5.2.1 Visibility and Visual Range

1. Horizontal Visibility Aspects

Horizontal visibility is defined as the maximum distance up to which prominent objects on the earth’s surface can be seen and identified by day and prominent lighted objects of a given brightness, on the earth’s surface, can be seen by night.

Horizontal visibility may be estimated by observing suitable objects or lights at known distances. The horizontal visibility value reported in meteorological reports — referred to as meteorological visibility — may be either the minimum visibility observed in any direction or the visibility which prevails over more than half the horizon (prevailing visibility). There is no common world-wide procedure in this respect.

Whenever conditions of restricted visibility exist, neither method meets the operational requirements, since, by virtue of the concepts of weather minima, what the pilot requires to know is whether adequate visual guidance will be obtained on the approach to the runway is use. This is, however, not a matter of horizontal visibility but of ’slant visual range’ (SVR). Until such time that slant visual range observations are available ’runway visual range’ (RVR) observations are made in conditions of restricted visibility.

For reports for take-off the meteorological visibility observations must be representative of the take-off and climb-out area, and for reports for landing the observations must be representative of the approach and landing area. Visibility observations made for reports disseminated beyond the aerodrome must be representative of the aerodrome and its immediate vicinity; in such observations special attention should be given to significant directional variations.

In reports in abbreviated plain language, the name of the element should be given and the units used for visibility should be specified clearly. When the visibility is 5 kilometres or less, it should be expressed in increments of 100 metres, in the form ’VIS 2300 m’; when it is more than 5 kilometres, but less than 10 kilometres, the units should be kilometres, in the form ’VIS 7 km’; and when it is 10 kilometres or more, it should be given as 10 kilometres, except when the conditions for the use of CAVOK apply.

2. Visual Range

The purpose of providing runway visual range (RVR) information is to permit pilots to appraise aerodrome visibility conditions and in particular to determine whether these conditions are above or below established aerodrome operating minima.

RVR is defined as the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway of identifying its centre line.

For this purpose, a height of approximately 5 metres is regarded as corresponding to the average eye-level of a pilot in an aircraft on the centre line of a runway.

In practice, RVR cannot be measured directly from the position specified in the definition but is an assessment of what a pilot would see from that position.

One should realize, however, that in assessing RVR no account is taken of the effect on vision of such variables and imponderables as:

• rain on the windshield of the aircraft; the level of cockpit lighting, which is adjustable;
• the illumination to which a pilot has been exposed during the preceding few minutes (for example, when passing over approach lights);
• any effect connected with the motion of a pilot with respect to the runway lights, e.g. the time taken for a pilot to react to a light coming into view;
• the pilot’s vision and any physical or psychological factors affecting it.

Thus, RVR is merely a method of assessing ’seeing conditions’ for take-off and landing and is not a statement of what a pilot would actually see.

There is no doubt that, for landing, an assessment of the visual guidance available to the pilot during final approach, i.e. ’slant visual range’ (SVR) would be even more useful than RVR. No practical system for assessing SVR has yet been developed, although several experimental SVR systems have been developed and are being tested. It is felt that progress might be made with the development of an observing system, if a specific reference point in space could be defined to which the SVR could be related. Such a value should relate to a particular height, on a particular approach slope and with a visual segment intercept on the ground of a certain length.

The longer term objective is to meet in full the complete aeronautical requirement, which includes the two basic concepts:

• the height at which ground cues become evident, i.e. probable contact height; and the degree to which this reference is maintained as the descent is continued.

ICAO and WMO are encouraging States to pursue efforts both to satisfy the long term ends, and to provide a more immediate limited solution.

Therefore, RVR is often used, not only for its primary purpose of indicating visual guidance along the runway but also for the secondary purpose of giving some indication of ’seeing conditions’ on final approach.

RVR information is issued for one or more observation sites along a runway but although ideally desirable, it is impracticable to make the observation from the pilot’s position over the runway. The various techniques generally approach as closely the ideal as is possible without risk either to aircraft or to human observers. The observations are used to assess values that are satisfactorily representative of what a pilot would see at the appropriate part of the runway.

3. Observing Techniques Of RVR

Three techniques for observing runway visual range (RVR) are currently in use:

a. Transmissometer

The transparency of the atmosphere is measured in terms of the transmission factor over a horizontal distance between a light projector and a photo-sensitive receiver. The transmission factor is converted to RVR taking into account the characteristics of the runway edge and centre line (if available) lights, background luminance, etc., available to a pilot for guidance, and the expected sensitivity of his eyes in the prevailing conditions of ambient illumination (day, twi-light or night).

b. Human observer

An observer counts the number of runway lights or day markers he can see from an observing position near the runway and this number is converted to RVR, making due allowance for the differences in light intensity, background, etc., from the different viewing positions of the observer and a pilot. Sometimes, where it is difficult to count runway lights, observations are made on a special row of runway of other lights, set up near the runway.

c. Television

A television camera near the runway is directed at the runway (or special) lights or markers and the observer monitors the television receiver in the observing station.

