New windshear wording

Since airport opening in 1998, the methodology of alerting windshear at the Hong Kong International Airport has been based on the so-called "first encounter, maximum intensity" principle, which was adopted for use after consultation with the local aviation community. The idea is that the microburst or windshear alerts passed by the air traffic controllers to the pilots should include the location (i.e. runway or 1, 2 or 3 nautical miles (NM) from runway threshold) of the first encounter of the microburst or windshear event (i.e. first encounter) and the maximum value of the computed wind speed difference (i.e. maximum intensity). This simplifies the alerting of windshear when more than one occurrence of microburst or windshear is detected over the same approach or departure corridor.Despite the simplicity of alerts, some pilots have expressed concerns over possible confusion this may cause. As an example, under the windshear conditions depicted in the figure below where there is a windshear event of +15kt at 3 mile final and a microburst event of -30kt at 1 mile final, the consolidated windshear alert based on the "first encounter, maximum intensity" principle will be:

"Caution. Microburst minus 30 knots 3 mile final".

Pilots not familiar with this alerting principle may be caught by surprise when on approaching the airport they first experience a gain event (windshear of +15 kt) instead of a loss event (microburst of -30 kt) at the "first encounter (3 mile final)" location only to encounter a loss of -30 kt a couple of seconds later at 1 mile final. This issue was raised by the Civil Aviation Department and subsequently discussed by the International Federation of Air Line Pilots' Associations (IFALPA) in late 2002. The pilots generally felt that an indication of the maximum intensity and an indication of location in terms of approach or departure corridor (without further details on which section of the corridor) would suffice for their preparation for landing or take-off. A new phraseology has thus been drawn up, and under the new arrangement the alert in the above example will become:

"Caution. Microburst minus 30 knots on final approach".

A trial started on 1 April 2003 and will last until 31 March 2004. Users are encouraged to provide feedback to the Hong Kong Observatory (telephone: 2926 8434, fax: 2375 2645, email: stchan@hko.gov.hk).