Runway visual range

Runway visual range (RVR) is in aviation meteorology the distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating the runway or identifying its centre line. RVR is normally expressed in feet or meters. RVR is used to determine the landing and takeoff conditions for aircraft pilots, as well as the type of operational visual aids used at airport.

Originally RVR was measured by a person, either by viewing the runway lights from the top of a vehicle parked on the runway threshold, or by viewing special angled runway lights from a tower at one side of the runway. The number of lights visible could then be converted to a distance to give the RVR. This is known as the human observer method and can still be used as a fall-back.

Today most airports use Instrumented Runway Visual Range or IRVR, which is measured by devices called forward scatter meters which provide simplified installation as they are integrated units and can be installed as single unit(s) at a critical location along the runway or transmissometers which are installed at one side of a runway relatively close to its edge. Normally three transmissometers are provided, one at each end of the runway and one at the midpoint. In the US, Forward Scatter RVRs are replacing transmissometers at most airports. According to the US Federal Aviation Administration: "There are approximately 279 RVR systems in the NAS, of which 242 are forward scatter NG RVR Systems and 34 are older Transmissometer Systems."

Because IRVR is not derived using human observers, the values obtained are not a reliable guide to what a pilot can actually expect to see. This can easily be demonstrated by the fact that when obscuration such as fog is variable, different values can apply simultaneously at the same physical point.

For example, a 2000m runway could have reported touchdown, midpoint and rollout IRVR values of 700m, 400m and 900m. If the actual RVR at the touchdown point (300m from the threshold) is 700m as reported, then a pilot could expect to see the light 700m away, at the runway midpoint. If the pilot taxies to the midpoint and looks back through the same air mass, he or she must also be able to see the light at the touchdown point. But since the midpoint RVR is reported as 400m, a light 700m away must be invisible. Similarly, looking forward he or she cannot see the light in the rollout area. But according to the rollout RVR of 900m, the light there is visible, and has already been so for the last 200m.

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Wikipedia