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.