ATC RVR readings without transmissometer
 

Question ATC lads,

At an airport where there is no transmissometer - Can the ATC legally give a RVR distance (as they normally just count the runway lights visible from the end of the runway for distance estimate). With shallow fog 100ft> this airport experiences low RVR's with high Viz.

I just experienced myself a few times whereby we are 10 miles from the field with the runway in sight with shallow fog - with tower reporting Viz of 800m yet the RVR of less than 350m. We needed a RVR of 550m to shoot the approach. It is strange to have Viz more than RVR as it should be the other way round. They would not let me attempt an approach due to the RVR distance reported, despite if I converted the Viz to RVR than I would be legal.

I am just trying to clarify if the airport can "count the lights" for an accurate RVR reading, or if their Viz estimate is their only legal reading, whereby the pilot can convert the Viz to RVR with the appropriate table. I was under the impression that only pilots did the "counting the lights" for an estimate for take off only. I have searched the publications to no avail...any help?

In the UK, CAP746 will get you started. Chapter 4 page 6 and appendix C seem to cover HORVR and IRVR.

Shallow fog is going to be a major hinderance or a 'thing of nothing' depending on where to look at it from. If the vis in the fog is 200m but it's only 10m deep, you'll easily see through it from above, but on or near the ground you won't see much.

CAP 746: Meteorological Observations at Aerodromes | Publications | CAA

The "count the lights" method of assessing RVR is referred to as the "Human Observer Method". Provided that the airport has lights calibrated for RVR assessments, a chart is provided to equate the number of lights to each RVR value and human observers who meet eye sight and hearing standards, the resultant RVR value reported by ATC is legal and would be used in a prosecution for any breach of UK civil aviation legislation.

Frustrating as it is when shallow fog persists, which will invariably reduce the reported RVR, if the reported RVR is below your instrument approach limits, even though you can see the runway as you go around the holding pattern, don't go there unless the reported RVR is 800 metres or greater, in which case you could probably legally opt to fly a visual approach.

I can only speak from experience in the UK but if ATC pass you a RVR it will have been measured in some approved and authorised way. If there are no such approved procedures you will only be passed a prevailing visibility (i.e. what most people will call met vis). I would expect the same to be true in other countries.

Although these days ATC will often be involved in the derivation of met info at aerodromes, it's not actually an ATC function. Officially all (in almost every situation) ATC does is to pass on met reports that are produced by someone else.

As far as I am aware, aircraft ops procedures make a RVR - if there is one - the controlling element when deciding whether or not an approach can be started or continued. If there is no RVR then the aircraft crew can factor the reported met vis in accordance with Ops Manual procedures.

Ah, the 'Human Observer Method'. :ok:
The cold, mist-shrouded lonliness of the Touchdown ROP.
"6 lights".
The location from which 'met vis' is measured is often a considerable distance from the runway. The observer reporting the 'met vis' is often in a different micrometerological environment (in the terminal complex/ATC Tower) to whom, or whatever, reports RVR, (out on the airfield, with less anthropogenic heat to evaporate any mist or fog).
Hence the often great difference between the 2 measurements.

When I was a "Runway Controller" at Manch, I counted the [special] lights...from the runway caravan. At Brum, the Fiermen were tasked to do it, from an obsevation post [elev 15ft] alongside the threshold of the active runway [usually 33]

When I last worked in the Tower at Manch, I produced Met reports via the SAMOS system...in real time. In bad wx it really wasa PITA:ok:

If there's no transmisometers or calibrated RVR reference lights, ATC will only pass met visibilty, and it's up to the pilot to use the requisite conversion factor to find RVR, ATC will not do the conversion for you so if you're passed an RVR by ATC, then it must be derived in some manner.

It is frequently the case that Met vis will be greater than RVR. Met vis is the prevailing visibility in a 360 degree arc around the observation point (ie the vis that is most prevalent in greater than 50% of the local atmosphere.)
At my airport the runway sits next to a river and so can be prone to shallow fog on the runway at dawn but the rest of the airport remains gin clear. Trust me we find it a PITA too. What should be being reported though is fog patches, fog banks, shallow fog or a reduced vis in a particular direction Provided it is BOTH less than 10km and less than half of the prevailing vis), together with any IRVR readings.
It would make more sense to me to disseminate an operational visibility and on occasion I have been known to sway my observations to this criteria. Ssssh.

I remember working at Edinburgh and from the tower we could see for miles. However, there was a bank of fog/mist over the runway only a few feet thick. From overhead, pilots reported being able to see the whole runway as they were looking down on it. However, when they got to short final they were looking along the fog bank i.e. it was a lot thicker, and they lost site of the runway.

Hence RVR can be a lot less than met visibility.