Because of the trend to automation of aerodrome observations, transmissometer systems have become the principal means of observing runway visual range and now outnumber the human observer and television systems.

It should be noted that the length of the baseline (the distance between the light projector and the sensor) sets limits on the range of RVRs that can be measured.

For CAT II, IIIA or IIIB operations the baseline must be very short. At Schiphol Airport transmissometers with baselines of 82 and 12 metres are installed.

The human observer and television techniques as primary means of RVR assessment, but may be used as substitutes, are used very little. In this system a camera is mounted at a height of 5 metres above the runway level and at a distance of 75 metres from the runway edge. It is directed at a series of lights, identical to the runway lights and placed on a line approximately parallel to the runway. An observer in the meteo-station, using a special lights switching technique, observes the RVR on a monitor.

4. Observing Of RVR

a. RVR observations must be representative of the touch down zone and, as may be selected by the authority concerned, of the middle and far sections of the runway.

b. RVR observations should be made on all runways intended for use during periods of reduced visibility and in particular on:

• Precision approach runways;
• Runways used for take-off and having high intensity edge lighting and/or centre-line lighting.

c. RVR observations should be carried out at a lateral distance from the runway centre-line of not more than 120 metres. The site for observations to be representative of the touch down zone should be located about 300 metres along the runway from the threshold. The sites for observations to be representative of the middle and far sections of the runway should be located at a distance of 1000 to 1500 metres along the runway from the threshold and at a distance of about 300 metres from the other end of the runway. The exact position of these sites and, if necessary, additional sites should be decided after considering aeronautical, meteorological and climatological factors such as long runways, swamps and other fogprone areas.

d. RVR observations should be made during periods when the horizontal visibility is less than 1500 metres.

e. ATS units and other users at an aerodrome should be kept informed without delay of changes in the serviceability status of the RVR observing system.

5. Availability Of RVR Observations At Ats Units

If transmissometer readings are converted into RVRs using computers, the RVR is usually presented automatically in the aerodrome control tower and approach control office using digital displays; similar displays are installed in the MET office or observing station (e.g. USA).

Tables may be used to convert the transmission factor into an RVR value. This is usually done in the meteorological observing station which then transmits the RVR reports to the users by telephone, telewriter, pneumatic tube, etc.

It will be understood that all systems used, which are not automated, give a certain time-delay in the availability of the the current RVR report. Most authorities have taken steps to keep this time-delay as short as possible.

6. Reporting Procedures Of RVR Reports

RVR reports reach aircraft via ATS units and / or aeronautical broadcasts.

a. The reporting scale should consist of increments between 25 metres, when RVR is less than 150 m, 50 metres for RVR up to 800 metres, 100 metres for RVR between 800 and 1200 m and increments of 200 m for RVR above 1200 m. Where the observations are made by counting runway edge lights, the reporting increments should be determined by the spacing of those lights. Any observed value which does not fit the reporting scale in use should be rounded down to the next lower step in the scale.

b. 50 metres should be considered as the lower limit for measurements and reports of RVR. Below this limit the reports should merely indicate that the RVR is less than 50 metres, in the form ’RVR BLW 50 m’.

c. RVR should be reported to the appropriate local ATS units, whenever there is a change in the value to be reported in accordance with the reporting scale. The transmission of such reports should normally be completed within 15 seconds after the termination of the observation.

d. In reports in abbreviated plain language the name of the element should be given in abbreviated form and the units should be included, for example, ’RVR 400 m’. When RVR is above the maximum value which can be determined by the system in use, it should be reported in the form ’RVR ABOVE 1700 m’; the use of this form of reporting should be limited to cases where RVR is above a value between 1500 m and 2000 m. When the RVR is below the minimum value which can be determined by the system in use, it should be reported for example in the form ’RVR BLW 150 m’ where the figure 150 is the minimum value that can be determined by that system. If RVR is observed from one location along the runway, about 300 metres from the threshold, it should be included without any indication of location. If the runway visual range is observed from more than one location along the runway, the value representative of the touch down zone should be given first, followed by the values for the other locations along the runway, together with an indication of these locations, according to the method by which positions are notified in the Aeronautical Information Publication (AIP), for example, ’RVR RWY 16 PSN A 600 M PSN B 500 M PSN C 400 M’. When there is more than one runway in use, the available RVR values for each runway should be given and the runways to which the values refer should be indicated, for example, ’RWY 26 RVR 500 M RWY 20 RVR 800 M’; if more than one runway is in use, but RVR is available only for one runway, that runway should be indicated in the form ’RWY 20 RVR 500 M’.

