Logan SF34 near Edinburgh on Jun 5th 2017, severe icing and three activations of stick shaker
Last Update: March 8, 2018 / 16:50:11 GMT/Zulu time
The United Kingdom's AAIB released their bulletin concluding:
After encountering severe icing, probably associated with a CB, the Saab340 stall warning system functioned as it was designed to by alerting the crew, through the stick shaker, of an AOA in excess of 5.9°. The crew actions did not initially address the problem sufficiently and the stick shaker occurred again. Following the third activation of the stick shaker the crew descended the aircraft to regain a safe airspeed. Although maximum power was not set, the aircraft did accelerate and the crew were able to clear the icing and turbulent conditions before continuing their flight without further incident.
The AAIB reported the stall warning logic had been changed:
Due to incidents within the worldwide fleet of the Saab340, in which the aircraft encountered a stall without any prior stall warning, the manufacturer developed a modified stall warning system which incorporated an improved stall warning logic. Fitment was mandated in 2014, with operators given 18 months to complete the fitment of the new stall warning computer.
Ice Speed modification
The modification to the stall warning computer adjusted the logic of the stick shaker and introduced the Ice Speed system. This increased the stall warning speed trigger levels to compensate for possible ice accretion on the wings. The trigger AOA for the stick shaker activation was lowered from 12.1° to 5.9° but the stick push logic remained unchanged.
This Ice Speed function is activated by switching on the engine anti-ice system. It remains activated even when the engine anti-ice system is selected off because a separate ice speed switch must be additionally selected off. The engine anti-ice system must remain on for five minutes after exiting icing conditions. Any time the Ice Speed system is active, the aircraft speed on approach must be increased in order to maintain the margin over the stick shaker activation AOA.
The AAIB analysed:
The aircraft encountered icing on departure from Edinburgh Airport. The severity and extent of ice accumulation on the upper wing surface led to a significant performance loss, which meant the aircraft was unable to maintain the selected airspeed at the aircraft altitude and a descent was necessary to regain lost airspeed. This level of icing would match the UK AIP pilot reporting definition of severe icing and this, combined with the information available from the Met Office, suggested that the crew probably encountered a CB on their route. Whilst these CBs were forecast, it is possible they did not show on the aircraft’s weather radar, which would have made it less likely that the crew would become aware of their presence before encountering them.
The severe icing and turbulence they encountered caused the AOA to increase beyond the 5.9° trigger for the stick shaker on three occasions due to short duration transient variations. On two occasions this caused the autopilot to disconnect (during the third activation the autopilot was not engaged). Re-engaging the autopilot without fully reducing the pitch attitude of the aircraft, meant that the stick shaker was triggered for the second time after 10 seconds. The subsequent descent saw the aircraft clear both the icing and turbulent conditions whilst increasing the airspeed.
The modification to the stall warning computer has had the unintended consequence of increasing stick shaker activation for the operator’s Saab340 aircraft on which it has become a regular occurrence. Therefore, it is possible that pilots no longer take the activation as seriously as they did previously as they may interpret it as a nuisance and not react. The AOM specifically allows the crew to disregard transient (less than 1 second) warnings as long as the aircraft is at a safe speed and height. In this case, for two of the stick shaker activations the speed was below VCM, and the stick shaker could not be disregarded. Whilst there is no suggestion that the crew on G-LGNB deliberately ignored the warning, or considered it a nuisance, it is human nature to give less regard to a warning known to be triggered regularly even when it is not required.
This article is published under license from Avherald.com. © of text by Avherald.com.
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