What happened
On March 19, 2001, at approximately 18:25 EST, a Comair Airlines, Inc. flight operating as Flight 5054 was traveling from Nassau International Airport, Bahamas, to Orlando International Airport, Florida. The Embraer EMB-120, registration N266CA, was in cruise flight at 17,000 feet mean sea level (msl) when it encountered icing conditions and departed controlled flight, descending to approximately 10,000 feet.
The aircraft departed in visual meteorological conditions. Flight data recorder analysis shows that seven minutes prior to the upset, the airplane was flying at 17,000 feet with the autopilot engaged and an airspeed of approximately 200 knots indicated airspeed (KIAS). Starting around 18:18, the airspeed decreased to 185 KIAS as the autopilot applied nose-up trim to maintain altitude. The airspeed continued to drop to about 137 KIAS over the following three minutes while the autopilot trimmed the aircraft up to 7 degrees.
Observing significant ice accumulation on the wing and spinner, the first officer adjusted the leading-edge deicing system to "heavy" and the propeller deicing system to "cold." At an airspeed of approximately 141 KIAS, the autopilot disengaged. The airplane then pitched down and rolled 80 degrees to the left. Over the subsequent 20 seconds, the aircraft underwent several violent oscillations, including rolls of 110 degrees to the left, 130 degrees to the right, and a full 360-degree roll to the right, before returning to near wings level. The crew successfully diverted to West Palm Beach, Florida, where they landed without further incident. All 28 occupants (2 flight crewmembers, 1 flight attendant, and 25 passengers) were not injured, though the aircraft sustained substantial damage to the elevators and horizontal stabilizer.
The investigation
The investigation determined that the aircraft's behavior was consistent with an ice-induced stall event. Meteorological data suggested the flight may have encountered super-cooled large droplets (SLD), conditions for which the EMB-120 is not certified. While the airspeed had decreased to 137 KIAS before control became difficult, this was still above the airplane's stalling speed of approximately 115 knots calibrated airspeed at its weight of 23,800 pounds.
While the first officer noted that the stick shaker and aural stall warnings activated, investigators could not determine the exact timing of these warnings relative to the upset.