What happened
On 3 October 2014, a Saab-Scania SF34/B, registration G-LGNM, was operating a scheduled passenger flight from Aberdeen to Sumburgh. During the climb to FL130, the aircraft encountered severe icing conditions and mountain wave turbulence. As the aircraft reached its target altitude, it failed to accelerate as expected, leading the crew to initiate a descent to FL110 to increase airspeed.
While descending, the aircraft's pitch attitude remained high despite the crew's attempts to use the autopilot's vertical speed mode. The crew then experienced a significant vibration, described as a buffet, followed by the activation of the stall warning system and the automatic disengagement of the autopilot. The pilot took manual control, pitched the nose down, and successfully increased the airspeed to a safe level. The aircraft subsequently completed a normal landing at its destination.
The investigation
The investigation examined the aircraft's performance, the effectiveness of the ice protection systems, and the crew's response to the deteriorating flight conditions. Investigators analyzed flight data recorder (FDR) and cockpit voice recorder (CVR) information, which revealed that the aircraft's angle of attack (AOA) had increased significantly during the descent.
It was established that the aircraft was experiencing severe icing, which placed it outside its certified operating envelope. The investigation also looked into the cockpit environment, noting that while the co-pilot recognized that power and propeller RPM should have been increased according to standard operating procedures, he did not voice this concern to the commander due to a perceived steep command gradient.
Findings
- The aircraft encountered severe icing and mountain wave effects, which significantly degraded aerodynamic performance.
- The crew reduced the airspeed to a level intended for exiting icing conditions but failed to increase power and propeller RPM to the required settings.
- The autopilot remained engaged during a period of significant performance loss, which prevented the crew from promptly pitching the aircraft down to arrest the speed decay.
- The crew's stall recovery training had previously focused on low-power approach configurations, which did not adequately prepare them for a high-power, cruise-altitude recovery.
- There was a lack of effective Crew Resource Management (CRM) as the co-pilot was reluctant to advocate for necessary procedural changes.