Why did the ATSB do this research
A number of unforecast weather episodes relating to flights into major Australian airports have led to unforeseen diversions, holding, and in some cases, landing below published safe limits. For example, on 18 June 2013, two flights encountered unforecast weather en route to Adelaide, South Australia, leading to a diversion to Mildura Airport, Victoria. Upon arrival, both encountered weather unsuitable for landing.
Aerodrome weather forecasts allow pilots and operators to develop a contingency plan during flight planning and en route (such as carrying additional fuel for holding or diversion) when there are indications of conditions potentially unsuitable for landing at the intended destination. Weather unsuitable for landing mostly involves thunderstorms, a low cloud base and/or low visibility, and to a lesser extent, strong winds.
This is the first report in a series covering Australian airports supporting regular passenger transport operations. The results will assist aircraft operators to focus on the highest risk seasons and times of day for weather reliability, facilitating better flight planning and support for pilots. They will also allow for more informed prioritisation of investment decisions about aircraft and aerodrome navigational equipment. This report focuses on Adelaide and Mildura Airports.
What the ATSB found
Weather conditions were reported as unsuitable for landing in about one in every 22 days at Adelaide Airport and one in 10 days at Mildura Airport. Considering the total time, episodes of weather below the landing minima were rare, accounting for only 0.23 per cent of the time at Adelaide and 0.99 per cent at Mildura.
It was very rare for forecasts not to provide sufficient indication of conditions unsuitable for landing (less than 0.1 per cent of all time). However, when weather conditions were unsuitable for landing, aerodrome forecasts (TAFs) did not provide sufficient indication of these conditions 13 per cent (Adelaide) and 9 per cent (Mildura) of that time. At Adelaide, using the shorter-term trend forecasts (TTF) alone, unsuitable conditions were not indicated 22 per cent of that time.
Taking into account aircraft traffic arrival patterns, an average of 15 (Adelaide) and four (Mildura) aircraft were expected to arrive during unforecast weather each year by these TAFs. For TTFs used alone at Adelaide, 27 aircraft were expected to be affected per year. Mornings had the most aircraft arrivals affected by unforecast weather, especially in June (Adelaide) or July (Mildura).
The potential impact on safety, measured by unexpected holding time required if a flight crew was unable to land due to unforecast weather varied considerably. Unexpected holding periods of 30 minutes or more were calculated in Adelaide mornings 53 per cent of the time when unsuitable conditions were not indicated by the TTF. For TAFs, this was 36 per cent of the time for Adelaide mornings, and 64 per cent of the time for Mildura mornings.
Retrieving the TAF closer to the intended landing time led to a lower incidence of insufficient indications of weather conditions that were unsuitable for landing, particular in the mornings. At Adelaide in the mornings, an increase of 2.3 aircraft arrivals per year were predicted during unexpected unsuitable conditions for every additional hour prior to arrival that a TAF is retrieved.
Safety message
In the morning at Adelaide and Mildura, it is relatively more important that forecasts are retrieved at the latest possible time (before the point where a diversion is no longer possible) prior to arrival. Using the alternate minima rather than landing minima for all decision making, both for pre-flight planning and in-flight, considerably improves the chances of not being exposed to unexpected unsuitable conditions for landing.