Shallow fog and approach ban

[ATP_Al]

Does anyone have any thoughts on this situation:

You can see the airfield 20 miles away but ATC are giving an RVR of 400m because their is shallow/patchy fog covering the transmissomitters. Until you've passed the outer marker the approach ban is still in force. The airfield wasn't expecting it so LVPs are not in force, so you can't do a CATII/III approach and have to go around and hold, and potentially divert if you don't have CATII. While all this is taking place, you can clearly see the runway, the only place on the airfield where the visibility is reduced is the transmissomitter used to measure touchdown zone RVR.

My question is, why is there not a common sense procedure to deal with this situation? Why can't ATC look out the window and declare the touchdown zone IRVR equipment inoperative so you can use the mid-point instead? Or have a "shallow fog" procedure where you can be cleared for a CATI approach or visual?

[Meikleour]

ATP Al: Transmissometers are designed to measure the prevailing visibility in the runway locality. It is difficult to imagine a situation where the fog is only at the side of the runway and not also on it! Besides, from 20 miles out, how do you tell?
Your scenario is an old hoary one. Many years ago pilots would in fact declare themselves `visual` and make an approach. Unfortunately many of them did not fully appreciate the problems associated with shallow fog and the attendant loss of visibility when very close to the runway resulting in many accidents, hence rules were introduced to prevent this impulsive behaviour. Most manuals will include a minimum RVR reading for a Visual approach to avoid this very situation. The same considerations apply to ATC when they are attempting to assess the visibility from the tower which may be some way off and considerably higher than the runway.

[suninmyeyes]

This exact situation happened in Glasgow many years ago with a HS 748 landing in the 1980s.

The aircraft was Cat 1 only. ATC warned that the RVR given was below Cat 1 limits due fog at the threshold. However the flight crew could see the runway clearly from a long way out and landed visually.

All was fine until after landing when ATC said they were going to file a violation about them. A phone call later from the Captain sorted it out and it was all dropped. If ATC had given no vis or RVR at all the flight crew could then have legally continued visually and landed. As it was technically they shouldn't have landed. UK rules allow a visual assessment by the Captain to override touchdown RVR but only for takeoff and not for landing.

[safetypee]

Other considerations might be that shallow fog is more associated with the fog formation phase, this leads to variable RVRs which can change rapidly. Also, shallow fog structure is likely to be layered, thus very thin stratus layers could be encountered below 150 ft causing potentially disorienting visual cues.

Another aspect of shallow fog – fog formation, is that if the fog is stirred up by preceding traffic or possibly a ground vehicle, the fog ‘thickens’ and the RVR falls quickly.
It’s best to believe the RVR. Although this is not always a common sense procedure, the measuring system should not suffer human weaknesses of error or bias.

[ATP_Al]

Thanks for the replies (well the sensible ones anyway), I asked the question because this happened at Bristol this morning, ATC reported 400m in shallow fog but at no point was it anywhere near the runway!

[Neptunus Rex]

The problem with shallow fog is not simply that of visibility. As you enter a shallow fog bank, there is an illusion of 'pitching up.' This can lead to the PF lowering the nose causing a large increase in sink rate, as well as a nose low attitude very close to touchdown. It caused a number of fatal accidents, and was the original reason for the 'Approach Ban' being introduced years ago.

Of course, we have more sophisticated equipment now, but the illusion is still there for the unwary.

[bearfoil]

I think the most obvious reason is being overlooked here. From a vertical vantage point, (aloft) one is penetrating (visually) only the depth of the fog bank. As one descends toward finals, one is attempting to penetrate the "length" (horizontally) of the bank, and visibility is emphatically different, If any fog is as deep as your seat is high, it occludes the RW. Even if it is only eight feet deep, it prevents visual rollout, and subsequent taxi. Fog is always a hazard, it is in its name "Foggy". Foggy awareness is not sufficient for safe visual landing. (CatII/III, different). Except for other aircraft, most hazards are less than eight feet tall, tugs, people, cars, Gazelles, etc. just a thought. Even CAT approaches don't generally include FOD.

regards,

bear

[Neptunus Rex]

Transmissometers are not foolproof, like all of us they need some TLC. Some time ago I was making an approach to a Cat 1 airfield somewhere in Asia. The RVR was passed by ATC and it was below minima. It was a clear moonlit night, and we could see the airfield from many miles out. There was not a patch of fog to be seen, and the wind was too strong for radiation fog to form.

