What happened
On 3 October 2014, a Saab-Scania SF304B, registration G-LGNM, was performing a scheduled passenger flight from Aberdeen to Sumburgh. During the climb to FL130, the aircraft encountered significant meteorological challenges, including forecast mountain waves and icing conditions. As the aircraft climbed, the crew observed ice accumulating on the windscreen wipers and noted a reduction in the rate of climb.
To manage the climb in icing conditions, the commander reduced the airspeed to 145 KIAS. However, the power settings were not increased to maximum continuous power as required by standard operating procedures. Upon reaching FL130, the aircraft failed to accelerate as expected. In an attempt to regain airspeed, the commander initiated a descent using the autopilot's vertical speed mode. During this maneuver, the aircraft' 145 KIAS airspeed continued to decay, and the angle of attack increased.
The crew then experienced a heavy vibration, which the co-pilot likened to driving over a cattle grid. Shortly after, the aural stall warning activated, the stick shaker operated, and the autopilot disengaged. The commander took manual control, pitching the nose down to 2 degrees to recover. The aircraft eventually regained a safe airspeed of over 190 KIAS, and the autopilot was re-engaged before the flight continued to its destination without further incident.
The investigation
An AAIB investigation examined the flight data recorder, cockpit voice recorder, and the aircraft's maintenance history. Investigators analyzed the aerodynamic drag forces, finding that the aircraft experienced drag levels nearly four times the certified datum, confirming the presence of severe icing. The investigation also reviewed the manufacturer's manuals and the operator's training protocols.
Key areas of focus included the effectiveness of the de-icing boots, the functionality of the stall warning system, and the crew's adherence to operating procedures. The investigation also looked into the impact of the crew's previous simulator training, which had focused on stall recovery from an approach configuration rather than the cruise configuration experienced during this event.
Findings
- The aircraft encountered severe icing conditions that exceeded its certified limits, leading to a massive increase in aerodynamic drag.
- The crew did not increase engine power to maximum continuous power when reducing airspeed in icing conditions.
- The co-pilot did not advocate for the necessary power increase due to a perceived steep command gradient with the commander.
- The autopilot was not disengaged during the period of significant performance loss, which prevented the crew from pitching the nose down more effectively.
- The crew's previous simulator training had not adequately prepared them for a stall recovery at cruise altitude using high power settings.
- Confusion existed regarding the significance of black rings on the propeller spinners, which were not officially documented as an icing indicator.