Norwegian investigators are seeking to improve pilots’ awareness of an impending stall during icing conditions on British Aerospace ATP turboprops, after a serious incident involving a West Atlantic aircraft last year.
Investigation authority SHT says that ice accretion on the wings can cause the ATP to stall “prematurely”, before the stick-shaker activates.
It has advised that ATP crews be made more aware of recommended minimum speeds in icing conditions, and says these thresholds should be “easily available” to pilots in the cockpit.
SHT says the incident occurred as the aircraft flew north-east of Lillehammer on 25 September 2014.
Ice-protection systems on the propellers and air intakes were active, and the crew engaged the airframe de-icing system at 15,000ft to remove ice from the wing leading-edge, fin and horizontal stabiliser.
But the ATP’s cruising speed bled away – falling by 58kt over 8min, to a minimum of 136kt – and the aircraft began to buffet and shake violently. The flight-data recorder shows that, at one point, the aircraft was rolling 32° while pitched up and descending at over 2,500ft/min. SHT says this “illustrates the seriousness” of the event.
“The first officer, who was at the controls, stated that he had to push the nose down by force,” says the inquiry. “The ailerons did not respond properly, and the buffeting was so violent that he could hardly read the instruments.”
SHT believes the loss of airspeed was probably due to higher drag on the aircraft as it flew into a region of icing combined with vertical air movement from mountain waves.
The aircraft’s lowest speed during the incident was 22kt below the recommended minimum speed in icing conditions, but still 16kt above the threshold for stick-shaker activation.
SHT says the aircraft lost around 1,000ft of altitude – although the underlying terrain was not a threat – and the crew regained control after about 30s.
BAE Systems, the type certificate holder for the ATP, has taken steps to improve clarity over minimum speeds in icing, says the inquiry.