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Runway visual range
From Wikipedia, the free encyclopedia
Runway Visual Range (RVR) is a term used in aviation meteorology to define 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. It is normally expressed in feet or metres.
It is used as one of the main criteria for minima on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. The maximum RVR reading is 2,000 metres or 6,500 feet, above which it is not significant and thus does not need to be reported. RVRs are provided in METARs and are transmitted by air traffic controllers to aircraft making approaches to allow pilots to assess whether it is prudent and legal to make an approach.
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 transmissometers which are installed at one side of a runway relatively close to its edge. Normally three are provided, one at each end of the runway and one at the mid-point.