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 airconditioning air
cycle machine, Part Number 728790, had failed internally, resulting
in the smell and smoke in the aircraft cabin.