The Australian Transport Safety Bureau (ATSB) has released a preliminary report detailing how two pilots managed to escape a Boeing 737 firefighting aircraft after it crashed in Western Australia.
The Boeing 737-300, registered as N619SW, belonging to United States (US)-based company Coulson Aviation, crashed in Fitzgerald River National Park, near Ravensthorpe on February 6, 2023.
While the aircraft impacted the terrain at the park and was later destroyed in a post-impact fire, the two pilots suffered only minor injuries and were able to escape the accident site, the ATSB noted.
“Despite extensive fire damage, ATSB recorders specialists in our Canberra technical facilities were able to download files from both the flight data recorder [FDR] and cockpit voice recorder [CVR],” stated Angus Mitchell, the Chief Commissioner of the ATSB.
Crashed during a second fire retardant drop
The crew of the Boeing 737, callsign Bomber 139, had completed two previous flights at the same location and were carrying out their second fire retardant drop on their third flight of the day when the incident occurred.
The Boeing 737 arrived at Busselton Airport (BQB) on December 13, 2023, where it was based before the accident.
On the day of the crash, the report noted that the WA Department of Biodiversity, Conservation, and Attractions (DBCA) of Australia requested fire suppression assistance from the State Operations Air Desk (SOAD), asking for “fixed-wing assets” under the criteria of “known high fuel loads and likelihood of excessive ROS [rate of spread] and/or extreme fire danger”.
Responding to the request, the SOAD identified all aircraft that would be able to help, established a fire common traffic advisory frequency (F-CTAF), briefed the pilots, and sent task messages at 10:12 AM, 11:27 AM, 14:07 PM, and 15:05 PM local time (UTC +8). Bomber 139 responded to the second, third, and fourth taskings, with the aircraft departing and returning to BQB twice on the day.
According to the ATSB, the Boeing 737 departed for its third flight of the day at 15:30 PM local time (UTC +8). 11 minutes prior to this, the bird-dog (BD) responsibilities had been transferred from BD125 to BD123, as the former had to refuel.
Per the ATSB, a BD is an aircraft used to assess the fire ground, determine the best flight path, and lead firefighting aircraft to locations where they can drop retardant with a smoke generator.
Bomber 139 contacted BD123 15 minutes from the fire site, and the crew was told to use an altimeter barometric pressure subscale setting (QNH) of 1003 hPa and to contact BD682 five minutes before arriving at the location of the fire, with BD682 clearing the Boeing 737 to enter the F-CTAF and to not fly above 2,500 feet (762 meters). The first officer of the flight told the captain that the target drop airspeed (Vdrop) was 133 knots, with the crew requesting BD123 to lead the way to the drop site.
Having successfully conducted the first retardant drop, the crew began to prepare for another circuit, completing another pre-drop checklist. The ATSB report stated that for the second drop, the Vdrop was 118 knots and BD123 told the pilots of Bomber 139 to “tag and extend all existing retardant, it is start at the hill as it pushes down, target altitude 500 [ft], descending 400 [ft]”. The Boeing 737’s crew set flap-40 and proceeded to follow BD123.
The ATSB continued: “During the second drop, Bomber 139 descended through 400 ft altitude (80 ft radio height) at about 110 kt computed airspeed and 30% N1 (engines at high idle) as the retardant line was extended downslope.” The captain of the Boeing 737 advancing the throttle levers 2 seconds before the aircraft’s peak of descent peaked, also pitching the nose upwards.
However, the ATSB pointed out that since the nose-up movement was done before the advancement of the throttle levers, the aircraft experienced “a reversal of the rate of descent, but also a decay of the airspeed”. Shortly after, the stick shaker of the aircraft activated, indicating a potential aerodynamic stall, and the Boeing 737 went through “an abrupt vertical acceleration associated with the aircraft impacting a ridgeline at an elevation of about 222 ft at 104 kt computed airspeed with the engines at 85-89% N1”.
Airlifted from the crash site of Bomber 139
Despite a fire igniting immediately after the crash, the pilots managed to successfully escape the Boeing 737 and were airlifted from the site.
“After the impact with the ridgeline, the aircraft cleared a small line of foliage before impacting the ground a second time and then sliding to rest,” the ATSB stated in the report, adding that BD123 immediately made an all-stations mayday call. Once the Boeing 737 was still, the first officer began the evacuation checklist. However, since the cabin door was “buckled” and the right-hand side window of the cockpit was stuck, the captain managed to open the left-hand side window on his second attempt. Shortly before, the commander of the aircraft saw that a post-impact fire had started.
“Both pilots then exited out of the left window and moved clear of the wreckage and fire. They were subsequently rescued by a helicopter involved in the fire control activities after 2 single-engine air tankers had dropped retardant on the aircraft fire believing the crew were still inside,” the ATSB report added.
The preliminary report also noted that the first officer did not notice any deviations during the second drop and accident sequence, later telling the investigators that “their focus of attention was likely on the airspeed indicator and radio altimeter, monitoring for any adverse trends”.
“About 2 seconds prior to impact, at a radio altitude of about 28 ft with the flaps at 40º, an increasing aircraft nose-up pitch attitude resulted in the vane angle of attack exceeding 20°, which triggered the stick shaker,” the ATSB said, adding that the Boeing 737 began climbing “just before impact with terrain”.
Following the preliminary report, the Australian authorities will look into the communication between BD aircraft and firefighting aircraft pilots, Coulson Aviation Crew Resource Management (CRM) procedures, and standards as well as safety margins for the Boeing 737 Fireline retardant drop.