The Boeing 747 aircraft was being operated on a scheduled
passenger flight from Melbourne to Perth with an estimated time of
arrival (ETA) at Perth of 0945 WST. The flight crew had been
provided with an aerodrome forecast (TAF) for Perth, valid until
0600 the following day, which indicated that the visibility and
cloud base would be above the alternate criteria throughout the
period of the forecast. As there were no operational requirements,
the aircraft departed Melbourne without alternate or holding fuel
being carried for Perth.
Three minutes after the aircraft's departure from Melbourne, an
amended TAF for Perth was issued with fog being forecast until
0800. After that time, conditions at Perth were forecast to improve
above the alternate criteria. A trend type forecast (TTF) for
Perth, also issued soon after the aircraft's departure, indicated
that the visibility would be reduced in fog until 0800. Subsequent
Perth TTFs issued from 0635 until 0759 indicated that the
visibility would be reduced in fog or mist until 0800 when the
conditions were forecast to improve above the alternate
criteria.
The FLAKE waypoint, 465 NM east of Perth, was the flight planned
point of safe diversion (PSD) based on a return to Adelaide. The
crew obtained the 0730 Perth TTF that indicated an improvement in
conditions above the alternate criteria from 0800 and, as the
aircraft proceeded to the FLAKE waypoint, the Perth TTFs continued
to forecast that improvement. The aircraft passed the FLAKE
position at 0824 and soon after, a Perth TTF was issued that
indicated the meteorological conditions would be below the
alternate criteria until 15 minutes after the ETA of the aircraft
at Perth.
As the aircraft was not fitted with an aircraft communications
addressing and reporting system (ACARS), the operator's flight
dispatch section attempted to contact the crew via high frequency
(HF) radio but the attempts were unsuccessful. At 0845, a message
from the operator about the 0825 TTF was provided to the crew by
air traffic services. As the aircraft had flown past the PSD and
fuel was not carried to divert to an alternate airport, the crew
decided to continue the flight to Perth. The aircraft made an
uneventful landing at 0938.
A report from the Bureau of Meteorology (BoM) indicated that a
weak cold front with a fragmented cloud band was expected to move
through the Perth area between 0800 and 0830. No showers were
expected ahead of the front and the BoM aviation forecasters
assessed the formation of fog in the expected weather situation as
being unlikely. However, a weak ridge formed overnight ahead of the
front and in the light wind regime caused by the weak ridge,
advection fog formed at Perth Airport.
As the front approached Perth it became apparent to the aviation
forecasters that the frontal passage was being enhanced by
north-easterly winds in the Perth area. This meteorological
situation resulted in reduced visibility and low cloud being
observed until 0930. The forecasters had not previously encountered
this type of situation, which produced a mix of advection fog and
frontal fog that was difficult to forecast.
