Shallow Fog - Approach Ban
 

Hi all,

Just wondering if anyone can help me out with this question. If there is a very shallow layer of fog over part of the runway and the airport is giving RVRs below CAT I minima; can anyone confirm that the approach ban does still apply, even if an aircraft were to call runway in sight early on in the CAT I ILS approach?

E.g. An aircraft 20 miles out can clearly see the runway threshold, all the approach lighting and the majority of the area around the airport, and calls visual. The aircraft is then cleared for a visual approach by ATC. Just want to confirm that the approach ban does still apply requiring at least CAT I minima (i.e. 550m in the touchdown zone); or is the minimum required touchdown zone RVR now 800m, since the aircraft called 'visual' early on in the ILS approach?

I presume the approach ban would still apply as RVR is always the main thing taken into account. Obviously you would either not commence an approach, or if you did; make a go-around at the outer marker unless the RVRs improved prior to reaching it. I am a little unsure though as to whether the minimum required touchdown zone RVR, in this example, would be 550m or 800m?

Many thanks,
All the best,

Atrflyer

If the rvr is as reported then lvp is in force. Therefore the option of a visual approach does not apply. Furthermore as you descend through decision you may find your vis not usable.
This was the case in man on sun Night. as we approached we could see the runway from 40 miles and clearly all the way to about 200 feet. Then it all went blurry. Hope this helps.

The preamble for lvp in your part A for cat 3 landings should include a section on the structure of fog which may expand on the reasons for this

Hey ATRflyer:

I can only speak for the US. Under 14CFR part 91 rules (Takeoff and Landing under instrument flight rules), if the pilot has been cleared for an “IFR” approach, a CAT I ILS in this case, only the ATC controller can issue a clearance for a “visual approach.” There has to be sufficient visibility from the tower controller’s perspective for that to occur. As long as the runway of intended landing is, as in your example, reported to be “RVR below CAT I minima,” I don’t believe there is any way that ATC is going to clear the pilot for “a visual approach” regardless of what the pilot says is "visible." Of course the pilot may ask for and may be cleared for a “visual approach” to land on an alternate runway – but only IF the prevailing visibility at the airport would allow it and the visibility on the alternate runway is at or above “VFR” minimums. In your example case, I don’t think the controller would agree to this because the prevailing visibility probably doesn’t exist to issue such a clearance.

If the pilot, in your example, continues the ILS approach, on the IFR clearance issued, s/he would be legal to go all the way down final approach to decision height and determine if the requirements to go below DH are present. Those requirements are 1) the aircraft is continuously in a position from which a descent to a landing on the intended runway can be made a normal rate of descent using normal maneuvers (and if operating under part 121 or part 135) that descent rate will allow touchdown to occur with the touchdown zone of the runway of intended landing; 2) the flight visibility is not less than the visibility prescribed in the standard instrument approach being used, AND 3) at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot: a) the approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable; b) the threshold; c) the threshold markings; d) the threshold lights; e) the runway end identifier lights; f) the visual approach slope indicator; g) the touchdown zone or touchdown zone markings; h) the touchdown zone lights; i) the runway or runway markings; or j) the runway lights.

If these requirements cannot be met, the pilot is obligated to miss the approach.

Hope this helps you.

_______
AirRabbit

A few years ago, I was a witness for a case in which the exact scenario you described occurred. The commander was found guilty, given an absolute discharge, and fined £500 with costs.
Answer your question?

The case happened in England, Ireland may or may not be different.

Under JARs, if the RVR is below minima, an approach ban is in force.

Visibility in fog is all about slant visual range. You may see the lights on the approach down to very low levels - below DA/H is not a problem. But when you flare and start looking through the fog, the lights disappear. It's happened to me and it's scary!

If wx reported below the applicable approach minima, then a visual approach is not available. Any request for same should be denied.

Any approach made when the RVR is below Cat 1 minima is automatically reported to the CAA. It follows that you will be investigated!

Sorry, Hudson Bay, that is not correct.

Firstly, such approaches are not "automatically" reported. Secondly, since the ATR is a Cat II-capable aircraft, it would look just a little silly to report the crew for carrying out something they are entitled to perform...

Thanks
 

Hi all,

Thanks very much for all your input. Just one final question regarding that JAR OPS requirement of Min RVR 800m for a visual approach. I presume this comes into play as soon as an aircraft operating on an IFR flight plan requests a visual approach?

Thanks again,
Atrflyer

ATRFlyer. Presumably this in the a/c ops manual? Unless the rules have changed quite recently, ATC cannot refuse a visual approach purely on the grounds of low RVR . The cloud base is the only criteria mentioned plus the ability of the pilot to complete the approach with visual reference.

UK ATC will inform pilots when the RVR is below the 'Absolute Minima' for a visual approach. Should a pilot still elect to continue, ATC are required to report the facts, having advised the pilot accordingly.

ATC don't normally impose Wx related 'Approach Bans'. They do give pilots the facts, and are sometimes obliged by the regulations to warn pilots, on the R/T, of the consequences of continuing a particular action.

