A final ATSB investigation report into a serious incident involving a Saab passenger flight in June 2002 has found that pilots lost control because of low airspeed, airframe icing and the operation of the aircraft autopilot system, and that they did not receive a prior stall warning.
As a result of this serious incident and an ATSB report released in May 2001 into a similar serious incident, the ATSB has made further safety recommendations to operators, to Saab, and to CASA.
On the evening of 28 June 2002, a Saab 340B, VH-OLM, operating as a regular public transport service from Sydney to Bathurst experienced an in-flight loss off control after it levelled out at its minimum descent altitude of 3810 feet.
The pilots inadvertently allowed the aircraft's power to remain at about 17 per cent when it should have been over 50 per cent and the stall speed was higher than normal because of airframe icing.
The aircraft initially rolled to the left and pitched down without warning and during the recovery from the first stall, the aircraft rolled to the right and descended to 112 feet before altitude was recovered.
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 pilots did not appreciate the seriousness of the incident until after it was reported to the ATSB by passengers and the ATSB had investigated the circumstances.
The investigation, classed as a serious incident report, identified a number of other occurrences involving Saab 340 aircraft stalling where little or no stall warning had been provided to flight crew while operating in icing conditions.
As a result of a 1994 fatal accident involving airframe icing to an ATR-72 at Roselawn, the US FAA issued an airworthiness directive, which applied to US Saab 340s, requiring that flight manuals warn that autopilot operations may mask problems in severe icing conditions. For Saab 340s in Canada, an ice stall warning protection option is fitted to aircraft.
The ATSB believes the Saab 340's defences should be enhanced to protect against situations of human error such as failing to take early action on power and speed and/or failing to notice ice accretion.
The ATSB has recommended that as a matter of priority Saab modify the stall warning system on the worldwide fleet to give sufficient warning of an impending stall during icing conditions.
The ATSB urges operators of the aircraft type to carefully note the circumstances of this recent serious incident as well as the previous Australian serious incident and international accidents and incidents and alert and train their crews accordingly.