada and the US swam to safety when the Sikorsky S-64E Skycrane, dubbed "Christine", crashed into narrow, steeply sided Wood Creek Dam near Jericho, Gippsland, on January 28 last year.

They were water-bombing bushfires west of the Thomson Dam at Aberfeldy when they crashed while refilling the Skycrane's water tank at 7.08pm.

One of the men was seriously injured. The other two were unhurt.

An Australian Transport Safety Bureau [ATSB] investigation sets out the dramatic moments leading up to the chopper slamming into the dam and the crew's frantic swim to safety through dark, fuel-contaminated water.

Crash training was critical to their survival, says the ATSB report.

"They all waited until the last moment to draw a breath and did not unbuckle and attempt to exit the helicopter until motion had ceased," it says.

The main pilot had to swim across the cabin and was helped out of the right door by the second pilot.

The rear door was jammed so the third man had to push out a window.

"All three crew successfully exited the aircraft, inflated their life jackets and swam to shore," the report says.

"The crew reported that it was not possible to see anything underwater and that jet-fuel contamination was present."

Narrow, steep, tall trees

The pilots had 62 years flying experience between them and took turns as main pilot every two hours.

Their "dip point" was a dam, seven kilometres west of the fire front at the eastern base of Mt Gregory in the Yarra Ranges National Park.

"(The area) had a narrow body and steep sides surrounded by tall trees," the ATSB report stated.

The helicopter had to hover above the dam for 45 seconds while a snorkel drew water into the tank.

The ATSB found the crew calculated how much water the helicopter could carry and then drew 91 kilograms less than that, to ensure they had better lift capability to rise up out of the confined space

But the report found that during the afternoon, the crew were sent to fight a new fire, and changed their flight path, shortening their approach to the dam.

The year long investigation by the ATSB found the pilots shortened their approach to the dam, creating a steeper descent to fill up. ( Supplied: Australian Transport Safety Bureau )

Chopper's tail hit water

"A witness to the event reported that the aircraft had an apparent high rate of descent and a nose-high attitude," the report says.

The chopper's tail struck the water, snapped off and the aircraft fell into the dam.

"The crew reported that they did not feel that any of the parameters were excessive, though speed and angle of bank were felt to be at the higher end of their normal range."

The crew said once they descended below the tree line, the aircraft generated no lift and fell into the water, stating that they had very likely encountered vortex ring state (VRS).

"The topography, high rate of powered descent, and steep flare that reduced the airspeed, created conditions conducive to the onset of VRS," the ATSB found.

It said the crew reported the tight operating area around the dam "did not provide enough time or space to manoeuvre sideways to effect a recovery".

Another crew flying overhead saw the crash and the trio walked through dense bush to a road where they were met by their rescuers.

Flying conditions on the day were favourable, with light northerly winds, mild turbulence and visibility greater than 10 metres.

The helicopter was one of 10 water bombers in action against the "Thomson catchment complex" fires, burning just north of Mount Baw Baw, about 100 kilometres east of Melbourne.

Extra man on board

The $30 million Skycrane was salvaged from the dam in Gippsland last year after crashing in January 2019. ( News Video )

The ATSB found the crew chief was not required to be on board the helicopter during firefighting operations and found his presence on board exposed the crew to unnecessary risks.

" While the Crew Chief had significant system and task knowledge, he was not required to be on board the helicopter."

Since the incident, Erickson, the company that operated the helicopter, had emphasised vortex ring state avoidance and recovery in training and banned non-essential personnel during firefighting operations.

The ATSB said the crash demonstrated the value of Helicopter Underwater Escape Training (HUET) in saving lives.

It said its findings should not be read as apportioning blame or liability to any organisation or individual.