As the aircraft approached runway 23 for landing, the crew observed a bank of fog drifting toward the aerodrome from the north-east. By the time the aircraft arrived at the aerodrome, the runway threshold was obscured by the fog. As a result, the crew elected to conduct a missed approach.
During the missed approach, the crew noticed that the threshold area of runway 05 was clear, so they requested an immediate visual approach to runway 05 before the fog drifted further to the south-west. Due to other instrument flight rules traffic, Air Traffic Control (ATC) could not issue an immediate clearance for the approach. By the time that clearance was available, the remainder of the runway was obscured by fog. A B737 aircraft had been able to land on runway 05 following a VOR/DME approach, so the A320 crew attempted to conduct a similar approach. However, that attempt resulted in a second missed approach. The aircraft tracked to the north-east of the aerodrome and the crew informed ATC that they would conduct an instrument landing system (ILS) approach to runway 23, and then land using the aircraft's autoland system. With 1,500 kg of fuel remaining, the aircraft landed without incident in the fog. Visibility was 250 to 350 m.
The aircraft was certificated for autoland approaches, but the ground equipment was not. The ILS transmitter was a Category 1 unit with a minimum visibility of 1,200 m required for landing. The crew decided to conduct an autopilot-coupled approach with automatic landing, as fog was also present at the Royal Australian Air Force base at Edinburgh, rendering that aerodrome unsuitable as an alternate. The crew considered that Whyalla, the nearest suitable aerodrome, was likely to have similar weather conditions to Adelaide.
Fog had not been forecast for Adelaide when the crew submitted their flight plan. Consequently, the aircraft did not carry fuel for holding at Adelaide or for diversion to an alternate.
However, fog had been forecast for both Edinburgh and Parafield. The Bureau of Meteorology (BoM) reported that this was not unusual, as records showed that in the past 20 years, fogs formed at both Adelaide and Edinburgh on about 50% of occasions, with Edinburgh proving to be the greater risk. On the day of the occurrence, moisture levels were higher to the north of Adelaide, with fog forming at Edinburgh at 0700 Central Standard Time. What was unusual about this event was that the advection of fog from the north took place at a greater speed than the surface wind and that the onset time of fog at Adelaide Airport was 40 minutes later than any recorded onset time at that location in the past 30 years.
BoM records showed that Adelaide Airport averaged 4.9 fog events per annum. The highest annual total for events was nine, recorded in both 1956 and 1983. At the time of the incident on 20 August, there had been 11 fog events recorded at Adelaide Airport during 1999.