The crew of the DHC-6-320 aircraft reported that during descent
to Cairns they smelt smoke in the cockpit. They then noticed the
battery load meter was at maximum discharge and the right generator
light was illuminated. The left generator switch was moved to the
OFF position, however, the generator remained on-line.
Subsequent in-flight inspection revealed that the smoke was
coming from behind the right cabin roof panel; which had begun to
melt and bubble. The pilot in command then contacted Cairns
approach control reporting a fire in the cabin and the aircraft was
cleared to track direct to Cairns. The co-pilot accessed the cabin
fire extinguisher and extinguished the fire. At approximately 4NM
from the airport, the co-pilot reported that the fire was an
electrical fire and had been extinguished. After landing, the crew
stopped the aircraft on the runway and shut down the right engine
to allow the fire fighters access to the cabin. Following
confirmation that the fire was extinguished the aircraft was taxied
to the terminal.
Investigation by the owner's maintenance organisation found that
the left reverse current relay had severe heat damage. Other
components and wiring near the left reverse current relay were also
heat damaged. The reverse current relay was disassembled by the
maintenance organisation's engineers, but they were unable to
determine the reason for the failure due to the severity of the
heat damage.
The Bureau did not conduct an on-site investigation of this
occurrence.