What happened
On 6 February 2023, a Boeing 737 large air tanker, operating under callsign Bomber 139, was engaged in aerial firefighting within the Fitzgerald River National Park, Western and Australia. The mission, requested by state authorities, involved dropping fire retardant to manage active wildfires.
During the third flight of the day, the crew attempted a second retardant drop. Following an initial partial release, the crew executed a go-around from high ground. During this maneuver, the aircraft struck terrain. The investigation confirmed that the aircraft's engines were at idle and the plane had entered a high rate of descent, leaving insufficient thrust and airspeed to clear a rising ridgeline in the flight path.
The investigation
Investigators examined wreckage, flight data, cockpit voice recordings, and crew interviews. The analysis found no mechanical failures or issues related to weather or crew fatigue. Instead, the focus shifted to operational procedures and human factors during the fireground maneuvers.
On this specific flight, the pilot flying opted for a right-hand circuit pattern, which placed the pilot on the outside of the turn, making it harder to monitor the drop zone. The crew also declined a 'Show-Me' profile, which could have provided better situational awareness. The investigation noted that the pilot's attention was heavily focused on identifying a specific road crossing to mark the start of the drop, which likely prevented the detection of rising terrain in the exit path.
Findings
- The aircraft entered a low energy state characterized by a significant drop in airspeed and altitude below standard targets.
- The pilot monitoring did not alert the pilot flying to deviations in altitude or speed.
- There was a lack of established minimum drop height requirements in the published procedures for both the operator and the tasking agencies.
- The crew did not utilize the radio altimeter to set a decision height reference.
- The practice of recalculating target speeds after partial drops reduced safety margins for stall speed and energy.
Safety message
- There is a critical need for standardized national operating procedures for large air tankers to ensure consistency between different tasking agencies.
- Crew resource management practices should be reviewed to ensure pilot monitoring announcements are not limited to narrow tolerances, allowing for earlier detection of unsafe flight states.