IRVR – what you need to know

“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 lights delineating the runway or identifying its centre line” (ICAO Annex 6: Operation of Aircraft).
Because it is not possible to measure RVR directly, an assessment is made, either by a human observer or using an automated system to provide what is called Instrumented Runway Visual Range, (IRVR). The reliable and accurate assessment of RVR is essential if airport operations are to run as safely and efficiently in low visibility conditions.

“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 lights delineating the runway or identifying its centre line” (ICAO Annex 6: Operation of Aircraft) Because it is not possible to measure RVR directly, an assessment is made, either by a human observer or using an automated system to provide what is called Instrumented Runway Visual Range, (IRVR). The reliable and accurate assessment of RVR is essential if airport operations are to run as safely and efficiently in low visibility conditions. Airports report weather conditions in the form of a METAR (Metrological Aerodrome Report) which give information on current conditions, including the runway visual range (RVR). This is shown in the below example with the RVR highlighted in red:

METAR YUDO 221630Z 24004MPS 0800 R12/1000U DZ FG SCT010 OVC020 17/16 Q1018

In the above example, R12/1000U tells the pilot that for Runway 12, the current RVR is 1000m, and the ‘U’ indicates this is increasing- but where does this come from? Information from Transmissometers, Forward Scatter meters and some form of background illuminance reading are fed into a calculation program, however, this is also dependant on the runway lighting. Why does runway lighting matter when calculating RVR? Most airports now operate an instrument approach procedure; a type of air navigation that allows pilots to land an aircraft in reduced visibility. RVR is used as one of the main criteria for low visibility measurement on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. This is determined using visibility sensors (either forward scatter instrument or transmissometer). The prevailing RVR is then calculated via algorithms which also consider ambient light conditions (in daylight) and an assumed value for ‘runway edge lighting intensity’.

Most airports now operate an instrument approach procedure; a type of air navigation that allows pilots to land an aircraft in reduced visibility. RVR is used as one of the main criteria for low visibility measurement on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. This is determined using visibility sensors (either forward scatter instrument or transmissometer). The prevailing RVR is then calculated via algorithms which also consider ambient light conditions (in daylight) and an assumed value for ‘runway edge lighting intensity’.

The ICAO standard for a high intensity runway edge light at maximum intensity is 10,000cd but the nominal intensity value used in the formula for calculating Instrument Runway Visual Range (IRVR) is usually reduced by a configurable degradation factor to allow for loss of runway light performance and any contamination. ICAO specifies this must be at least 20% to avoid overstating the published RVR but in the UK, this value was until recently, de-rated to only 2000cd (or 20% of the standard). Recognising the potential unfairness of this formula the UK CAA has introduced a system of IRVR Credits published in CAP670 Part C, Section 2: NAV01. This enables airports to realise the benefits of investing in an optimised AGL maintenance program by rewarding those that maintain their runways to a high standard. NAV01 states that airports can apply to the CAA for IRVR Credits by submitting a minimum of 4 photometric inspections (one in each quarter) taken during the last 12 month period to evidence photometric performance commensurate with the credits claimed plus satisfactory evidence of ongoing inspection and subsequent maintenance program Note 2 of NAV01 states that “CAA staff will determine whether continued operation at the initial de-rating factor is justified or whether the de-rating factor can be increased upward towards the ICAO recommended maximum of 80% (Doc 9328 Chapter 6.5, “Light and Light Intensity”)”

There have been various studies and reports into the true cost of low visibility procedures
(LVPs) to airports, with each outlining the financial losses to both airports and airlines during periods of fog. The point at which an airport goes into LVPs is dependant on the RVR measured and calculated at a point in time. If the calculated RVR can be increased, through the IRVR credits program outlined above, there must therefore by gains to be made for all stakeholders:

1. Improved airport safety – Pilots approaching in reduced visibility will be given improved IRVR readings
2. Reduction in length of time Airport’s would be operating under Low Visibility Procedures by reason of a less-pessimistic IRVR
3. Increased airport capacity because the onset of LVO would be delayed, and their cessation hastened
4. Chances of a successful approach increased and costs associated with diversion reduced
5. Reduced CO2 emissions for aircraft holding unnecessarily
6. Reduce the number of occasions when a pilot will complain of glare
7. Airports with IRVR credits would be recognised by Stakeholders for their efforts in raising standards and gain an operational advantage

A study published in April 2019 by Atmosphere Journal investigated the “Loss to [the] Aviation Economy due to winter fog in New Delhi during the winter[s] of 2011-2016”. It was concluded that “The total number of flights cancelled, diverted, and delayed during this period was 351, 398, and 567, respectively. This led to a total economic cost of approximately 3.9 million USD (248 million Indian rupees) due to flights affected by heavy fog spells at IGI Airport over five years. In summary, IRVR credits and the ability for airports to benefit from proper and regular AGL maintenance is a good thing. However, this does still need investment in both time and equipment. Remember, if an airports average AGL intensity is below that programmed into its IRVR equipment, the declared Runway Visual Range will be overstated and safety will be compromised. For further information into RVR and IRVR credits, or how MALMS can help your airport, please contact info@malms.aero