Flight Crew Report
The flight crew reported that while conducting an instrument landing system (ILS) approach with the autopilot engaged in approach mode, they noted that the flight director displayed a climbing right turn, yet the airplane remained on the localizer and glideslope. The airplane's ice protection system was active, and no visible ice accumulation was observed. After disengaging the autopilot, the airplane unexpectedly rolled to the right. The crew attempted to regain control by increasing engine power and applying counteractive control inputs, but the airplane subsequently impacted the ground in a near-wings-level attitude.
Post-Accident Examination
Examination of the airplane's primary flight control system and engines after the accident revealed no defects. The rudder trim was found in the neutral position, and the pitch trim was set to airplane nose up. The aileron trim position could not be determined. Inspection of the autopilot components identified deficiencies in the yaw damper system that rendered it inoperative; however, the yaw damper was an optional component and not necessary for normal operation. Testing of other autopilot components revealed some deficiencies that could have degraded performance but would not have resulted in a complete failure of the automatic flight control system.
Performance Data Analysis
A performance study using ADS-B data showed that the airplane intercepted the localizer and glideslope for the ILS approach and was descending in a level attitude. While maintaining ILS guidance, the airplane decelerated below the desired approach speed of 130 knots as stated by the crew. The airspeed continued to decrease to approximately 102 knots, at which point the ADS-B data indicated a slight right roll, likely corresponding to the crew's description of events following the autopilot disconnect. The airplane then decelerated below 100 knots, reaching a bank angle of 27 degrees right wing down. Subsequently, the descent rate increased to over 4,000 feet per minute, and airspeed increased while ground speed remained between 80 and 90 knots. The airplane then rolled sharply to the left. The sudden roll and altitude loss after reaching a low airspeed were consistent with an aerodynamic stall.
Conclusions
Based on the available evidence, the airplane entered an inadvertent aerodynamic stall due to exceedance of the critical angle of attack after the flight crew allowed the airspeed to decay during the instrument approach. Although an unknown anomaly in the flight director system may have caused the crew to become fixated on an erroneous indication at the expense of airspeed control, the post-accident component examination was unable to explain the errant flight director indication described by the crew.