Annex 3 Trend Forecast, Amendment Criteria, and a Volcanic Ash Encounter
A MET office at an international airport issues a trend forecast appended to a METAR at 12:00 UTC: BECMG 1212/1214 4000 TSRA BKN008. The prevailing conditions at 12:00 UTC are 9999 SCT020 24/18 Q1012. At 12:40 UTC a volcanic ash advisory is received indicating an ash cloud from a Chilean volcano expected over the airport between 15:00 and 19:00 UTC. The airport has no radar and relies on a single ceilometer and a visibility sensor. The ATC unit asks whether the trend forecast can be amended and whether a SIGMET is required. The aerodrome has one instrument runway with a 200 ft decision height.
1. Evaluate whether the BECMG group is correctly constructed and whether it should be amended given the ash advisory.
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The BECMG group is correctly formed: it gives a validity period of two hours from 12:00 to 14:00 UTC and specifies the elements that are expected to change, with visibility falling to 4,000 m and a broken ceiling at 800 ft accompanied by thunderstorm rain. The group is valid because the change is expected to occur within the two-hour window and the elements are given in the correct order. However, the trend forecast is a description of expected conditions at the aerodrome and cannot simply be rewritten to include volcanic ash, because ash is not a standard trend element and its presence requires a SIGMET rather than a trend group. The correct action is to leave the BECMG group in place and issue a SIGMET for volcanic ash covering the affected area and period, with a separate aerodrome warning. If the ash is expected to reach the aerodrome within the trend validity period, the trend forecast should be amended to reflect the deterioration in visibility and ceiling that the ash will cause, but the ash itself must be communicated through the SIGMET and the aerodrome warning.
2. What operational advice should be given to ATC and the operator given the 200 ft decision height and the absence of radar?
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With a 200 ft decision height and no radar, the aerodrome has very limited ability to detect the leading edge of the ash cloud or to distinguish ash from ordinary cloud. The ceilometer and visibility sensor will show a fall in visibility and a lowering ceiling, but neither instrument identifies ash, so the first indication may be a pilot report. The advice to ATC is to treat the period 15:00–19:00 UTC as a high-risk window, to require pilot reports of ash, sulphur smell, or engine surging, and to be prepared to close the runway if ash is confirmed. The operator should be advised that dispatch to the aerodrome during the window is not recommended unless the aircraft has a demonstrated ash avoidance capability and the crew has a diversion airfield with adequate weather. The 200 ft decision height means that any reduction in visibility below about 1,500 m or any ceiling below 300 ft will make the approach unusable, so the practical trigger for closure is lower than the ash advisory boundary. A SIGMET and aerodrome warning should be issued, and the MET office should coordinate with the VAAC for updates every hour.