Incident Overview
Following departure and during the cruise climb phase, the flight encountered airframe ice and departed from controlled flight. The crew managed to recover control after the aircraft underwent significant roll and pitch excursions.
Flight History
Before the incident takeoff, the crew reviewed the dispatch package weather and noted two AIRMET reports for icing in clouds. The captain decided to perform the departure and turn control of the airplane over to the first officer after completing the climb checklist at the acceleration altitude. The aircraft was dispatched with the continuous mode of the boot deice system inoperable, per the operator's minimum equipment list, requiring manual operation of the deice boots. Company procedures require activation of the deice system at the first sign of ice accretion and operation of the deice boots continually thereafter while in icing conditions.
Weather and Icing Conditions
The departure was into level 2 weather conditions, defined as 10°C or colder with visible moisture. The crew had encountered light rime icing on the inbound leg while descending from 9,000 to 5,000 feet. During the climb, as the airplane passed through 11,700 feet, the captain noted light rime ice accumulating on the windshield wiper blades and about a 1/2-inch-wide area of ice on the left wing.
Cockpit Actions
The airline's Airplane Operating Manual called for computation of a minimum speed (Vcln+15) in icing conditions, which for this flight was 141 KIAS. The limitations section stipulates that indicated airspeed is the only authorized flight director/autopilot mode while climbing when ice accretion is occurring or with residual ice. Despite this, the captain engaged the autopilot in the medium climb mode, and the first officer changed the autopilot to vertical speed mode shortly after taking the controls at 2,500 feet. As the captain began to activate the manual deice boot system, he felt a heavy vibration, the windscreen turned white with ice, and the airplane's nose and left wing dropped, disconnecting the autopilot. The clacker sounded (indicating an imminent stall), the stick shaker activated, and the ground proximity warning system emitted a "bank angle" aural warning.
Flight Data Analysis
Digital Flight Data Recorder (DFDR) data showed that the indicated airspeed went from 144 to 130 KIAS over the 26 seconds before the upset, with the rate of airspeed decay accelerating in the final 10 seconds. The airplane departed controlled flight at 130 KIAS, before the stall warning activated. The data revealed a series of roll and pitch excursions, reaching maximum values of 86° left wing down, 140° right wing down, 23° nose up, and 40° nose down before recovery. About 26 seconds before the stall, while at 144 KIAS, the airplane began to experience a likely ice-induced slight rolling anomaly counter to aileron input; the autopilot arrested this motion. Fourteen seconds after the initial stall, both ailerons simultaneously traveled to the full up position for approximately 14 seconds.
Safety Board Conclusions
The Safety Board believes that the initial stall that occurred prior to stall warning, the upset, and the aileron upward deflections were caused by ice accreted on the wing in supercooled liquid droplets (SLD) conditions. The period of simultaneous upward deflection of the ailerons was caused by airflow separations over the ailerons, not by opposite control wheel inputs. This conclusion was based on past experience with similar SLD ice accretion incidents, higher airspeed and dynamic nature of the events, crew statements, and extrapolation from high speed taxi testing. The Board issued several recommendations, including urging the FAA to require operators of Saab SF340 series airplanes to maintain a minimum operating airspeed of 1.45xVs during icing encounters, to install modified stall protection logic and icing detection systems, and to instruct pilots to disengage the autopilot in icing conditions. The Board also reiterated recommendations for low-airspeed alert systems.