At the time the Boeing 747-400 departed Los Angeles for Sydney, operational considerations including weather forecasts did not require the carriage of additional fuel to enable flight from overhead Sydney to an alternate airport.
At 0240 EST when the aircraft was approximately 4 hours from Sydney, an amended weather forecast for arrival at Sydney was issued. The forecast indicated that flight crews could be required to divert to an alternate airport. At the time of the amended forecast Noumea and Brisbane were available to the crew as alternate airports.
At 0430 the trend forecast indicated that Brisbane remained suitable as an alternate aerodrome for the flight. Thirty minutes later, when about 1.5 hours from Sydney and after the aircraft could no longer divert to Noumea, an amended forecast indicated that the Brisbane weather was deteriorating and the airport was now not suitable as an alternate airport. The crew proceeded to Sydney where they landed without incident.
Specified minimum weather and visibility conditions are required for aircraft landing in Australia in order to land safely after conducting an instrument approach. The worst weather conditions under which a particular aircraft may land safely are called the landing minima. If a pilot attempts to land in weather that is worse than the landing minima, a safe landing cannot be assured.
Before an aircraft takes off, the crew must satisfy themselves that they are confident that the aircraft will be able to land safely. One of the requirements that must be considered is that the weather will be good enough at the destination for the crew to see enough to be able to land safely. In order to satisfy themselves that the weather is expected to be good enough, they will assess the appropriate weather forecast.
A weather forecast is a prediction of what the weather will be at some point in the future. It is possible that the actual weather on arrival will be close to, but not exactly the same as the forecast weather; the actual weather on arrival could be better or worse than that which had been forecasted. The crew will need to satisfy themselves that the weather will not be worse than the landing minima when they arrive at their destination. Because of the variability between actual and forecast weather conditions, the crew will only assume that a safe landing can be assured at the destination if the weather forecast for the destination is better than the landing minima. A different set of weather criteria are used when the weather conditions at the destination are being assessed from a forecast, instead of from an observation. These weather criteria are called the alternate minima.
If the forecast weather at the destination is worse than the alternate minima for the destination, then the aircraft must carry sufficient fuel to continue from the destination to another airport where the weather is better than the alternate minima for that alternate location. In this way, a crew may attempt to land at a destination, and remain confident that if the weather has deteriorated below the landing minima, they can still safely continue to another airport where a landing would be assured.
Based on the weather report current at the time of their departure from Los Angeles, the crew of the B747 had ensured that the aircraft carried sufficient fuel to enable flight to an airport where a landing could be assured. Prior to the time when Brisbane became unsuitable as an alternate airport the crew had been able to divert to an alternate airport if a landing at Sydney was not assured.
At the time when the aircraft landed at Sydney, the weather was better than the criteria for the landing minima, but worse than the alternate minima.