# Why is there a disparity in the conversion from runway visual range (RVR) to statute miles?

TBL 5-4-1 in the Aeronautical Information Manual (AIM) provides conversion from RVR to visibility in statute miles. I've reversed the first two columns from the AIM table to allow for comparison of the listed RVR to the actual distance in resulting from multiplying 5,280 by these fractions.

Why would there would be a difference of more than 100 feet? I understand that the RVR equipment may not be accurate enough to measure a value like 1,320, but why use 1,600 rather than 1,300 or 1,400?

Furthermore, the lack of RVR actually allows flight in lower true RVR for the 1600/quarter mile level. Let's say that the RVR was 1500 on an approach with mins of 1600. If suddenly the RVR equipment failed and a report 1/4 statute miles was substituted the approach would again be available. Usually the failure of a more precise piece of equipment substituted for a less precise calls for added margin of safety. Why the opposite approach here?

SM          RVR          ACTUAL
-           -            -
1/4         1600         1320
1/2         2400         2640
5/8         3200         3300
3/4         4000         3960
7/8         4500         4620
1           5000         5280
1 1/4       6000         6600


Some light on the mysteries of conversion...

Types of devices used for RVR measurement

A RVR device is composed of a light source and a light sensor. The light source is an incandescent lamp in a transmissometer, the oldest device, while in the more modern scatter-effect device it is an infrared LED. The sensor can determine RVR in a certain range, with the accuracy varying in the range:

• Incandescence: Down to 600 ft by 200 ft increments within 600-3,000 ft, 500 ft above 3,000 ft.

• Infrared: Down to 0 ft by 100 ft increments below 800 ft, 200 ft within 800-3,000 ft, and 500 ft above 3,000 ft.

RVR unit conversion

To make RVR ft-m conversion easy, rough equivalences are established:

• 100 ft = 25 m
• 200 ft = 50 m
• 500 ft = 100 m

Visibility unit conversion

Countries can also report visibility, which is different from RVR, in statute miles or nautical miles. Again rough equivalences are in place:

• 1/4 SM = 400 m
• 1 SM = 1,600 m.
• 1 NM = 11/4 SM.
• 1 NM = 2,000 m.

Substituting visibility to RVR

With all these approximate conversion factors defined, we can build a table for visibility by increments of 1/4 SM or 400 m (on the right hand of the table below), and use meter values, the common unit, to establish approximated RVR-visibility pairs.

(Source: Jeppesen)

Visibility, contrary to RVR, is not an exact measure. It's based on eye performance and available distinctive features of the surrounding environment. Moreover, visibility isn't generally evaluated for the same volume of atmosphere than RVR.

Sources:

• Technically, one mile is 1609.3 meters. Like in American (non-professional) track and field, they seem to be making a "good enough" approximation here that 1600 m = 1 mile, since the difference is 0.5%. – Cody P Aug 6 '16 at 18:48

Because visibility and RVR is not the same thing.

Visibility for aeronautical purposes is the greater of:

a) 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;

b) 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)

(From Skybrary)

• I get that they aren't the same thing, but why aren't the ratios of the RVR and visibility somewhat constant across the conversions int he table? Something doesn't smell right about that. – ryan1618 Aug 2 '16 at 17:25
• @RyanBurnette: That's because RVR is measured by increments (25m/100ft, then 50m/200ft, etc) then converted into fractions of SM. See this more complete table from Jeppesen. – mins Aug 2 '16 at 18:08
• @mins What a mess! That sheds a lot of light onto it though. It would be a pretty solid answer if you'd embed that chart. – ryan1618 Aug 2 '16 at 23:38
• @mins, in the UK itself, the common sense is already well on its way to overcome the archaisms. They now use SI units for most purposes, distances and speed limits on road signs being the main exception. The USA are the only place holding on to it. – Jan Hudec Aug 5 '16 at 16:45
• @JanHudec: "Aside from that, Mrs. Lincoln, how was the play?" – Sean Feb 20 at 22:10