Flight Details
The aircraft was on an instrument approach to the destination airport when it encountered light rime icing. The copilot was serving as the pilot flying at the time of the accident.
Events During Approach
Upon intercepting the instrument approach glide slope, the airframe anti-icing system was turned off. This occurred shortly before the airplane descended below the cloud layer. During the approach, the copilot observed light frost on the outboard wing and tip tank. On short final, the stick shaker activated. Subsequently, the airplane impacted left of the runway centerline, then departed the right side of the runway pavement, crossed a slight rise, and came to rest in the grass.
Cockpit Voice Recorder Analysis
The cockpit voice recorder transcript indicated that the pilots operated in icing conditions without the wing anti-ice system activated for approximately 4½ minutes prior to the stick shaker activation.
Postaccident Examination
A postaccident examination of the airplane revealed no anomalies consistent with a preimpact failure of the flight control system or a loss of anti-ice system functionality.
Performance Study
A performance study determined that during the final 30 seconds of the flight, the airplane’s airspeed was about 114 knots. The angle of attack ultimately met the stick shaker threshold. The expected stall speed for the airplane was approximately 93 knots.
Airplane Flight Manual Guidance
The airplane flight manual stated that anti-ice systems should be turned on prior to operation in icing conditions during normal operations. It warned that even small accumulations of ice on the wing leading edge can cause an aerodynamic stall prior to activation of the stick shaker and/or stick pusher.
Probable Cause
The official probable cause is the pilot’s decision to conduct an instrument approach in icing conditions without the anti-ice system activated, contrary to the airplane flight manual guidance, which resulted in an inadvertent aerodynamic stall due to an in-flight accumulation of airframe icing.