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.