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Analysis

Summary

The reason for the cabin depressurisation was likely to have been due to the moisture shrouds not being fitted after the removal of the airstairs. This permitted the ingress of water into the E/E bay and the pressurisation controller, resulting in a malfunction of the operating modes of the unit. The electrical faults found in the yaw damper coupler electrical filter and the rudder power control unit may have led to the rudder pedal movement detected by the flight crew during the descent into Brisbane.

Extensive research has shown that the effect of oxygen deprivation can be insidious and, as such, cabin crew may not be able to correctly judge their oxygen intake. Research conducted by the US Federal Aviation Administration (FAA) Civil Aeromedical Institute indicates that physical activity such as that performed by a cabin crewmember will significantly shorten the time of useful consciousness during an aircraft depressurisation. Based on that research, the FAA's recommended procedures for cabin crew during depressurisation are to immediately don the nearest oxygen mask, sit down, or grasp a fixed object and hold on in order to brace themselves until given clearance to move about the cabin by the flight crew.

The operator's emergency procedures for immediate action for cabin crew following depressurisation reflected that advice. However, in this occurrence, two cabin crew, while returning to the rear crew seats, assisted some passengers before taking oxygen themselves. A further delay to oxygen intake occurred as oxygen masks, that had failed to deploy automatically above the rear crew seat, had to be manually released before use.

Some cabin crew appeared to have judged that the angle of the cabin during the descent was not very steep. The operator's emergency procedure manual referred only to a `very steep' angle of descent and other more severe characteristics of depressurisation. The manual did not discuss the possibility of an emergency descent that may be less than `very steep' or indications that may be less severe than those associated with a rapid or explosive depressurisation. This may have led some cabin crew to believe that in the absence of other characteristics associated with depressurisation, they could safely assist passengers while moving to the rear of the aircraft before using crew oxygen. Remaining where they were, using oxygen until advised by the flight crew that a safe level had been reached, may have been a safer practice than moving through the cabin to reach the rearmost crew seats before taking oxygen.

 
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