ATSB implicates icing in 2002 Saab 340 incident

 - February 6, 2007, 3:59 AM

The Australian Transport Safety Bureau (ATSB) listed it as merely a serious incident but considered it significant enough to issue a full report. The incident involved the loss of control a Saab 340 experienced when it encountered icing. There were no injuries and no damage to the aircraft, but the pilots did not recover from the loss of control until the aircraft was only 112 feet above the ground. The incident happened June 28, 2002, on a scheduled airline flight.

The flight had left Sydney, New South Wales, and was descending from an altitude of 12,000 feet for the GPS approach to Runway 17 at Bathurst. Conditions on approach alternated between VFR and IFR, reported the pilot-in-command, and the airplane descended through clouds. The copilot, who reduced power on the left engine to about 15 percent and the right engine to about 19 percent for the circling approach, conducted at 135 knots, said that the engine anti-ice system was on, but the propeller de-ice and the airframe boot de-ice system were not activated.

Wing Icing Causes Stall

There was ice on the windshield wiper but neither pilot noticed wing icing. However, when the airplane reached the minimum descent altitude (MDA) of 3,810 feet, the autopilot captured the altitude and commanded the trim system to raise the nose of the aircraft to maintain MDA, as the airspeed was decreasing. The PIC overrode the autopilot to turn to the right to enter downwind for the runway. The copilot noticed the speed was decreasing and called “Speed.”

The pilot applied power but the aircraft rolled to the left and pitched down. As the pilot tried to recover, the airplane rolled to the right and descended to 112 feet agl. The pilot managed to control the aircraft and climb back to the missed approach altitude and landed without further incident.

When the aircraft pitched down, the stall warning system did not activate, but the autopilot did disconnect when the airplane rolled to the right. An experienced pilot who was a passenger on the flight reported pre-stall buffet before the aircraft stalled. After landing, the crew did not find ice on the airframe.

Investigators determined that when the autopilot captured the MDA, the speed continued to decrease because the power, which should have been at 50 percent, had been reduced, causing a stall. The lack of warning from the stall warning system was probably caused by the airframe ice accumulated during descent. A passenger reported the incident to the ATSB, which began an investigation. The operator immediately grounded the aircraft, which had been operating on its scheduled routes for 27 hours.

The ATSB’s investigation found that it is possible for the aircraft to stall before activation of the stall  warning system if the aircraft has accumulated ice on the wings. Among the recommendations from this investigation was that Saab modify the stall warning system of the worldwide fleet of 340s to include the ice-speed modification, and that aircrews be notified of the consequences of this incident.

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Canadian-registered Saab 340s were already required to have an added input to the stall warning computers, designated as ice speed, controlled by a separate switch, activation of which causes the stall warning computer to operate on lower triggering. After the 1994 fatal accident involving airframe icing to an ATR 72 at Roselawn, Ind., the FAA issued an airworthiness directive, which applied to U.S.-registered Saab 340s, requiring that flight manuals warn that autopilot operations might mask problems in severe icing conditions.