What happened
On January 27, 1998, a Finnair SAAB SF340, registration OH-FAE, was performing a scheduled flight from Helsinki to Lappeenranta. While approaching the airport, the crew was executing an ILS approach to runway 06 with the intention of transitioning to a low approach for runway 24 due to strong winds.
During the transition from the glide path to level flight, the aircraft's airspeed decreased significantly. As the aircraft entered a left turn, a gust of wind caused a bank angle of approximately 25 to 30 degrees. This loss of airspeed triggered the aircraft's stall warning and stick shaker. The captain subsequently took manual control of the aircraft, disconnected the autopilot, and increased engine power to stabilize the flight and continue the landing on runway 24.
The investigation
The investigation examined the actions of both the pilot flying (the first officer) and the monitoring pilot (the captain). Investigators reviewed air traffic control recordings and flight data from the flight recorder, although some cockpit voice recorder data was unavailable. The investigation also reviewed the airline's specific operating procedures for the SAAB aircraft group, particularly regarding the division of tasks between the flying and monitoring pilots during autopilot use.
Findings
Several contributing factors led to the loss of airspeed and the subsequent stall warning:
- The first officer, acting as the flying pilot, failed to increase engine power when transitioning from the glide path to level flight.
- The captain, acting as the monitoring pilot, selected the 'MODE HDG' and 'MODE ALT' functions on his own side of the flight control panel rather than on the side controlling the autopilot. This caused the autopilot to execute a left turn that was not as expected by the flying pilot.
- The simultaneous occurrence of the turn, the lack of power application, and a gust of wind reduced the aircraft's speed margin to the point of activation of the stall warning systems.
- The crew's attention was diverted toward managing the flight control panel settings rather than monitoring airspeed.
Following this event, the airline updated its procedures for the SAAB aircraft group to ensure that flight control panel selections are made on the side controlling the autopilot, aligning with the practices used in their other aircraft groups.