Shallow fog and approach ban
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Shallow fog and approach ban
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?
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?
Last edited by ATP_Al; 1st Nov 2010 at 14:50. Reason: Spelling
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.
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.
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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.
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.
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.
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.
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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!
Last edited by ATP_Al; 1st Nov 2010 at 15:52. Reason: clarification
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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.
Of course, we have more sophisticated equipment now, but the illusion is still there for the unwary.
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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
regards,
bear
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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.
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.
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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.
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.
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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!
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!
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ATP Al wrote:
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
I'm asking why ATC can't make an exception
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.
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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.
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.
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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,
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.
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.
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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.
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.
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<<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.
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.
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who gets it in the neck???
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'
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'
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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.
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.
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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.
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<<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..
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..