ATSB report details sequence of events leading to 737 accident in Western Australia

An ATSB preliminary report has detailed the sequence of events leading to a Boeing 737 large air tanker’s impact with a ridgeline in Western Australia’s Fitzgerald River National Park while conducting an aerial fire-fighting task on 6 February 2023.

The preliminary report outlines factual information collected in the on-going investigation’s early evidence collection phase, and details that the modified Coulson Aviation-operated 737 air tanker, callsign ‘Bomber 139’, with two pilots on board, had departed from Busselton Airport to assist fire control efforts near Hopetoun, about 600 km south-east of Perth.

“Arriving at the fire ground, the 737 crew was briefed by a bird-dog aircraft that the tasking was to tag and extend an existing retardant line,” said ATSB Chief Commissioner Angus Mitchell.

“The retardant line was to extend downslope, with the bird-dog briefing the 737 crew that their target altitude (above sea level) was 500 feet descending to 400 feet.”

Flight recorder data shows that Bomber 139 descended to about 400 ft and completed a partial drop of three-quarters of their tank before the captain, who was pilot flying, stopped the drop because their retardant line was entering a burnt area.

Bomber 139 then repositioned to commence another drop to use the remaining retardant to further extend the retardant line.

During the second drop, Bomber 139 descended through 400 ft altitude – or 80 ft above ground height – at an airspeed of 110 kt with the engines at high idle as the retardant line was extended downslope.

“Flight recorder data shows the throttles were advanced and the engines had accelerated just before the aircraft struck a ridgeline with the stick shaker activating. The aircraft then cleared a small line of foliage, before impacting the ground a second time and sliding to rest,” said Mr Mitchell.

“Fortunately, and remarkably, both pilots were able to exit the aircraft through a cockpit window, and suffered only minor injuries.”

The aircraft was subsequently consumed in a post-impact fire.

“Despite extensive fire damage, ATSB recorders specialists in our Canberra technical facilities were able to download files from both the flight data recorder and cockpit voice recorder,” Mr Mitchell said.

Key points:

• 737 had been tasked to tag and extend a line of fire retardant which extended downslope
• Flight data and cockpit voice recorder downloads instrumental in establishing accident sequence of events
• Preliminary report outlines factual information collected in the on-going investigation’s early evidence collection phase

“As well as information from those recorders, interviews with the flight crews, a 3D map of the accident site created using a drone, and other recorded flight information will be instrumental to the ongoing investigation.”

ATSB investigators have also collected documents and recorded data from the operator as well as records from the WA Department of Fire and Emergency Services.

“The investigation is continuing and will include validation of the recorded data and reviews of the communication procedures for bird-dog and large air tanker pilots, the operator’s crew resource management procedures and practices, and standards and safety margins for 737 retardant drops.”

The preliminary report notes that following the accident, the operator increased their large air tanker minimum retardant drop heights and airspeeds.

Mr Mitchell said that a final report, containing findings and the analysis to support those findings, will be released at the conclusion of the investigation.

“However, 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.”