What happened
During the departure and cruise climb phase, the flight crew encountered light rime icing while descending from 9,000 to 5,000 feet. Although the dispatch weather reports indicated two AIRMETs for icing in clouds, the aircraft was dispatched with the continuous mode of the deice boot system inoperable, requiring manual operation by the crew.
The captain initially engaged the autopilot in medium climb mode, and the first officer later changed the autopilot to vertical speed mode. As the aircraft climbed through 11,700 feet, the captain observed light rime ice accumulating on the windshield wiper blades and a 1/2-inch area of ice on the left wing. The crew failed to detect a decaying airspeed caused by the accumulation.
While attempting to activate the manual deice boots, the crew experienced heavy airframe vibration and the windscreen became obscured by ice. The aircraft's nose and left wing dropped, and the autopilot disconnected. Following this, the stick shaker activated and a bank angle warning sounded. The Saab SF340 departed controlled flight at an indicated airspeed of 130 knots. The aircraft underwent several roll and pitch excursions, reaching a maximum of 140 degrees right wing down and 40 degrees nose down before the crew recovered control.
The investigation
Digital Flight Data Recorder (DFDR) data showed that the indicated airspeed decreased from 1rag44 to 130 KIAS over the 26 seconds preceding the upset. The data also revealed that approximately 26 seconds before the stall, the aircraft experienced a rolling anomaly counter to the direction of aileron input. Additionally, 14 seconds after the initial stall, both ailerons moved to the full up position for approximately 14 seconds.
Findings
Investigators concluded that the initial stall, the upset, and the upward aileron deflections were caused by ice accreted in supercooled liquid droplets (SLD) conditions. The simultaneous upward deflection of the ailerons was attributed to airflow separation over the surfaces rather than conflicting control inputs from the crew. The investigation also noted that the crew failed to detect the decaying airspeed and that the aircraft was operating with an inoperable continuous deice boot mode.