IRVR 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 lights delineating the runway or identifying its centre line” (ICAO Annex 6: Operation of Aircraft)
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?
On approach to a runway, “a decision height to the required visual reference is that specified for the particular procedure and operation. (ICAO Annex 6, and PANS-ATM).” Pilots will look for a visual reference on the ground, normally the runway centreline lights, touchdown zone or runway edge lights. If these cannot be identified, the approach will be aborted, and the aircraft will perform a go-around.
The ICAO standard for a high intensity runway edge light at maximum intensity is 10,000cd, however, in order to avoid overstating the published RVR; considering that the lighting may not be up to the required standard, in the UK, these were de-rated to only 2000cd (or 20% of the standard). Recently, this has been uprated to only 4000cd (or 40%), however, this has not been widely adopted. Airports can now however apply for ‘IRVR Credits’, where, through “submission to the CAA of results obtained by either photometric measurement or following the block replacement of those lights”, 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”)”
Finally, Note 3 highlights that “A minimum of 4 photometric inspections (one in each quarter) should be taken during each 12 month period…”
What are the benefits?
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 parties, both financial and environmental.
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.
Aside from the obvious financial losses to an airport, there are other benefits which could be gained from the ability to avoid implementing LVPs until a later time. Aircraft holding around an airport produce CO2, and reducing holding times would greatly decrease the amount of greenhouse gasses released into the atmosphere.
In summary, IRVR credits and the ability for airports to benefit from proper and regular AGL maintenance should be a good thing, however, this does still need investment in both time and equipment. Referencing the report from Atmosphere Journal again, it was calculated that one diversion of one international flight to New Delhi would cost an airline almost $22,000. With this in mind, the costs of a MALMS system or regular MALMS testing would ‘pale into insignificance’ if flights were able to land given more accurate and improved RVR figures.
For further information into RVR and IRVR credits, or how MALMS can help your airport, please contact info@malms.aero