What happened
During the initial climb phase following takeoff, a malfunction occurred when a low torque sensing switch failed. This failure triggered the automatic feathering of one engine. During the ensuing emergency, the crew did not correctly execute the necessary procedures to manage the propeller, which led to the propeller unfeathering and entering a windmilling state.
Following this event, the crew retracted the flaps. This action deactivated a spring tab on the rudder, a component intended to provide increased rudder effectiveness during periods of high power and low airspeed. The combination of the windmilling propeller and the reduced rudder authority caused the minimum controllable airspeed, V(mca), to rise significantly from 101 kts to 138 kts.
As a result of these aerodynamic changes, the aircraft rolled into an inverted position and struck the ground. Evidence found on the propeller of the functioning engine suggested that the impact occurred while the aircraft was traveling at a speed below 90 kts. The accident resulted in nine fatalities, consisting of four crew members, four students, and one instructor.
Findings
- A failure in the low torque sensing switch initiated an unintended auto-feather sequence.
- Improper handling of the feathering procedure allowed the propeller to windmill.
- The retraction of flaps disabled a critical rudder spring tab, leading to a loss of directional control at speeds below the newly increased V(mca).