What happened
On 5 June 2017, a Saab-Scania SF340B, registration G-LGNB, departed Edinburgh Airport for a scheduled passenger flight to Sumburgh. During the climb, the aircraft encountered intense turbulence and rapid ice accumulation, likely associated with an embedded cumulonimbus cloud.
As the aircraft climbed through FL100, the turbulence intensified, causing the autopilot to disconnect and the stick shaker to activate for the first time. The pilot attempted to accelerate by reducing the pitch and re-engaged the autopilot; however, the warning system triggered a second time 13 seconds later. A third activation occurred shortly after, prompting the crew to initiate a descent to regain airspeed. During this maneuver, the aircraft lost approximately 500ft before recovering to normal flight. No injuries were reported among the 33 passengers and 3 crew members, and no damage was sustained by the aircraft.
The investigation
The AAIB examined flight data recorder (FDR) parameters, including angle of attack (AOA), pitch attitude, and engine power. Analysis by the aircraft manufacturer confirmed that the aircraft did not enter an aerodynamic stall, but rather that rapid fluctuations in pitch and AOA caused by turbulence momentarily exceeded the stall warning threshold.
Investigators also reviewed the aircraft's "Ice Speed" system, a modification designed to increase the stall warning trigger level during icing conditions. The investigation found that the operator's fleet had seen a significant increase in stick shaker events following this modification. The crew noted that such activations were a relatively regular occurrence for their operations, which often involve using engine anti-ice systems on shorter runways.
Findings
- The primary cause of the incident was the encounter with severe icing and turbulence, which led to a significant loss of performance and an inability to maintain the selected airspeed.
- The stick shaker was triggered because the AOA exceeded the 5.9° threshold due to transient turbulence-induced variations.
- The crew did not follow the full recovery procedures outlined in the Aircraft Operations Manual, specifically failing to verbalize the "Stall – Max Power" call or apply maximum continuous power.
- The frequent occurrence of similar warnings in the fleet may lead to a diminished sense of urgency when such alerts are triggered.
Safety action
EASA, the manufacturer, and the operator considered making the deactivation of the Ice Speed logic independent of the engine anti-ice system's five-minute post-icing requirement to prevent repeated warnings during the approach and landing phases.