San Francisco  - A probe was launched Thursday into the deadly helicopter crash that killed eight firefighters and a pilot Tuesday night, as a survivor said a rotor hit a tree branch as the aircraft was taking off from a small forest clearing.

The crash of the Sikorsky S-61 was the deadliest firefighting incident in the US since 1994 when 14 firefighters were killed in a Colorado wildfire.

It came amid one of the most devastating fire seasons on record in California where over 450,000 hectares have already burnt even though the peak fire months are still to come.

Survivor Richard Schroeder, 42, told the Los Angeles Times that it seemed that the helicopter's rotor hit a tree as it was taking off.

Schroeder said someone screamed a warning and the helicopter pitched forward and plummeted. He blacked out on impact, and when he came to, a body was lying on him and the tail of the aircraft was on fire. Schroeder exited through a partially broken window and scrambled up a hillside before the helicopter exploded.

The crash came three months after the Federal Aviation Authority issued an "airworthiness directive" regarding Sikorsky S-model helicopters after one developed fatigue failures in the main rotor shaft, the paper reported.

Bob Madden, an executive of Carson Helicopters which operated the crashed aircraft, said the S-61 helicopter was inspected twice a day as part of its agreement with the Forest Service and was in good shape. (dpa)