de Havilland Canada DHC-8-315, VH-TQE, 41km E Scone NSW

REPORTED INFORMATION

On 2 February 2005, a de Havilland Canada DHC-8-315 (Dash 8) aircraft, registered VH-TQE, was being operated on a scheduled passenger service between Sydney and Tamworth, NSW, at flight level 160. At 150 km south-east of Tamworth the aircraft's cabin rapidly depressurised. That was followed by the failure of navigation aids and the illumination of warning and caution lights on the aircraft's caption panel ¹. The pilot in command declared an emergency and conducted a rapid descent to 10,000 ft. Aerodrome emergency services at Tamworth were activated and the aircraft landed without incident.

The airline commenced an investigation into the circumstances of the incident. The problem was isolated to the number-one starter-generator unit. It was determined that the output of the direct current generator had decreased to the extent that systems connected to the left main electrical bus dropped off line. The under-voltage condition resulted in multiple failures, including the release of the pressurisation outflow valve and the loss of all navigation aids except the number-two automatic direction finding indicator. The release of the pressurisation outflow valve depressurised the aircraft. There was no low voltage warning associated with those systems.

Functional testing of the generator found an open circuit within the number-one shunt coil. A broken wire was found at approximately 28 turns on the 100-turn coil. The broken wire would have caused the voltage output of the generator to fall to a residual level, typically of 2 or 3 volts. The wire could have been damaged either during manufacture or routine maintenance. A conclusive determination could not be made due to the condition of the wire.

The starter-generator had operated for 3,947 flight hours prior to the failure. It was last overhauled on 4 September 2004 and failed 920 flight hours later.

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1. The caption panel on the flight deck contains a series of caution and warning advisory lights designed to alert the flight crew to an abnormal condition.

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It is likely that the failure of multiple systems in the aircraft alerted the flight crew to a possible electrical problem. However, their initial response in dealing with the cabin depressurisation event and associated checklist items would have distracted them from attempts to locate the source of the problem. The subsequent failure of other systems would also have required specific checklist responses. These actions, when combined with the need to follow navigation and air traffic control requirements, as well as the absence of an under-voltage warning system, minimised opportunities for the crew to identify the problem.