After landing, I visited ATC and saw the read-out from the transmissometers showing clearly ridiculous figures. I asked the controller to have the transmissometers cleaned. Big smiles as the indicators passed 1,500 and switched to standby.

[ATP_Al]

Just to clarify, i'm not asking why we can't land in shallow fog conditions as I'm well aware of the of the dangers in that situation. I'm asking why ATC can't make an exception in cases where something obviously very localized is obscuring the transmissometers but not any other part of the airfield.

In my opinion, the situation I encountered today (which is not that uncommon in the west country) has more in common with Neptunus Rex's story than most of the other posts.

[Thunderbug]

Are they still able to send a vehicle down to the threshold and count how many lights can be seen? Is that still a legally valid way of determining an RVR figure?

I've had the case where fog was forming over the damp grass beside the runway i.e. where the transmisiometer was sited. ATC was giving RVR as 200m, yet on the runway the vis was good enough to see the stop end lights. Funny stuff that FG!

[Data Dad]

ATP Al wrote:

I'm asking why ATC can't make an exception

Speaking from the ATC side, I know exactly where you are coming from with this - I have seen this situation many times. However, the question should more correctly be directed to the regulator (CAA SRG) as they make the rules. Since the Coventry Air Algerie accident many years ago there is no leeway allowed to ATC with the application of Absolute Minima etc. if the IRVR is showing a reading then it has to be used.

Ultimately though, it is one of the downsides of increasing automation and cost-cutting. Instead of employing someone who can go out on the airport and count runway lights (human observer method) we have IRVR - which, most of the time is great. In these shallow fog 'over the grass' situations though, the Human method is better because its actually looking along the runway NOT the grass.

DD

[Neptunus Rex]

ATP Al
Before the days of transmissometers, there was a tale of a RAF Argosy flying to an airfield in India, who reported fog, which surprised them. They saw the fog, but it did not affect the runway, so they landed. As they taxied in, they reported to ATC that the localised fog bank emanated from a local river, and just covered the control tower, the adjacent Met office and the Stephenson screen. The rest of the airfield was clear in the emerging sunrise. You can't believe that it had not happened before, but rules are rules for the Jobsworth.

On the other side of the coin, a very sharp duty QFI in the tower at Linton-on-Ouse, noticed a wind change late on a winter's afternoon. He issued an immediate recall, and all the JPs landed within fifteen minutes or so. I was the last to land, and just beat the fog bank to the runway.

A good call - we had Samuel Smith's OBB off the wood in the Mess at Linton, it was Theakston's Old Peculier in metal kegs at Leeming, which would have been our diversion airfield.

[Tom355uk]

The problem with shallow fog is not simply that of visibility. As you enter a shallow fog bank, there is an illusion of 'pitching up.' This can lead to the PF lowering the nose causing a large increase in sink rate, as well as a nose low attitude very close to touchdown. It caused a number of fatal accidents,

Sounds suspiciously like what happened earlier this year to the Afriqiyah A330.

:sad:

[145qrh]

Used to happen in Gla regularly, a long time ago mind.

Switched on ATC would switch off trasmiss, claim it was U/S and give assessed RVR.

The meters used to lie in a wet ditch close to the runway, no fog on the RWY.

[con-pilot]

These conditions happened quite a lot at Vandenberg Air Force Base in California. Vandenberg sits very near the coast on a high bluff and is affected by coastal fog in the winter months.

I have landed there may times with an RVR under 300 feet. However, the runway is 15,000 feet long* and RVR measuring unit was at the north end of the runway. What would happen was that fog would roll in and cover the north half of the runway, while the southern half was clear and when I say clear, that was exactly what it was, totally clear, no clouds or restrictions to visibility for the first 7,000 to 8,000 feet of the runway.

So I would just fly a visual approach, land and then turn off the runway before going into the fog. Takeoff basically the same, except we would be airborne prior to going into the fog.




* Vandenberg's runway was lengthened to make it a alternate Shuttle landing site. In fact there is even a Shuttle hangar near the runway. Then someone pointed out the problems that the winter fog would cause and the Base was dropped as a Shuttle alternate.

There is not much traffic that goes into Vandenberg, sometimes we were the only traffic that they saw in a week.

[HEATHROW DIRECTOR]

<<I'm asking why ATC can't make an exception in cases where something obviously very localized is obscuring the transmissometers but not any other part of the airfield.>>

Possibly because many controllers are not qualified met observers. At Heathrow I had a foreign pilot tell me he wanted no more RVR checks once the vis was above his minima. He got them. I wasn't putting my neck on the line for an idiot.

