What happened
On 6 April 2006, a DHC8-Q 400, registration LN-RDA, was performing a scheduled commercial flight from Stockholm/Arlanda to Kalmar. During the descent, the crew observed a warning lamp indicating a propeller electronic control failure on the right engine, which was accompanied by a significant increase in propeller speed.
While the co-pilot suggested following the emergency checklist to feather the propeller and shut down the engine to mitigate drag, the commander elected to maintain the engine at flight idle to avoid interrupting the approach. This decision resulted in the propeller blades remaining at a flat angle, creating massive aerodynamic drag. As the aircraft continued its descent, the asymmetric power caused the autopilot to disconnect and the aircraft to begin a rapid descent and right turn. The crew was forced to increase power on the left engine to 125% Torque, exceeding the maximum permitted limit, to maintain control. The aircraft eventually touched down at Kalmar airport after a highly unstable approach, with the crew struggling to maintain the centerline.
The investigation
The Swedish Accident Investigation Board (SHK) examined the flight data, cockpit voice recordings, and the aircraft's maintenance history. The investigation focused on the crew's decision-making process, the effectiveness of the existing emergency checklists, and the technical nature of the propeller overspeed. Investigators also reviewed previous maintenance logs, which revealed that the same aircraft had experienced three similar propeller overspeed incidents in the three weeks prior to the accident.
Findings
- The primary cause of the incident was the failure to complete the emergency checklist, specifically the instruction to feather the propeller.
- The commander's decision to keep the engine at flight idle created severe asymmetric drag, leading to an unstabilized approach and significant control difficulties.
- The crew was not fully aware of the aerodynamic risks associated with an unfeathered propeller in flight idle.
- The emergency documentation regarding propeller overspeeding was found to be unclear.
- There was a lack of adequate follow-up regarding previous similar technical failures on this specific aircraft.