Shortly after take-off, in rain and IMC, the master warning
lights, right tailpipe hot light on the central warning panel, and
the associated aural alarm of the Saab 340 activated. The crew
declared a PAN and advised the air traffic controller that they
wished to return to the airport.
After completing the appropriate checklist items, the crew shut
down the right engine and the captain advised the passengers that
the aircraft was returning to Melbourne. The flight attendant then
entered the cockpit and advised the flight crew that there appeared
to be smoke in the cabin. The captain declared a MAYDAY while the
flight attendant returned to the cabin to attempt to identify the
source of the smoke. The flight crew put on their oxygen masks.
Shortly afterwards, the flight attendant returned to the cockpit
and advised that there was no heat in the smoke and it appeared to
be mist or water vapor. The flight crew subsequently conducted a
normal single-engine landing.
An engineering inspection of the aircraft did not identify any
evidence of fire or problems with the heat detector wiring harness.
Subsequent testing identified anomolies with one of the heat
detectors. The detector was removed, tested, resealed and replaced,
and the system functioned normally.
There have been a number of previous incidents of false tailpipe
hot warnings involving Australian-registered Saab 340 aircraft, and
the aircraft manufacturer has issued a service bulletin that was
intended to improve the moisture protection and strength of the
heat detection wiring harness. The aircraft had been modified in
accordance with the provisions of the service bulletin.
Mist or water vapour emanating from the cabin air conditioning
system vents is a relatively common occurrence in Saab 340
aircraft, particularly when the aircraft is operating in conditions
of high humidity. The drying function of the environmental control
system is not completely effective in these conditions and moisture
(in the form of water vapour) may enter the cabin.