Icing- and Autopilot-related Incidents
• In January 2006, two pilots flying an American Eagle Saab 340 out of San Luis County Regional Airport in San Luis Obispo, Calif., nearly lost control after the autopilot shut off during icing conditions on climb-out. The Saab 340 lost 5,000 feet during the recovery. NTSB recommendation: Require all operators of turboprop- driven airplanes to instruct pilots, except during intermittent periods of high workload, to disengage the autopilot and fly the airplane manually when operating in icing conditions.
• March 2001, West Palm Beach, Fla. A Comair Embraer EMB-120 Brasilia was cruising at 17,000 feet in icing conditions. The autopilot kept trimming the nose up as airspeed slowed then suddenly disconnected. The EMB-120 descended to 10,000 feet before the pilots regained control, and according to the NTSB, “the airplane sustained substantial damage to the elevators and horizontal stabilizer.”
While there is no NTSB recommendation attached to this incident report, investigators discovered something interesting. Comair EMB-120 pilots were in the habit of resting a leg against the trim wheel so they could feel it move as the autopilot automatically trimmed nose up to counter airspeed loss in icing conditions. In this incident, the nose increased in pitch by seven degrees, which would have rotated the trim wheel 1.3 revolutions. After this incident, Embraer issued a service bulletin recommending installation of a placard telling pilots to disengage the autopilot in icing conditions, and the FAA made this mandatory with an AD.
• A Simmons Airlines ATR 42 flight crew allowed the airplane to stall during an IFR approach while flying on autopilot on June 25, 1990. The airplane stalled and rolled 80 degrees to the left and 60 degrees to the right, according to the NTSB, then lost 600 feet during the recovery. No recommendations associated with this incident could be found.