A Boeing 737-300 Air Tanker, callsign Bomber 139, collided with ridgeline at an elevation of about 222 feet after completing a fire-retardant drop while engaged in a fire fighting mission in Fitzgerald River National Park, Australia. Both occupants survived the crash. The aircraft was consumed by fire. The aircraft had been contracted by the Australian National Aerial Firefighting Centre (NAFC) and was flown to Australia in December 2022. == Contributing factors ? During the retardant drop downhill, the aircraft descended significantly below the operator?s standard target drop height and airspeed and entered a high rate of descent with the engines at idle. While the engines were starting to accelerate at completion of the drop, the airspeed and thrust were insufficient to climb above a ridgeline in the exit path, which resulted in the collision with terrain. ? Prior to the retardant drop, the aircraft captain (pilot flying) did not detect there was rising terrain in the exit from the drop, which likely contributed to the captain allowing the aircraft to enter a low energy state during the drop. ? After arrival at the fireground, the aircraft captain (pilot flying) declined a ?Show-Me? run and was briefed by the Birddog pilot that it would be a downhill drop. Bomber 139 then conducted a go-around from the high ground after the first drop and was led to the target through the smoke on the second drop. These factors likely contributed to the captain not expecting or detecting the rising terrain in the exit path. ? The co-pilot (pilot monitoring) did not identify and announce any deviations during the retardant drop, which could have alerted the aircraft captain (pilot flying) to the low-energy state of the aircraft when it descended below the target drop height with the engines at idle. ? The flight crew did not brief a target retardant drop height and, contrary to published standard operating procedures, did not set it as a decision height reference on the radio altimeter. Subsequently, the co-pilot (pilot monitoring), who did not believe there was a minimum drop height, did not alert the aircraft captain (pilot flying) to the low-energy state of the aircraft. ? Coulson Aviation and the relevant Western Australian Government Departments had not published a minimum retardant drop height in their respective operating procedures for large air tankers. Consequently, the co-pilot (pilot monitoring), who did not believe there was a minimum drop height, did not alert the aircraft captain (pilot flying) to a drop height deviation prior to the collision. (Safety issue) Other factors that increased risk ? The Coulson Aviation practice of recalculating the target retardant drop speed after a partial drop reduced the post-drop stall speed and energy-height safety margins. (Safety issue) ? The Coulson Aviation crew resource management practice of limiting the pilot monitoring (PM) announcements to deviations outside the target retardant drop parameter tolerances increased the risk of the aircraft entering an unrecoverable state before the PM would alert the pilot flying. (Safety issue) ? Australian states and territories that engage in Large Air Tanker (LAT) operations have developed their own separate standard operating procedures (SOPs) for LATs and aerial supervision assets. This can result in safety requirements being omitted or misunderstood by the different tasking agencies, such as a minimum drop height, resulting in inconsistencies in the development and application of LAT SOPs. (Safety issue)