<<Switched on ATC would switch off trasmiss, claim it was U/S and give assessed RVR. >>

In both the cases mentioned above, what happens if you crash? Who gets it in the neck? There's nothing "switched on" about being stupid.

I worked at an airfield where we did not have IRVR. Met were giving x meters in shallow fog. Inbound aircraft saw the runway from 30 miles and made a visual approach. He got to about 500 ft and lost the lot.... went around to take a look at the other end and flew it into the ground - all dead.

[stator vane]

that example you give is obvious!

the idiots that couldn't manage a go around!!!! that has nothing to do with getting an approach clearance.

we cannot legislate common sense and airmanship.

and all the people warning us about how deceptive fog can be, well, i know it when i see it--we should be allowed to make the approach and if we ball it up on the runway, it's pilot error. simples!

i had a right barney (i think that's what brits call it-i might mis-spell it) with the tower controllers in the states some years ago, when they were saying the runway was below mins for takeoff. i told them to look out the window and see for themselves. and the other aircraft waiting behind me, backed me up. it was only fog over the wet grass where the rvr meters were. the tower finally agreed and let us take off.

maybe a 'take off at your own risk'

[PPRuNe Pop]

I recall several instances at ABZ when at times when it was really busy. In particular a cracking training captain noting the tranmiss was giving an RVR of 400' 50 yards off the runway when the viz down the runway was as clear as a bell. He landed without mishap and the CAA "wished to talk to him" - he argued very succinctly the reasoning for his decision and since the CAA had no answer than the book they let it go - because the system was flawed. It soon became a case of experience with discretion. I did it myself and ATC were not phased at all. At the time there were Tridents doing reverse thrust above the ground, heleos taxying across active runways, plenty of GA and plenty of schedules and plenty of charters. NO ACCIDENTS. Minor incidents yes, but a truly brilliant ATC.

The worst happening at ABZ was a drunk (there were enough of those!) who walked into turning prop!

The Skean Dhu was a place of much discussion on this and that in regard to ops and whoopsies.

[bearfoil]

Isn't it after all a question of command. As to take-off, that's "elective" and commercial (financial). Landing is quite another, and Captain is in command. A TOGA is hazardous, as is continued flight (in any conditions, by definition more hazardous than sitting penalty box.) It seems a simple question, if visual, land. The "visual" is the commander's call, not the tower's. Otherwise they wouldn't ask if the Captain was visual in the first place. "Pilot's Discretion" after all, also his/her responsibility.

[HEATHROW DIRECTOR]

<<the idiots that couldn't manage a go around!!!! >>

It was a perfect go-around into a "visual" circuit for the other end. It flew into the ground on final approach. Flight deck crew included a Check Captain..

[demomonkey]

For a CatI landing the visual requirements are quite vague in that you must only see any part of the approach/runway edge/centrelines lights. If passing 1100' on the approach or FAF and despite the iRVRs reading 400m, the pilot has full CatI visual requirements and the runway is in full and continuous sight then is it admissible to land?

[captainsmiffy]

Have seen the self-same fog phenomena on a few occasions at BRS myself. No fog on the rwy at all but only over the transmissometers. Much discussion as to what to do with the approach on the first occasion, with the older and more experienced captain in the RHS urging me to land and ATC implying that it was below minima....managed to persuade the ATCO, in the end, that his transmissometer needed looking at prior to the approach ban point so he declared it u/s and we landed with everybody happy. It is a stupid situation though and one where common sense should prevail. A case of where 'rules are for the guidance of wise men and the obedience of fools....'

[moggiee]

I got caught by this landing a VC10 at Gutersloh in the late 80s. ATC were giving 400m RVR which was OK for us because our military rules permitted a Cat I ILS at that RVR. We could see the lights from miles away and we made the decision to land when still some distance away (it was my landing). All was well until we reached the flare at which point we hit dense fog and the reflection from the landing lights blinded me. My landing was a bit firmer than normal due to loss of visual references and I found myself doing about 140kt with practically no view out of the front window. The fog was so dense that we missed our turn off but as it was only about 40ft deep, tower could see our tailfin sticking out of the top of it like some sort of landshark and were able to guide us. 10 minutes later the fog was all gone and the whole airfield was covered in a thick layer of dew.

