Precursors

Prior to the accident, there were six icing related incidents involving EMB-120 aircraft. Excerpted from the NTSB report and details of the incidents are as follows:

" In April 1995 near Tallahassee, Florida, an EMB-120 was in cruise flight at FL 250 when the pilots noticed the airspeed decrease from 180 KIAS to 140 KIAS and pitch increase to 5o nose up with only trace icing observed on the leading edge of the wing. They activated the deice boots; the airspeed subsequently increased and the pitch decreased.

In October 16, 1994 near Elko, Nevada, an EMB-120 was in cruise flight at 13,000 feet msl, at 160 KIAS. The pilots checked for ice on the wings and propeller spinners but did not see a significant amount. Moments later, as they entered a right bank with the autopilot engaged, the stick shaker and pusher activated. The pilots resumed manual control of the airplane and recovered. Post-flight inspection of the aircraft revealed clear ice on the leading edge and spinner. The deice boots were not activated during the flight because the crew did not believe the ice was of sufficient thickness. Review of the FDR data indicated that the airplane's airspeed had decreased to 138 KIAS before stick shaker activation.

On April 23, 1993 at Pine Bluff, Arkansas, while climbing on autopilot, an EMB-120 stalled and experienced an upset event and subsequent separation of propeller blades. FDR data indicated that the airplane's airspeed had decreased to 138 knots before stick shaker activation and autopilot disconnect. The Safety Board concluded that an accretion of ice on the wing was the only reasonable explanation for the occurrence of stick shaker activation and subsequent loss of roll control at higher than expected airspeeds. There was no evidence that any ice protection systems were activated before, during, or after the upset, and the flight crew did not recall seeing evidence of icing before the loss of control.

On November 22, 1991 in Clermont-Ferrand, France, an EMB-120 was in an autopilot-controlled descent when the captain disconnected the autopilot manually to slow the descent, stabilizing the aircraft at 4,500 feet. FDR data indicated that the airplane's airspeed decreased to 150 KIAS, the stick shaker activated, the airplane rolled to the right and lost about 1,000 feet of altitude. During recovery, the engine power was increased and the deice boots were cycled by the first officer. Post-flight inspection revealed clear ice on the horizontal stabilizer, wing tips, and inboard section of the wing.

In September 1991 at Fort Smith, Arkansas, an airplane was in level flight with the autopilot engaged at 19,000 feet msl when the pilots felt vibration through the floorboards. The pilots inspected the wings, propeller spinners, and engine inlets for ice, but did not observe excessive amounts of accumulation. Thirty seconds after the first vibration, the stick shaker activated and the captain called for all ice protection equipment to be turned on. The airplane entered a right bank, nose-down descent; the pilots regained control at 16,000 feet msl.

On June 28, 1989 at Klamath Falls, Oregon, an EMB-120 was operating at 16,000 feet msl in icing and turbulence with the autopilot engaged. When the airplane descended to 15,000 feet msl, the pilots observed light mixed rime and clear ice followed by a rapid decrease in airspeed from 180 to 160 KIAS, stick shaker activation, and an uncommanded roll/upset. The pilots resumed manual control of the airplane and applied maximum power; they stabilized the airplane at 12,000 feet msl. Both pilots reported that the airplane was operating in light icing conditions when the upset occurred. Although the pilots reported light icing, Air Line Piolots Association's (ALPA) report of this incident stated that a United Airlines Boeing 727 flight engineer occupying seat 3C in the cabin described the icing conditions as "moderate" and indicated that up to one inch of ice may have accumulated on the wing leading edges. There was no indication in the FDR data or the ASRS report that any ice protection equipment was used."

Following these incidents, in the fall and winter of 1995, Embraer conducted a series of EMB-120 controllability tests to evaluate the airplane in super-cooled large droplet (SLD) icing conditions. Also in November 1995, Embraer presented the results of these tests at a flight crew awareness seminar. In December 1995, Comair issued the previously discussed interoffice memo entitled "Winter Operating Tips," and Embraer conducted icing tanker tests in SLD conditions. In April 1996, Embraer distributed Operational Bulletin 120-002/96, which suggested in part that deicing boots be activated at the first sign of ice accretion. Later the same month, Embraer produced AFM Revision 43, and in May 1996, the FAA issued Airworthiness Directive AD 96-09-24, mandating requirements of SLD conditions. In July 1996, Comair issued Flight Standards Bulletin 96-02 titled "Severe Icing Conditions" followed in October by Flight Standards Bulletin 96-04 titled "Winter Flying Tips."

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