Shades of Always
On 6 February 2023, a Boeing 737-3H4 aerial tanker owned by New South Wales-based Coulson Aviation—a company specializing in the provision of aircraft and aircrews for aerial firefighting, intelligence, emergency personnel/cargo transport, and industrial heavy-lift operations—went down in the Great Southern Region of Western Australia’s Fitzgerald River National Park while fighting multiple fires.
The aircraft’s sole occupants—its two pilots—survived the harrowing accident and were transported to hospital with minor injuries. The instance occasioned the first hull loss of a Boeing 737 on the Australian continent.
On 04 May 2023 the Australian Transport Safety Bureau (ATSB) made public its primary report on the accident. Subject report, while containing neither analyses, findings, nor definitive conclusions, details facts established by investigators during the investigation’s evidence collection phase, and provides timely information to the industry and public.
The accident aircraft departed Busselton Margaret River Airport (BQB) at 12:08 AWST (Australian Western Standard Time/UTC+8) on the first of three planned missions to fight wildfires burning in the vicinity of Hopetoun—a coastal town some 319-nautical-miles southeast of Perth, Western Australia’s capital city.
En-route to the fires, the 737 climbed to FL290 before descending to approximately seven-hundred-feet for purpose of discharging fire-retardant over the fire-zone.
The aircraft returned to BQB, took on a fresh load of fire-retardant, and departed on the day’s second mission at 13:50 AWST.
Returning to the fire-zone, the aircraft again discharged fire-retardant and returned to BQB, landing at 15:08 AWST.
The 737 departed on a third sortie, rising from BQB’s runway at 15:32 AWST and proceeding some 220-nautical-miles southeast toward the fire-zone. Upon arriving over the blaze, the aircraft performed a split-run, dropping half its load of fire-retardant in one area and the remainder elsewhere. While executing the run’s second pass, the 737 struck a ridge and “pancaked” into terrain. Despite the violence of the impact and a post-crash fire that destroyed all but the 737’s forward fuselage section, the two, reportedly Canadian, pilots escaped the burning wreckage and were rescued by helicopter. Assessed in hospital, the two were found to have suffered only minor injuries.
The downed aircraft was a 28-year-old Boeing 737-300, registration N619SW, serial number 28035, delivered new, by the Boeing Company, to Southwest Airlines in November 1995. In August 2017, having accumulated 69,016 hours in commercial service, the aircraft was retired by Southwest. On 08 August 2017, ownership of the 737 was transferred to Coulson. The aircraft’s last certificate of registration was issued 14 November 2017 and was valid until 30 November 2023.
On 10 November 2018, Coulson Aviation was issued a supplemental type certificate (ST04050NY) for the installation of the Coulson Aerial Firefighter Tanker Modification to Boeing model 737-300 series aircraft, which limited passengers to persons essential to firefighting activities. The tanker modification was completed on 30 May 2022. At the time of the accident, the airframe had accumulated 69,187.6 hours.
On the day of the accident, a low-pressure trough was transitioning southern Western Australia, spawning widespread thunderstorms and wind direction changes. The graphical area forecast for the accident-site included moderate turbulence from the surface to ten-thousand-feet with thermals. A SIGMET was issued at 14:00 AWST for frequent thunderstorms. While the northwestern boundary of the SIGMET was close to the accident site, satellite and radar imagery indicated no thunderstorms in the vicinity of the accident-site at the time of the mishap.
The 737’s approach to the ridgeline left two, distinct jet-blast lines evinced by broken trees fallen opposite the aircraft’s direction of travel. Nearer the ridgeline, trees were broken in the direction of the aircraft’s travel—likely from contact with the 737’s engines and airframe. Upon contacting the ridgeline, the aircraft became airborne for approximately 69-meters, shedding engine, wing, and fuselage debris before impacting the ground a second time in a slight nose-down attitude on a heading of 140º —more or less.
The aircraft’s forward progress ceased some 176-meters from the ridgeline. Prior to stopping completely, the 737 yawed port, coming to rest on a heading of approximately 80°. The impact and subsequent inertia-event opened a fracture in the jet’s fuselage slightly forward of its empennage, and separated the aircraft’s port engine from its pylon. The 737 was consumed by fire.
Evidence of neither a fuel-spill nor a fire-trail was observed prior to the point at which the aircraft came to rest. No evidence was found of aircraft components or debris having separated from the 737 before it impacted the ridgeline.
The aircraft’s flight data recorder (FDR) and cockpit voice recorder (CVR) were recovered from the crash-site and retained by the ATSB for examination and download. Extensive fire damage precluded comprehensive inspection of the wreckage. Investigators did, however, establish the positions of the 737’s leading-edge slats, trailing-edge flaps, and horizontal stabilizer—with which no anomalies were observed. The jet’s trailing-edge flap ball-screws were in the fully-extended position, consistent with a 40° setting.
The 737’s left main landing gear was found adjacent the main part of the aircraft’s wreckage, yet parts of its support structure were scattered throughout the debris-field—indicating the aggregate landing gear assembly had been torn from the aircraft prior to the latter’s coming to rest. The plane’s nose and right main landing gear were retracted in place. An inspection of the 737’s cockpit and flight-instruments was rendered impossible by fire damage.
Analysis of the aircraft’s Flight Data Recorder indicated that during the final low-level retardant drop, at a radio altitude of less than one-hundred-feet AGL, the jet’s power-levers were positioned at high idle (0°). As the 737’s engines’ N1 speeds decreased to about thirty-percent, the aircraft’s rate of descent increased to 1,800-feet-per-minute. The power-levers were advanced during the flight’s final seconds, albeit to little avail. Approximately two-seconds prior to impact, at a radio altitude of 28-feet and a flap setting of 40º, an increasing nose-up pitch attitude resulted in an Angle of Attack (AOA) vane deflection in excess of +20° and a subsequent triggering of the aircraft’s stick-shaker. The 737 attained a positive rate of climb immediately prior to its impact with terrain.
Following the accident, Coulson Aviation issued operations bulletin 2023-01 advising air-tanker pilots operating in Australia that the company’s minimum retardant drop altitudes and Vdrop airspeeds had been increased from 150-feet AGL and 1.25 Vs to two-hundred-feet AGL and 1.35 Vs. Additionally, Coulson’s B-737 normal checklist was amended to reflect the revised minima.
The ATSB investigation is ongoing, and expected to conclude in 2024’s third-quarter.
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