What happened
During an instrument approach to the destination airport, the aircraft encountered light rime icing. At the time of the incident, the co-pilot was acting as the pilot flying. The crew had deactivated the airframe anti-icing system just as the aircraft intercepted the glide slope, shortly before descending below the cloud base. During the approach, light frost was observed on the outboard wing and the tip tank.
As the aircraft reached the short final phase of the flight, the stick shaker activated. The aircraft subsequently impacted to the left of the runway centerline. The momentum of the flight caused the plane to depart the right side of the runway pavement, cross a small rise, and eventually come to a stop in a grassy area.
Findings
Analysis of the cockpit voice recorder revealed that the pilots had been flying in icing conditions for approximately four and a half minutes without the wing anti-ice system engaged before the stick shaker triggered. A post-accident inspection of the aircraft found no evidence of mechanical failures regarding the flight controls or the anti-ice system functionality.
Data from a performance study showed the aircraft was traveling at approximately 114 knots during the final 30 seconds of the flight, and the angle of attack reached the threshold for the stick shaker. While the expected stall speed was roughly 93 knots, the failure to activate the anti-ice system allowed ice to accumulate on the wing leading edge, which can trigger a stall even when operating above standard stall speeds.