de Havilland Canada DHC-8-103, VH-TQX

The pilot in command of a de-Havilland Dash 8 aircraft reported that on the downwind leg of the circuit for a landing on runway 18 at Narrabri aerodrome, the crew received a spurious GPWS mode 4A warning. As they established the aircraft in a climb to cancel the GPWS warning, the co-pilot reported that he could smell smoke. At that time the pilot in command could not see or smell any smoke.

A short time later, the flight attendant entered the cockpit to report that she could smell and see smoke in the cabin. She described it as a grey mist when the sun was shining through it.

The pilot in command could smell the smoke when they were on late downwind and decided to stop the aircraft on the runway and disembark the passengers. They declared a PAN to air traffic services and requested the attendance of fire fighting and rescue services. The aircraft was stopped on the runway and the passengers were disembarked through the main cabin door. The smoke and smell dissipated after the aircraft was stopped and the door was opened. At no time did the crew receive any warnings or observe tripped circuit breakers.

An engineering examination revealed that the air conditioning air cycle machine, Part Number 728790, had failed internally, resulting in the smell and smoke in the aircraft cabin.