In aviation, the runway visual range (RVR) is 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

meters The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its p ...

feet The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made ...

. RVR is used to determine the landing and takeoff conditions for aircraft pilots, as well as the type of operational visual aids used at the airport.

Measurement

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 (IRVR), which is measured by devices called

scatterometer A scatterometer or diffusionmeter is a scientific instrument to measure the return of a beam of light or radar waves scattered by diffusion in a medium such as air. Diffusionmeters using visible light are found in airports or along roads to measur ...

s which provide simplified installation as they are integrated units and can be installed as a single unit(s) at a critical location along the runway or

transmissometer A transmissometer or transmissiometer is an instrument for measuring the extinction coefficient of the atmosphere and sea water, and for the determination of visual range. It operates by sending a narrow, collimated beam of energy (usually a ...

s 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." File:AGVIS FSI Forward Scatter Runway Visibility System.jpg, An AGVIS FSI

. File:AGIVIS 2000 Runway Visual Range System.jpg, An AGIVIS 2000

Data reliability

Because IRVR data are localized information, the values obtained are not necessarily a reliable guide to what a pilot can actually expect to see. This can easily be demonstrated when obscuration such as fog is variable, different values can apply simultaneously at the same physical point. For example, a 2000 m runway could have reported touchdown, midpoint and rollout IRVR values of 700m, 400m and 900 m. If the actual RVR at the touchdown point (300 m from the threshold) is 700 m as reported, then a pilot could expect to see the light 700 m 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 400 m, a light 700 m 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 200 m.

Usage

RVR is used as one of the main criteria for minima on

instrument approach In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landi ...

es, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. The maximum RVR range is 2,000 metres or 6,000 feet, above which it is not significant and thus does not need to be reported. RVRs are provided in

METAR METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting. Raw METAR is the most common form ...

s and are transmitted by

air traffic controller Air traffic control specialists, abbreviated ATCS, are personnel responsible for the safe, orderly, and expeditious flow of air traffic in the global air traffic control system. Usually stationed in air traffic control centers and control ...

s to aircraft making approaches to allow pilots to assess whether it is prudent and legal to make an approach. RVR is also the main criterion used to determine the category of visual aids that are installed at an airport. The

International Civil Aviation Organization The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international sc ...

ICAO stipulates in its Annex 14 that for RVR values above 550 m, CAT I lighting shall be installed, if RVR is between 300 m and 549 m then CAT II lighting is required. CAT IIIa is installed for RVR values between 175 m and 300 m. CAT IIIb is required for RVR values between 50 m and 175 m while there is no RVR limitation for CAT IIIc visual aids.

External links

FAA - Lighting Systems Group

Transport Canada - Canadian RVR Monitors

FAA - American RVR MonitorsFlight Crew Guide - Conversion of reported meteorological visibility to RVR
* {{cite news , url= https://aviationweek.com/business-aviation/when-visibility-and-ceiling-reports-differ-pilot-s-perception , title= When Visibility And Ceiling Reports Differ From A Pilot's Perception , date= Jun 25, 2019 , author= Patrick Veillette, Ph.D. , work= Business & Commercial Aviation Aviation meteorology Air traffic control Visibility