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Analysis

Summary

The circumstances of this accident were consistent with a loss of control by the pilot at night, resulting from inoperative attitude and directional indicators. The combination of a dark night, high level cloud, and limited ground lights, would have provided the pilot with few external visual cues. This would have required him to quickly modify his instrument scan to allow him to control the aircraft by sole reference to the remaining flight instruments. One of those instruments, the electric turn coordinator, was probably the 'electric backups' that the pilot referred to in his call to FS.

To achieve a desired flight performance, the aircraft is placed in a particular attitude, together with an appropriate power setting. Precise attitude information can either be gained by reference to the natural horizon, or to a gyro-stabilised attitude indicator, when external indications are either not available or are unreliable. The altimeter, air speed indicator and vertical speed indicator can, in combination, also provide limited attitude information.

The loss of vacuum to the air-driven gyroscopic flight instruments would have resulted in those instruments providing erroneous aircraft attitude and heading indications to the pilot. It is possible that the pilot did not mask the failed attitude and directional indicators. Consequently, the pilot may have inadvertently continued to respond, however briefly, to the erroneous indications from the failed instruments. His instrument scan proficiency, with the attitude indicator as the primary focus, would have been developed over several years. Such a scan could not easily have been modified to ignore the very powerful stimuli from erroneous attitude indications.

The pilot was dependent on alternative instruments for aircraft attitude information and it is likely that while attempting to control the aircraft as well as calculate flight time and distance to Mildura, the pilot became spatially disorientated and lost control.

 
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