ATC Low Viz Procedures

[grow45]

Listening to the Edinburgh ATIS at home this morning before going to work (non aviation job so I know I'm sad) it was giving 10k viz with patches of fog and Low Viz Procedures in operation.

As I understand it the viz is measured automatically and fed into the computer that generates the ATIS " voice". Would fog patches be measured in the same way and the computer decide Low Viz Procedures were needed or does a Met Observer make the decision having looked out the window.

Just curious as a combination of 10K viz and Low Viz Procedures seems a bit odd although understandable with fog patches on the runway.

TIA

[Moneyshot]

Can't answer your question fully but the 10k vis and 'fog patches' assessment would be made by the met man (at the larger airports where one is available).
I have noticed that the automatic systems which determine RVR (runway visual range) are notoriously unreliable. I have been flying at night with low visibility procedures in force when I have been able to see the airport from many miles away. Furthermore, throughout the whole approach, the runway has been in view perfectly throughout. This has been reported to ATC who seem to be slaves to the automatic system.
This has the potential for commercial disaster as crews make an assessment of the suitability of an airport based on ATIS information. They may decide to divert at great cost rather than risk a go-around from a much lower altitude.

[Jerricho]

If I may, what cloud was there about?? The cloud ceiling can also cause LVPs to be in force.

[Spitoon]

As Jerricho says, LVPs can be triggered by low cloud or, perhaps, RVRs. With shallow fog about one could easily get RVRs even though the met vis is 10k - there's a small steam that runs alongside the runway that can generate local fog patches that are just large enough to cover the transmissometers.

Even where the met is largely generated automatically there is still an observer who checks what the automatics are reporting and visually does the bits that the automatic sensors can't do.

There was a time when if a pilot could see the runway he could make an approach regradless of the RVR. I seem to recall that this was stopped when there was an accident because even though the runway was visible from height, the slant visibility on final approach was pretty much zero. I guess it was a bit of a commercial disaster too.

The impact of slant vis on approach is basic pilot training (and has been for years).

[1261]

At EGPH I'm afraid our hands are tied; there's no met office in central Scotland now (our METARs go to Belfast!) and although our "SAMOS" observers can override an obviously erroneous horizontal visibility, we have no mechanism for overriding our (automatic) RVR equipment. Spitoon is quite correct, both the Gogar Burn and the River Almond run along side 06/24; often a little mist creeps just far enough through the fence to trigger a reading on our IRVR transmissometers. This can frequently show as a touchdown RVR of 250m, even though we can clearly see (as much as we can ever see of) the runway from the tower. If the IRVR system gives out less than 600m, regardless of the met vis, then we have to implement LVPs.

I realize that this is deeply annoying for those crews flying G-plated aircraft (I notice that for PH registered aircraft it doesn't seem to be an issue if they can see the runway) but we are forbidden under pain of death from making any comment on the r/t that might suggest that the met situation is better than that indicated by the auto met systems.

And bear in mind that at NATS scottish airfields, there is not a single full met observer on the unit!

[grow45]

Thanks 1261 and others. I guess that answers the question and confirms my suspicion that the machines are taking over :(

[yachtpilot]

Edinburgh is an airport particularly prone to this problem....some years back when operating our night mail schedules EDI was our final stop early morning...with cold air,clear skies and superb vis all around it was very common to be able to see the entire airport lit up including the runway edge lighting, however these conditions were prone to creating a shallow layer of fog... due to ( I believe ) a small brook or ditch somewhere in the vicinity of the RVR measuring equipment it would record very low vis ( only experienced by us on the SW runway ....It was tempting to call visual and continue but on a couple of occasions we were almost caught out as although the vis from altitude was fine ( looking down through a thin layer of mist ) over the threshold looking through the mist layer we would lose sight of the ground at a critical point...
A favourite trick at EDI was to await an early morning Jet departure and be close in behind him as his jet-blast would clean out the air for a few minutes and we were in...

[radar707]

Unfortunately we are legally obliged to implement LVP's if cloud is BKN or OVC at 200ft or if the TD RVR is 600m or less, regardless of how much of the runway is visible, it is really annoying when you get a patch of shallow fog sitting clear of the runway but over the transmissometers thereby giving RVR readings.

Glasgow is sited at the confluence of 3 rivers, and at this time of year we are prone to overnight fog which lingers on through the morning rush before burning off. Thankfully over the last few years it's not been too bad, but on occasion has gotten so bad we can't even see aircraft taxying past the tower.

From a radar controllers point of view at Glasgow, during LVP's when there are departures we need to use 15 mile gaps between inbounds (to ensure that departing a/c are clear of the localiser by the time the inbound gets to 6 miles), and 10 mile gaps when there are no outbounds, so delays are inevitable.

One thing that would be of benefit is if we are in LVP's tell us your minima on first contact, at least if you need 450m we can try and sneak you in ahead of someone who only needs 250m (should the RVR go up). I had a guy flying round LANAK who needed 550 but never told me (I was too busy to ask him his minima), I could have sneaked him in, but he ended up diverting to PK