Visibility is a measure of the distance at which an object or light can be clearly discerned. Visibility may vary according to the direction and angle of view, and the height of the observer. Visibility is affected by the presence of fog, cloud, haze and precipitation.
Visibility for aeronautical purposes is the greater of:
- the greatest safe distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognised when observed against a bright background;
- the greatest distance at which lights in the vicinity of 1000 candelas can be seen and identified against an unlit black background.
Runway Visual Range (RVR) is the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the light delineating the runway or identifying its centre line. (ICAO Annex 3)
Units of Visibility
Visibility is measured in metres (or kilometres) or in Statute Miles, depending on the country concerned.
Visibility and/or RVR are reported in routine and special reports using abbreviated plain language, or the Meteorological Terminal Air Report (METAR) or SPECI code; at aerodromes, visibility and RVR is also reported in abbreviated plain language in Automated Terminal Information Service (ATIS) transmissions.
Importance of Visibility
Visibility is an important factor in all phases of flight, but especially when the aircraft is manoeuvring on or close to the ground, i.e. during taxi-out, take-off and initial climb, approach and landing, and taxi-in.
Aircraft departure and arrival is limited by the visibility (or RVR) to an extent that depends on the sophistication of ground equipment, the technical equipment fitted to the aircraft and the qualification of the flight crew. Many aerodromes and aircraft are fitted with equipment that makes possible a landing in very low visibility conditions provided the flight crew are suitably qualified; however, in very low visibility, it may prove impossible for the pilot to navigate the aircraft along the runway and taxiways to the aircraft stand.