A Boeing 737 firebombing aircraft which crashed in Western Australia's south earlier this year hit a ridge line while dropping retardant, a preliminary investigation has revealed.
The Large Air Tanker, a civilian aircraft that was converted for firefighting and operated by Coulson Aviation, had been helping crews fight a bushfire in the Fitzgerald River National Park when it crashed on February 6.
At the time, the Australian Transport Safety Bureau (ATSB) confirmed it was the first time a 737 had been "lost" in Australia.
Both pilots escaped with minor injuries before most of the huge plane was destroyed by fire, with Premier Mark McGowan describing their survival as a "miracle".
Investigators from the ATSB found the pilots attempted to pitch the plane up from the low-altitude drop before its engines could accelerate, seconds before it hit the ridge line.
"Flight recorder data shows the throttles were advanced and the engines had accelerated just before the aircraft struck a ridge line with the stick shaker activating," ATSB Chief Commissioner Angus Mitchell said.
"The aircraft then cleared a small line of foliage, before impacting the ground a second time and sliding to rest.
"Fortunately, and remarkably, both pilots were able to exit the aircraft through a cockpit window, and suffered only minor injuries."
In its preliminary report, the ATSB said the two pilots had been tasked with tagging and extending an existing retardant line, with a target altitude of 500 feet descending to 400 feet.
Pilots made mayday call
Flight data shows the air tanker emptied three-quarters of its load, before the captain stopped the drop because the retardant line was entering a burnt area.
The pilots then repositioned the aircraft to drop the remaining retardant, which was when the accident occurred.
The report found the co-pilot did not announce any deviations during the accident and later reported they were likely focused on the air speed indicator and radio altimeter.
After they hit the ridge, the aircraft cleared a small line of foliage before impacting the ground a second time and then sliding to rest, with the crew issuing an all-stations mayday call.
Cabin door stuck
"Both pilots were unable to open the cabin door as it had buckled and the co-pilot was unable to open the right-side window," the report read.
Realising the plane was on fire, the captain managed to open the left side window on a second attempt, allowing the pair to escape.
Crews believed the pilots were trapped inside the aircraft and dropped retardant on the tanker, before the duo were rescued by helicopter with minor injuries.
Despite the extensive fire damage to the aircraft, the ATSB was able to download files from both the flight data recorder and cockpit voice recorder.
Investigations are continuing and a final report will be prepared.
"Should a critical safety issue be identified at any time during the course of the investigation, the ATSB will immediately notify relevant parties so appropriate and timely safety action can be taken," Mr Mitchell said.
The ATSB said Coulson Aviation had increased its large air tanker minimum retardant drop heights and airspeeds as a result of the preliminary findings.
Opposition calls for review into firebombers
WA Shadow Emergency Minister Martin Aldridge called for "deep consideration" from the government and regulator over the use of large aircraft to fight bushfires.
"Operating these large air tankers in these types of conditions isn't without risk," he said.
"That's why the opposition has been calling for the government and indeed, the Civil Aviation Safety Authority (CASA) as the regulator, to review the operation of firebombing aircrafts, particularly large air tankers in an Australian context."
Mr Aldridge said he was yet to hear a response from CASA since the crash.
"It will be interesting to see, noting that Coulson themselves have adjusted their operating conditions, whether or not CASA will reinforce those with regulations to ensure that similar aircraft operating within Australian jurisdictions remain safe over fire grounds."
Transport Minister Rita Saffioti said she had not been fully briefed on the preliminary report, but acknowledged the "inherent risks" involved in aerial firefighting.
She said the WA government would "continue to work with the federal government in relation to the safety of our skies".
"In particular, the safety of aerial firefighting, which is now an essential part of managing fires across the state," she said.
