What happened
In northern Saskatchewan, a wheel-ski equipped de Havillend DHC-3 Otter, registration C-FASV, was performing a routine mission to transport road construction workers. After dropping off a crew at a remote lake, the pilot returned to the main base at Points North Landing to refuel the aircraft for a planned four-day deployment.
Upon returning to the lake for the afternoon pick-up, the pilot encountered slushy conditions on the ice surface, with approximately five to six inches of slush present. The pilot initiated a first take-off attempt but, noting slow acceleration, aborted the run. He then repositioned the aircraft to utilize a different path and moved a passenger to a forward seat to assist in raising the tail.
During the second attempt, the pilot continued the acceleration beyond his established rejection point. As the aircraft moved forward, the engine revolutions per minute (rpm) decreased by approximately 150 rpm. The aircraft failed to become airborne and struck the low shoreline and rocks. The impact triggered an intense fire that consumed the engine and much of the fuselage. While the pilot and passengers successfully evacuated, one passenger sustained minor burns during the escape.
The investigation
Investigators examined the aircraft's tracks on the lake, which revealed that during the second attempt, the left ski remained in constant contact with the slush. The investigation also reviewed the pilot's recent flight history and the aircraft's mechanical performance.
It was established that the pilot had experienced a similar decrease in engine rpm during take-offs over the preceding ten days and had accepted this as a routine occurrence. Furthermore, the investigation looked into the physics of ski operations in slush, noting that viscous drag and the accumulation of slush in front of the skis significantly increase resistance. The investigation also considered the pilot's decision-making process regarding weight, balance, and the use of the available runway.
Findings
- The pilot continued the take-off run with the left ski firmly embedded in the slush beyond the point where a safe abort could have been performed.
- A decrease in engine rpm during the take-off contributed to the loss of performance.
- The pilot's decision to continue the run was influenced by a belief that the aircraft would become airborne, despite the lack of sufficient acceleration.
- The movement of a passenger to change the center of gravity did not sufficiently reduce the resistance caused by the slush.