e. In reports disseminated beyond the aerodrome only the value representative of the touch down zone should be given and no indication of location on the runway should be included. When there is more than one runway in use and there are significant differences in RVR between those runways, values for more than one runway should be included in accordance with agreement between the authorities and the operators concerned and the runways to which the values refer should be indicated in the form ’RWY 26 RVR 500 M RWY 20 RVR 800 M’.

f. At an aerodrome with several runways used simultaneously for landing and take-off, the RVR should be computed separately for each runway: these computations should be based on the runway light intensities currently used for each runway. When one or more of the runways are temporarily out of use, the RVR for these runways should be computed on the basis of the light intensity currently used for the runway or runways in service or, the highest light intensity currently being used for any of the runways in service, if the intensities are different. When all runway lights are switches OFF, the RVR should be computed on the basis of the appropriate light intensity which would normally be used.

7. Accuracy Of RVR Reports

As already indicated previously, an RVR report is not more than an ’assessment’ what a pilot would see form the position on a particular runway for which the RVR is issued.

For the following reasons inaccuracies in the reports are unavoidable:

a. Fog is seldom uniform. Transmissometer(s) may be located in areas of good/bad visibility compared with general visibility.

b. Locations of observation is, in nearly all cases not on the runway (see also subpara 4.-c.).

c. Observations are seldom made at the eye-level of the pilot who uses the RVR report. Since most runway edge lights are inclined a few degrees above the horizontal the pilot’s eyes are, depending on circumstances, close to or away from the axis of the beam of the lights and therefore visual range respectively increases or decreases.

d. Components used in the theoretical conversion of transmissivity of the air into RVR may change in actual practice (e.g. runway edge lights used by the pilot may need cleaning).

e. Changes in visual range with a pilot’s eye-level can be caused by variations in atmospheric transparency with height.

f. Processing of RVRs involves averaging. Transmissometer readings are averaged over a period not less than 30 seconds and not more than one minute.

g. Reporting of RVRs is in increments (see also subpara 6.-a.).

h. RVR sometimes fluctuates rapidly, occasionally changing by several hundred metres in less than one minute. This causes difficulties to pilots landing, although some of the fluctuations may be of little significance, for example, those experienced when passing through small patches of fog. Rapid changes in visual range also raise difficulties for ATS units when passing information to aircraft. Some smoothing of observations, by averaging over a certain period or otherwise, is therefore desirable (see also subpara 7.-f.).

i. In all cases where the RVR observing/reporting procedures are not automated a delay in reporting to aircraft concerned is unavoidable. (See also subpara 5).

8. Slant Visual Range

The operation requirement for ’slant visual range’ (SVR) has been discussed in subpara 2. Also an indication was given for the ideal situation re-availability of this kind of information. The ICAO All Weather Operations Panel is of the opinion that a possible approach to such a complete solution may be to determine and to report to arriving aircraft the height at which a pilot will see and continue to see a minimum ground segment equivalent to 5 light bars of approach lights at 30 metres spacings.

The Panel realized, however, that for the time being it was not possible to build a SVR observing system capable of providing this kind of information.

The ICAO 8th Air Navigation Conference (Montreal, April 1974) recommended therefore that States be encouraged to develop a ’limited’ system sufficient to determine and to report to arriving aircraft the slant distance to the farthest high intensity runway edge light, or approach light, which a pilot will see from a height H above runway threshold elevation on the approach path. (The value of H should depend on aerodrome conditions and types of aircraft operations).

Several countries adhere to ICAO’s request and are doing research work on SVR observing systems. Denmark, for instance, is investigating possibilities using backscatter techniques, other States focus attention on LASER.

9. The Visual Segment

From previous paragraphs we learned that no practical method exists for measuring the SVR available to the pilot at decision height and that, for low visibility approaches, RVR measured along the runway is used to provide a reasonable likelihood of visual contact on the approach at or before DH and to provide an adequate visual segment to the flare, touch down and roll-out. The length of the visual segment, required as external visual reference, determines, together with the length of the obscured segment, due to the cockpit cut-off angle, the total SVR.

