Incident Overview
A scheduled commuter flight transporting seven passengers and cargo departed from a remote airport during hours of arctic, predawn darkness. The pilot, who also served as the station manager, had arrived earlier than other company employees to prepare for the flight. Heavy frost was observed on vehicles and airplanes that morning, and the lineman who serviced the airplane noted a thin glaze of ice on the upper surface of the left wing.
Preflight Actions
The pilot was not observed deicing the airplane prior to flight and was described by other employees as being in a hurry to depart on time. The pilot directed the lineman to place fuel only in the left wing, resulting in a fuel imbalance between 450 and 991 pounds, with the left wing being heavier.
Flight and Accident Sequence
After takeoff, the first turn was made into the heavy left wing. The airplane was observed climbing past the end of the runway and then descending vertically into the water. No preimpact mechanical anomalies were found with the airplane or its powerplant. The aileron trim indicator was found in the full right-wing-down position.
Postaccident Testing and Research
Postaccident flight tests with left-wing-heavy lateral fuel imbalances disclosed that approximately one-half of right-wing-down aileron control deflection was used to maintain level flight, thus leaving only one-half right-wing-down aileron control efficacy. Research has shown that frost on airfoils can result in reduced stall angles of attack (often below that required to activate stall warning devices), increases in stall speeds between 20% and 40%, asymmetric stalls resulting in large rolling moments, and differing stall angles of attack for wings with upward and downward deflected ailerons (as when recovering from turns).
Probable Cause
The official probable cause is: The pilot's disregard for lateral fuel loading limits, his improper removal of frost prior to takeoff, and the resulting inadvertent stall/spin. Factors involved in this accident were the improper asymmetrical fuel loading which reduced lateral aircraft control, the self-induced pressure to takeoff on time by the pilot, and inadequate surveillance of the company operations by company management.