An AAIB investigation revealed that a Saab 340B stalled in icing conditions during a flight from Aberdeen to Sumburgh, U.K.. The aircraft was recovered in a non-standard manner and continued without incident to the planned destination.
On October 3, 2014, a Saab 340B passenger plane, operated by Loganair, departed Aberdeen carrying 25 passengers and a crew of three. On departure at 09:00 hrs UTC, for the second flight to Sumburgh, there was a strong south-south-westerly airstream and severe mountain waves were forecast between FL40 and FL280.
The planned cruising level was FL150 and ATC approved a climb to FL130. Close to FL65, the aircraft entered cloud and the crew switched on the engine anti-icing systems, together with the wing and stabiliser de-icing boots. Above FL110, the rate of climb reduced. A small amount of ice had formed around the windscreen wipers but the commander initially assessed that the reduced rate of climb was due to downdraughts caused by mountain wave effect.
Ice was seen to accrete on the propeller spinners, and propeller de-icing was selected to norm when the outside air temperature reached -5ºC. No propeller vibration was apparent and no ice from the propellers was heard to strike the fuselage. The commander, who was pilot flying, used the autopilot’s vertical speed (VS) mode and reduced the Indicated Air Speed (IAS) from more than 160 KIAS, the normal minimum speed for climbing in icing, to 145 KIAS, which was the appropriate V-ERICING speed for use in exiting icing conditions. The half-bank mode and IAS (hold) mode were engaged and the aircraft continued to climb at 145 KIAS. The commander realised the aircraft was being affected by ice, as well as by mountain wave effect, but was confident that it was close to the cloud tops and would climb above the icing level.
The pilots later recalled that the indicated rate of climb varied from a maximum of about 800 ft/min to a slightly negative rate. Climb power was maintained and the propeller rpm were also kept at the normal setting of 1,230 rpm. The co-pilot later stated that he realised that the power and propeller rpm should have been increased when the speed was reduced below 160 KIAS, in accordance with standard operating procedures (SOPs), but he had not suggested this at the time. Approximately 3/4 inch of ice could be seen on the windscreen wipers, while the de-icing boots on the wing appeared to clear any ice that formed on them.
The aircraft reached FL130 at 09:25 hrs but it only accelerated to 164 KIAS, rather than the expected 180 KIAS or greater. The commander’s reaction was to climb the aircraft another 100 ft, then descend back to FL130 using VS mode. This was not a manoeuvre the commander had previously employed, but he had seen it used by another pilot. Before commencing it, he and the co-pilot had a brief discussion about what was intended. The commander thought that the angle of attack (AOA) and drag would reduce during descent and the aircraft would accelerate. Instead, during the short climb, the IAS reduced quickly towards 150 KIAS and did not increase in the descent.
On regaining FL130, the autopilot remained engaged and the active vertical mode was ALTS which is the altitude hold mode displayed when the aircraft is maintaining a pre-selected altitude. The pilots believed they were experiencing moderate icing and should descend to FL110 to increase airspeed. After ATC had approved the descent, the commander commented that the ice conditions were more than moderate and that the airframe was accumulating a lot of ice. FL110 was entered in the altitude pre-selector and the commander selected a rate of descent of 1,000 ft/min, using VS mode.
The commander noticed that, despite the selected rate of descent, the aircraft’s pitch attitude remained high (around 5º nose-up) and he increased the selected rate to 2,500 ft/min. The airframe then started to vibrate and the commander said “feel that, that’s a stall… i think… icing stall”. Approximately 10 seconds after the vibration started, at 09:28:49 hrs, the aural stall warning sounded for approximately one second, the stick shaker operated and the autopilot disengaged.
The commander took manual control and pitched the aircraft to 2º nose-down. He later reported that a little more force than usual had been required to lower the nose of the aircraft but it then responded normally as the speed increased to greater than 190 KIAS. The vibration ceased and the aircraft seemed to be in trim, without any pitch trim adjustment by the commander. The autopilot was re-engaged approximately 9 seconds after disconnection and, shortly afterwards, an elevator mis-trim annunciation was displayed on both Electronic Attitude Director Indicators (EADIs). It was acknowledged by the crew and cleared within a few seconds.
The flight was continued to its original destination, without climbing again, and without further incident.
- AAIB report (PDF)