For calculation purpose a visual segment of 150 m (500 ft) is assumed to be adequate and this results in the following formula:

By using a correlation factor, depending on the required approach success probability, the required SVR can now be transformed into a RVR value. Experience has shown that the correlation factor between the measured RVR and the SVR available at decision height can vary. During homogeneous fog or low visibility conditions, a SVR/RVR ratio of 1.0 can be used since the measured RVR closely approximates the SVR.

However, the correlation factor can vary considerably (up to 0,5) in other conditions, such as inhomogeneous fogs, snow, blowing dust, shallow fog and torrential rain. All conditions should therefore be taken into account in setting the SVR/RVR ratio for deriving RVR.

One must also take into account the capabilities, limitations and accuracy of RVR reports, as discussed in previous paragraphs.

Furthermore in low daytime visibilities, a bright light can be seen further than an object. As daytime visibilities increase, even a high intensity light will fade into the background and substitution of runway visibility would therefore be more accurate. Under certain conditions of daytime restrictions to visibility, runway paint markings are of greater help than the lighting system.

The use of landing lights at night, specially during deep fog conditions, will adversely affect the visibility, although a nightlanding without landing lights is not particularly recommended. Landing lights should therefore be used judiciously.

company requires the availability of RVR measurements by one or more transmissometers or human observers. If for a particular runway more than one RVR value is given in the weather report, the usability of that runway for landing must be based on the RVR value for the touch down zone. The RVRs for the mid point roll-out end of that runway may be used as an extra information, but should never be less than 125 m for those cases where a manually roll-out may be required.

10. Variation In Visual Segment

In order to assimilate and interpret the visual cues which are being presented in such extremely dynamic perspective, the pilot needs two things:

• A sufficient exposure time to the lights, and
• A minimum visual segment during his final approach.

The exposure time depends on the decision height and the glide slope angle. From an altitude of 200 ft to the point where the runway threshold lights disappear from view under the bottom edge of the windscreen, takes from 10 to 12 seconds. At a 100 ft decision height the exposure time drops to between 3 to 5 seconds and it is more than likely that another two or three seconds will be spent integrating the visual cues. In this respect it should be noted that there is an inability of the human eye to instantly change from short range vision to long range vision and back again and a certain time lag should be taken into account.

The present situation is such that a pilot can expect as many different combinations of glide slope angles and touch down point locations as there are ILS installations. If his decision height is 200 ft on 30 different approaches he can expect to see 30 variations in the view of the lighting system when reaching his decision height on each different approach. Though this does not make his task any easier, it can be appreciated that a difference of a second or so in a total exposure time of some 12 seconds is not critical. At a decision height of 100 ft however, the same variations in a total exposure time of some 3 seconds may well be vital!

The ’visual segment’ is the length of the approach light pattern in view at any one instant of time. This is not the visibility or SVR. The SVR merely determines the distance between the pilot’s eye and the farthest light he can see, whilst the distance to the nearest light depends on the altitude and the cockpit cut-off angle.

The distance between the nearest and the farthest lights visible is the visual segment, and a rough guide for the absolute minimum acceptable is a 150 m segment (5 lights) ahead of the aircraft. This is probably the minimum number which should be seen by the pilot to ensure that the lights have been correctly identified.

It should be noted that, because the amount of obscured ground below the aircraft decreases as descent continues, provided the fog is homogeneous, the visual segment should gradually increase during the final approach, till at touch down it is almost equal to the RVR. Thus, a pilot can expect his visual segment to increase continually from the point at which he goes visual until touch down. This is normal sequence!

However, most radiation fogs are comparatively shallow when forming and seldom extend to a height above 200 ft, so that a pilot flying in clear air above the fog top, and looking down through this layer, viewing the ground ahead just over the bottom of the windscreen, may well have quite a good visual segment. As the aircraft enters the fog top, the visual segment will decrease sharply and most or all the visual cues may be lost. This is an abnormal sequence!

A non-homogeneous air mass or a patchy fog condition can produce a dangerous situation during the final portion of the approach. If the pilot, who has been in visual contact, suddenly flies into a thicker mass of fog or a ’blob’ of fog, the farthest lights of the approach light system or runway light system may well now be obscured. The rapid foreshortening of his visual segment, or his loss of visual cues, may immediately cause him to believe that the nose of the aircraft has been pitched up, obscuring the nearest two or three lights. In actual fact, the lights farthest away have been lost from view! The natural reaction is to lower the nose, thus increasing the rate of descent, and of course also increasing the chances of an undershoot.

Once all of these factors and the destination weather are understood, the pilot will possess the knowledge to effect a safe and smooth transition from instruments to visual reference and a successful landing.