An all-American flight crew was on board the RFS air tanker which crashed in NSW on Thursday.

Three US firefighters, who had flown to Australia to assist with bushfire relief efforts, died when the plane plummeted into dense bushland near the Snowy Mountains township of Peak View.

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RFS Commissioner Shane Fitzsimmons described the trio as “absolute professionals”.

“Our thoughts and prayers are with the three occupants who routinely crew that specialised aircraft,” he said.

Lost contact

The NSW Rural Fire Service lost contact with the Hercules C-130 in the Snowy Monaro area just before 1.30pm.

Fitzsimmons says the aircraft “was operating as it routinely does with waterbombing activities”.

The plane had been chartered from a Canadian firm, Coulson Aviation, to undertake bushfire relief work for several years.

The NSW Rural Fire Service lost contact with the C-130 Hercules just before 1.30pm. Credit: Flight Aware

The Coulson Family released a statement saying their “thoughts and prayers are with the families of the three crew members on board”.

“The accident is reported to be extensive.”

The firm is in the process of contacting the victim’s families and has grounded all large air tankers as a precaution.

Difficult terrain

RFS Commissioner Shane Fitzsimmons said it took “some time” for ground crews and aerial surveillance to locate the wreckage which is surrounded by difficult terrain and visibility.

Witnesses say the aircraft was a fireball as it plummeted into dense bush in the Snowy Monaro.

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7NEWS has been told the plane broke up on impact but the tail section remained intact.

Flight radar shows it took off from Richmond Airbase at 12.05pm and was due to return two hours later.

The aircraft plummeted from an altitude of around 1,600m, hitting the ground at over 285km per hour.

At this stage the cause of the crash is unknown.

The Hercules C-130 water bomber is Coulson’s leading fire attack aircraft with a 15,000-litre capacity.

Investigation

The Australian Transport Safety Bureau (ATSB) said it will investigate the incident and “analyse available recorded data, review weather information, and interview any witnesses”.

“The ATSB is deploying a team of transport safety investigators with experience in aircraft operations, maintenance and data recovery to the accident site, to begin the evidence collection phase of the investigation.”

It will release a preliminary report in approximately 30 days.

‘Ever grateful’

“We are mourning the three American firefighters who died in the Large Air Tanker crash in the Snowy Monaro area today,” Minister for Foreign Affairs Marise Payne said in a statement released on Thursday evening.

“Our hearts go out to their loved ones.

“They were helping Australia, far from their own homes, an embodiment of the deep friendship between our two countries.

Chevron Right Icon ‘They were helping Australia, far from their own homes.’

“I have conveyed Australia’s condolences to the United States Ambassador, Arthur B. Culvahouse.

“Thank you to these three, and to all the brave firefighters from Australia and around the world.

“Your service and contribution are extraordinary. We are ever grateful.”

‘Thoughts and prayers’

The NSW Premier Gladys Berejiklian said her “thoughts and prayers and heartfelt condolences go to their families”.

The public also shared their condolences for the three lost.

“They left their homes and flew across the world to defend someone else’s country,” one man said.

“Just devastating that they’ll never come home.”

Fitzsimmons says the crash is a “confronting and sobering reminder of the inherent dangers and risk associated with firefighting”.

“The firefighting fraternity as we keep saying is a tight-knit family, a fairly small family and the crew on board were well known, not just to their colleagues here in Australia.”

More than 1700 volunteers and personnel remain in the field with 80 fires, four currently at an emergency warning level, still burning in NSW.