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Summary

Summary

On the evening of 28 June 2002, a Saab 340B, VH-OLM, was being operated on a regular public transport service, from Sydney to Bathurst, NSW. The pilot in command (PIC), the flying pilot, had commenced a descent from 12,000 ft for a Katoomba-Bathurst Global Positioning System (GPS) arrival and subsequent landing on runway 17 at Bathurst. The PIC reported that as the aircraft descended to the minimum descent altitude (MDA), visibility alternated between visual and instrument flight conditions. During the descent, the PIC had retarded the power to about 17 per cent and slowed the aircraft to about 135 kts in preparation for a Category B circling approach.

The copilot, non-flying pilot, reported that during the descent the engine anti-ice was on, but not the propeller de-ice, nor had the airframe boot de-ice system been activated. The PIC reported that during descent, they entered cloud a number of times and noted ice accretion on the windshield wiper. The flight crew reported that they did not observe any wing ice during the descent.

At the MDA (3,810 ft), the aircraft's Flight Guidance and Autopilot System (autopilot) captured the altitude and, as the airspeed was decreasing due to the reduced power setting, commanded the trim system to progressively raise the nose of the aircraft to maintain the MDA. The PIC commanded the autopilot to roll the aircraft to the right to begin tracking downwind for runway 17. At about this time, the copilot observed that the airspeed was decreasing and called 'speed'. As the PIC applied power to compensate for the decreasing airspeed, the aircraft rolled to the left and pitched down without warning. During the recovery from the steep pitch and bank angles, the aircraft rolled to the right and descended to 112 ft AGL. The PIC regained control of the aircraft and climbed it to the missed approach altitude and carried out an uneventful landing.

The aircraft's aerodynamic stall warning systems of stick shaker, audible alarm, visual warnings and stick pusher, did not activate during the initial roll to the left. However, theautopilot disconnected during the subsequent roll to the right, due to activation of the stall warning.

The investigation determined that following capture of the MDA by the autopilot, the aircraft speed continued to decrease due to the reduced power setting. As a consequence,the aircraft stalled. However, this occurred prior to the stall warning system operating due to the likely presence of airframe ice that had accumulated during the descent.

The investigation found that it is possible for the aircraft to stall prior to the activation of the stall warning system if the aircraft has accumulated ice on the wings.

The investigation, classed as a serious incident, identified a number of other occurrences involving Saab 340 aircraft stalling where little to no stall warning had been provided to flight crew while operating in icing conditions. This included a Saab operated by an Australian operator, which resulted in a number of ATSB recommendations being issued in that investigation, not all of which were accepted and acted upon. Some of those recommendations have been re-issued.


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