“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)
Early in the development of the Precision Approach concept it was recognised that the prevailing visibility at and close to the runway must be part of the operational regulations. However, the conventional meteorological data available was deemed to be inadequate for the task. What was needed was real-time visibility/visual range reporting at the aerodrome.
In early experiments the Runway Visual Range was measured by human observation of the number of lights visible to an observer positioned on the centreline at the runway threshold at an eye height of 5m (15 ft) as the aircraft began the final approach. This was accomplished by the observer standing on a tall vehicle.
This methodology was clearly impractical for commercial operations. To overcome some of the practical problems the viewing position was moved to be located on an elevated platform alongside the runway adjacent to the nominal touchdown point. However, this procedure was not without short-comings. For example, recording the correct number of lights was open to error, particularly in Category 1 conditions where the number of lights to be counted is large. Also the system had to be calibrated to take account of the viewing position being off-set from the centreline. During periods when the fog was forming or clearing rapidly, the manually observed and reported RVR was susceptible to significant lags in the reporting. The IRVR concept was devised to overcome the various operational problems outlined above.
Because it is not possible to measure RVR directly, an assessment is made 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 lighting intensity.
The ICAO standard for a high intensity runway edge and runway centrelines at maximum intensity are 10000cd and 5000cd respectively (white lights). However 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 factor must be at least 20% (Doc 9328 Chapter 6.5, “Light and Light Intensity”) to avoid overstating the published RVR, but National regulators may de-rate this figure by as much as 60% (or 40% of the standard).
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.
It is the responsibility of every airport to maintain its runway lighting so that the actual light intensity is the same or above the value programmed into the IRVR equipment.
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. 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 by more effective maintenance of runway lighting there benefits for all stakeholders:
1. Improved airport safety – Pilots approaching in reduced visibility will be given reliable IRVR readings
2. Reduction in length of time Airport’s would be operating under Low Visibility Procedures by reason of lower 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. Airports with improved IRVR readings 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, 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. Pilots, passengers and airports all benefit from effective AGL maintenance.
For further information concerning IRVR and how MALMS can help your airport improve safety and compliance please contact Robert Shapton r.shapton@malms.aero.