To be frank, I reckon that the quoted RVR was more than a touch "generous" in fact I would be surprised if it was more than 200m and I'm sure that ATC were trying to help us out, but it gave us a false impression of what to expect. That said, if it had been quoted as less than 400m we would almost certainly have made a visual approach because we could see everything we needed from miles away!

The requirements for visual references give a great deal of scope for making that decision to land - the hardest thing could well be getting a civilian ATCO to approve the visual approach on the first place.

[aerobat77]

For a CatI landing the visual requirements are quite vague in that you must only see any part of the approach/runway edge/centrelines lights. If passing 1100' on the approach or FAF and despite the iRVRs reading 400m, the pilot has full CatI visual requirements and the runway is in full and continuous sight then is it admissible to land?

afaik basicly yes. when you have positive visual contact with the runway or approach lights you may continue to decision altitude. at decision altitude always the crew makes final decision if to land or not regardless of the reported RVR .

if you cannot see anything and the weather is reported below minimums you are not allowed to approach until DA.

of course at extreme low RVR reports you should be extreme careful even when you see the runway from far away since you may like described run into extreme shallow fog on flare , loose orientation and get into severe problems when you nearly made it.

[captainsmiffy]

...the point raised, though, was that the rwy was not in fog. Only the transmissometer was...the question is about the legality of the approach when common sense points out the shortcomings of the rules. We are not really discussing loss of visual in a shallow fog type of situation.

[aterpster]

captainsmiffy:

...the point raised, though, was that the rwy was not in fog. Only the transmissometer was...the question is about the legality of the approach when common sense points out the shortcomings of the rules. .

That is a very unusual condition. Even when it exists how does the flight crew safety and positively assess it; i.e., transmissometer(s) "fogged in" but the runway is "CAVU?"

[aerobat77]

the question is about the legality of the approach when common sense points out the shortcomings of the rules. We are not really discussing loss of visual in a shallow fog type of situation.

strictly due to law you may continue when you have a solid visual contact with the rwy or the app lights to decision altitude, and then finally decide regardless what the weather report says.

but i think its a theoretical situation that just the transmissometer is completely fogged with anything else good for a safe landing, never saw that in real life.

[captainsmiffy]

It very definitely is a real situation; have seen it with my own eyes. Mind you, been flying since 1986! Appreciate that there could be problems in assuming that it was only the transmissometer fogging-in but that was our situation indeed, and we could see BRS 27 from many miles away and could also see a small patch of fog obscuring the area south of the runway. This the transmissometers were giving very poor RVR measurements whilst the rest of north somerset was basking in a glorious evening sunshine. The controller, however, was a 'numbers man' and had not recently looked out of the window (am being kind here!). It took some persuasion before he finally relented and stated that the machine must be broken! This saved me a difficult decision as I was new to command and I had a right seat recently demoted captain who was urging me to land anyway! The ATCOs late decision saved me from a tricky call.....

[demomonkey]

Dammed if you do and dammed if you don't!

If you pass the OM/FAF/1000' point with RVR < 550m you have broken the approach ban despite being meeting all the requirements for a catI landing. Fail.

So if you do land, expect a call from 'The Authority' and if you divert expect a call from Dir of Flt Ops. Either way you're going to be short on the tea and biscuits front. Do like the idea of the proactive ATCO realising the paradox of the situation and declaring the RVR equipment U/S and thus allowing pilots to continue to DA. However from a flight safety perspective this approach whilst legally acceptable does require a cloud base to be of sufficient height to allow reasonable time to assess the lighting requirements. As such it should be treated with caution.

[jalbert1]

If you get passed an RVR then I'd be inclined to believe it.
It's on tape for a start and they're closer to the situation than you are.
Slant visibility and all that.Never nice to loose visual reference in the flare at 20 odd feet.
http://www.aaib.gov.uk/cms_resources.cfm?file=/S4-2008%20G-VAIR.pdf

[RAT 5]

