REX SF34 at Perth on Jul 6th 2021, stick shaker activation

Last Update: March 9, 2022 / 19:55:02 GMT/Zulu time

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Incident Facts

Date of incident
Jul 6, 2021

Classification
Report

Flight number
ZL--2131

Aircraft Registration
VH-ZLJ

Aircraft Type
SAAB 340

ICAO Type Designator
SF34

A REX Regional Express Saab 340, registration VH-ZLJ performing flight ZL--2131 from Perth,WA to Albany,WA (Australia) with 16 passengers and 3 crew, was climbing out of Perth when the crew, before entering icing conditions, activated the engine anti-ice and wing de-ice systems. Shortly afterwards the crew received indications of a fault in the de-ice systems. The crew levelled off at 7000 feet and worked the relevant checklists without resolving the issue. The crew requested a descent to 5000 feet out of icing conditions and a return to Perth. While the aircraft was levelling off at 5000 feet on autopilot, ATC instructed a right hand turn, about 20 seconds after beginning the turn the stick shaker activated. The first officer, pilot flying at that time, initiated the stall recovery procedure, the captain took control and completed the recovery. The aircraft returned to Perth for a safe landing.

On Mar 9th 2022 the ATSB released their final report concluding the probable causes of the serious incident were:

- The aircraft’s right wing inboard de-ice boot probably delaminated shortly before encountering icing conditions, triggering the de-ice system fault that led to the flight crew’s decision to return to Perth.

- The pilot flying became task saturated due to high workload and did not notice the reducing airspeed, which was also missed by the pilot monitoring due to a focus on other tasks until the stick shaker activated.

The ATSB analysed:

Shortly after the aircraft departed Perth, the flight crew received cockpit indications of a de-ice system fault. The fault was probably triggered by a delaminated de-ice boot on the underside of the right wing that failed during its inflation cycle shortly before encountering icing conditions. With a low freezing level and forecast icing conditions along the planned route, the de-ice fault indication led to the flight crew’s decision to return to Perth.

While manoeuvring towards Perth, engine power was reduced to descend the aircraft from 7,000 ft to 5,000 ft. However, when the aircraft levelled off at 5,000 ft, engine power was not increased. Consequently, as the airspeed reduced due to the low engine power, the autopilot maintained the selected altitude (5,000 ft) by pitching the nose up, increasing the AOA and reducing the airspeed further. This condition went unnoticed until one of the AOA sensors reached the level required for stick shaker activation.

The first officer, the pilot flying in the time leading up to the stick shaker activation, probably became task saturated while managing flying tasks (changing flight state to descend, level flight and turn) as well as communicating with, and following ATC instructions. This task saturation reduced the attention that the first officer paid to managing the airspeed and engine power. At that time, the captain (pilot monitoring) was also not monitoring these key parameters due to a focus on other communication tasks related to the return to Perth. The crew’s reduced awareness of airspeed resulted in the potential stall going unnoticed until the stick shaker activated.

In addition, during that rapid sequence of events, neither pilot recognised the first officer’s high workload and task saturation. Consequently, no attempt was made to alleviate the situation, for example by discontinuing the approach or deferring ATC requests for information until the aircraft was straight and level. Such actions would have provided additional time and opportunity for the crew to refocus on flight instruments and key parameters.
Aircraft Registration Data
Registration mark
VH-ZLJ
Country of Registration
Australia
Date of Registration
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Airworthyness Category
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TCDS Ident. No.
Manufacturer
S.A.A.B. AIRCRAFT CO
Aircraft Model / Type
340B
ICAO Aircraft Type
SF34
Year of Manufacture
Serial Number
Maximum Take off Mass (MTOM) [kg]
Engine Count
Engine
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Main Owner
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Main Operator
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Incident Facts

Date of incident
Jul 6, 2021

Classification
Report

Flight number
ZL--2131

Aircraft Registration
VH-ZLJ

Aircraft Type
SAAB 340

ICAO Type Designator
SF34

This article is published under license from Avherald.com. © of text by Avherald.com.
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