Incident Overview
On March 19, 2001, at approximately 1825 eastern standard time, a scheduled international passenger flight operated by Comair Airlines, Inc., as flight 5054, encountered icing conditions while in cruise flight at 17,000 feet mean sea level. The aircraft, an Empresa Brasileira de Aeronautica, S/A (Embraer) EMB-120 registered as N266CA, departed controlled flight and descended to about 10,000 feet. The flight crew regained control and diverted to West Palm Beach, Florida, where the airplane landed without further incident. The two flight crewmembers, one flight attendant, and 25 passengers were uninjured. The airplane sustained substantial damage to the elevators and the horizontal stabilizer.
Flight Details
The flight operated under 14 Code of Federal Regulations Part 121 as a scheduled passenger service from Nassau International Airport, Bahamas, to Orlando International Airport, Florida. It departed in visual meteorological conditions (VMC). In post-accident interviews, the captain stated that the airplane flew normally in VMC.
Events Leading to the Upset
Flight data recorder (FDR) data indicate that about 7 minutes before the upset, the aircraft was at approximately 17,000 feet with the autopilot engaged and airspeed stabilized near 200 knots indicated airspeed (KIAS). Around 1818, the airspeed slowed to 185 KIAS over one minute as the autopilot began trimming nose-up to maintain altitude. Over the next three minutes, airspeed decreased to about 137 KIAS as the autopilot continued trimming from about 0 to 7 degrees nose-up.
The first officer stated that immediately before the upset, he switched the leading-edge deicing system from "light" to "heavy" and the propeller deicing system from "norm" to "cold" because he observed more ice accumulation than he had ever seen on the wing and spinner.
Upset and Recovery
FDR data indicate that when torque for both engines was about 55 percent and airspeed was about 141 KIAS, the autopilot disengaged. The airplane then pitched down and rolled about 80 degrees left, then rolled back to near level. Over the next 20 seconds, engine torque increased to about 98 percent on both engines. The airplane rolled about 110 degrees left, returned to level, rolled about 130 degrees right, returned to level, then rolled 360 degrees right before returning to near wings-level, with torque stabilized at about 22 percent. The aircraft's behavior is consistent with an ice-induced stall event.
The first officer stated in interviews that the stick shaker and aural stall warning activated, but he could not indicate whether it was before, during, or after the upset. The Safety Board's investigation could not precisely determine whether or when these warnings activated.
Meteorological Conditions
Meteorological data at the time suggest the flight may have encountered an area of icing conducive to super-cooled large droplets (SLD). The EMB-120 is not certified for flight in SLD conditions.
Stalling Speed
FDR data show airspeed had decreased to about 137 KIAS before control became difficult. According to the Embraer EMB-120 Airplane Flight Manual performance section, the stalling speed for the approximate gross weight (23,800 pounds) is about 115 knots calibrated airspeed.