I can well appreciate the question and understand the frustration. It's generated by the apparent inability to use discretion, common sense and be flexible. Trouble is, and as noted by the stories, there are sometimes when you can land OK and some when you shouldn't try. How do you know which is when?
Myself, at SPL, crossed the OM at 3000' perpendicular to the rwy en-route to the hold. A/C CAT 1. RVR 500m. Oops. Last flight of the night and div was BUS. Fully visual with the full length of the rwy; tower 3km's from the threshold and zillions of feet in the air. What to do? Luckily a KLM was also inbd. Discussed if he was doing an autoland; luckily yes. Radar vectors to 4.5nm behind and a PIREPS on landing please. KLM declared that "RVR was well in excess of what was being given"; we declared visual, landed. Filed a report to effect and nothing heard. Lucky for us. But it was slant range visual at OM. Recently a colleague at GRO rwy 20 ILS, CAT 1, saw everything at DA. As PF (F/O) flared it all went very dark and correctly they made a G/A. landed on the other end NPA.
It is not an exact science and thus the multitude of answers. "Where there's doubt there is no doubt" come to mind. Saved many a foolhardy thinking pilot that one, and ignoring has cost many too much.
Finally the pilot at LTN, many moons ago, Christmas evening, CAT 1 a/c, RVR 300m. Flew overhead 3000', declared field in sight and requested visual. Granted, landed, CAA reviewed the meteo and landings a few days later. More than a slapped wrist I remember. Long before SAIR's etc. Back in the dark days of mission accomplished = star pilot. But he still had his come-uppance. Things are more legally framed and the risk to very high. Your defence has to be more solid than a politician's promise.

[captainsmiffy]

Gentlemen, when you are in the position that was posed here initially, and that I, too, recounted, then you will have no doubts about the approach. Slant range doesn't even come into it! If you are in widespread fog then, yes, agreed, take extreme care as you might be 'suckered in' and fall foul of the slant range etc - but the situation described was completely different. We were in CAVOK conditions, as was the rest of the county but a little light fog was rising from the damp grass adjacent to the runway. This is quite common in the UK near rivers, streams etc. Fog was not forecast and neither was it occurring except for a few feet around the transmissometer. This post is all about what to do when clearly there is a paradox from the RVR transmissometers.....they happen to be poorly placed and are very obviously giving erroneous readings (well, they might be giving a correct reading for their immediate environment but if they reside in the only small packet of fog then you have a problem!)

Without a sensible ATCO then you will be damned by the authority for making the approach and you might be damned by the company for not 'taking the common sense decision'! The only answer, insofar as I can see is to elicit the ATCOs help and get it declared u/s. Believe me, you will not be thinking 'slant range' etc in the described conditions; more like "WTF! Why doesn't he look out of the window....." This phenomenon is much more widespread than you might think in the UK in the autumn on those beautifully crisp, clear and cold days.

[bookworm]

strictly due to law you may continue when you have a solid visual contact with the rwy or the app lights to decision altitude, and then finally decide regardless what the weather report says.

Not according to EU-OPS:

OPS 1.405 (a) The commander or the pilot to whom conduct of the flight has been delegated may commence an instrument approach regardless of the reported RVR/Visibility but the approach shall not be continued beyond the outer marker, or equivalent position, if the reported RVR/visibility is less than the applicable minima (see OPS 1.192).
...
(e) The approach may be continued below DA/H or MDA/H and the landing may be completed provided that the required visual reference is established at the DA/H or MDA/H and is maintained.

The visual reference mentioned in paragraph (e) does not exempt you from the restriction of paragraph (a). If it did, the approach ban would be practically unenforceable from outside the cockpit, as the crew could always claim that they saw the lights despite the low reported RVR.

[TolTol]

Sorry to hijack the thread but I have another question regards the approach ban and it relates to paragraph (d):

(d) Where no outer marker or equivalent position exists, the commander or the pilot to whom conduct of the flight has been delegated shall make the decision to continue or abandon the approach before descending below 1 000 ft above the aerodrome on the final approach segment. If the MDA/H is at or above 1 000 ft above the aerodrome, the operator shall establish a height, for each approach procedure, below which the approach shall not be continued if RVR/visibility is less than applicable minima.

Does this mean that the commander is entitled to continue the approach past 1000ft even if the RVR is below minima, they just have to decide by 1000ft what they are going to do?

Thanks.

[captainsmiffy]

You can pass the approach ban point if you have the requisite RVR. Once past it, it doesn't matter what the RVR drops to but you must have the required visual references at your decision point in orfer to land.

This thread is about the rather unusual case where the RVR recorder lies in fog but the aerodrome is actually CAVOK. It is thus not legal to pass the approach ban point without clearing up the RVR issue with the ATCO..

[bookworm]

Does this mean that the commander is entitled to continue the approach past 1000ft even if the RVR is below minima, they just have to decide by 1000ft what they are going to do?

It's not intended to mean that, but the wording is awful, isn't it?! The intention, of course, is that the 1000 ft point substitutes for the OM for the purposes of paragraph (a).