<<UK ATC will inform pilots when the RVR is below the 'Absolute Minima' for a visual approach.>>

Where does that that appears in any documentation? So far as I can see the "Absolute Minima" procedure refers only to instrument approaches and does not mention visual approaches. Therefore, once an aircraft passes 1000ft if the measured RVR drops below his minima but he reports that he has the runway in sight and can continue visually ATC a) has no option but to approve and b) there is no procedure for filing a report.

(Not trying to be awkward but this is an interesting discussion).

Have a look at ATSIN No.19. Scroll down to the Attachment - 'Calculation of Absolute Minima', para 6 - 'Visual Approaches'.

"A value of 800 metres RVR shall be used at all airports as the Absolute Minimum for visual approaches"

OK spekesoftly.... humble apologies. Now found it!

Thanks for that Heathrow Director and spekesoftly.

Regards,
Atrflyer

Captain Stable I am not wrong!! You may have mis-understood me but I am not wrong! I will try to explain.

If a particular runway is designated CAT 1 and lets say the RVR is calculated at 550 metres (as published in the AIP which is the same as what is published in the Aerad plates) this RVR is known as "absolute minima" If the RVR is below 550 and an aircraft now commences an approach, after an initial warning ATC will pass the following message

" (Callsign) If you continue the approach and descend below 1000ft above aerodrome level, It is believed that you will be contravening UK legislation and I shall be required to report the facts, acknowledge".

No landing clearance will be given, only advice on traffic and wx conditions.

This absolute minima applies to any approach whether it be visual or instrument.

To conclude, if you pass 1000ft and the RVR reported is below the absolute minima for that runway you will be reported for breaking the law.

Hudlson Bay... I know what you're getting at but the confusion arose because your earlier statement: "Any approach made when the RVR is below Cat 1 minima is automatically reported to the CAA" is patently wrong!! Otherwise, how would thousands of Cat II and Cat III approaches be made?

OK HD I take your point. I was a bit brief in the first post!

Just to clear up another point from singleseater, ATC are NOT permitted to stop an aircraft making an approach due weather. They can for traffic reasons but they are not responsible for making sure minima is complied with. They just advise and report if the minima is broken. The decision to make an approach in specific weather conditions rests solely with the pilot whether it be legal or not.

My understanding (and that of our CAA approved ops. manual) is that:

You may commence an approach regardless of the reported RVR but may not continue beyond the OM or equivalent position unless if the RVR is below minima. ie. approach ban.

Once past the OM if the RVR falls below minima the approach may be continued to MDA/DA/DH and IF required visual reference is established and maintained at that point you can land perfectly legally.

So basically once past the OM it's up to the commander and if you are visual at DH/DA you can land quite legally, a report may very well have to be filed but there is no proof that visual reference was not obtained and that the landing was illegal. If you were in a 2/3 crew aircraft you may well get reported, quite rightly, by the other crew members but they and you are the only ones who really know.

The situation is no different from ANY instrument approach, the tower may well be reporting a value much higher than minimum but due to the uneven nature of fog patches you may not see the runway at DA/DH so you go around. If you continue below DA and land you would have broken the law regardless of the reported vis.

In short, landing with reported vis. below the minima for the approach is NOT illegal, landing without the required visual reference at DA/DH most certainly is.

Our manual also has a note saying that visual approaches may not be conducted with the RVR less than 800m so in answer to orginal question my opinion is that if the reported RVR is less than 800m you can't start a visual approach. It is grey area though because once you have started one (inside OM?, what measure do we use? it's a visual after all) and the RVR falls below 800m what do you do?, if you can "maintain visual reference" there is no difference between that and the visual segment following an SRA or NDB that might terminate some way out at say 800ft. If you loose visual reference in either case you have to go-around and you could be well below MDA or DH when you do.

Anyone care to comment, can't find anything in my documents and manuals that covers it.

I agree with Max Angle, except that my (ATC) instructions don't refer to any outer marker, but to a height of 1000' above aerodrome level as Hudson Bay says.

I can't help thinking that the Absolute Minima thing is a red herring and simply confusing the issue. Absolute minima are an ATC thing and have no bearing on how
a pilot operates an aircraft. The absolute minimum is supposed to be the lowest minimum that any pilot can ever calculate, therefore it may be lower than the minimum in a particular Ops Manual or AIP plate. The idea is that if a pilot has calculated a minimum below the absolute minimum then he or she has made a mistake and ATC pass the message as a prompt to encourage the pilot to check that the minimum has been correctly calculated. The absolute minima procedure/message is not followed if Cat II/III approaches are available - usually indicated by LVPs being declared in force.

As to the original question, to me as a simple controller, it's all very straightforward. If the visibility at the time the approach is being made is at or above the minimum set out in the pilot's Ops Manual for a Cat I approach then the approach can be made. It really doesn't matter what's going on on the ground - LVPs or whatever. OK, I know there are some variables like whether the flight is operating to JAR-OPS and there's now this thing about the OM or equivalent position but as a controller I don't worry about them. Then there's some legislation about the approach ban that applies to all aircraft in the UK and UK registered aircraft wherever they are just to make the situation even more complex.

All this is only valid for the UK of course. And I don't think any other country has introduced anything as daft as the absolute minima procedure for ATC!

I agree with Max Angle, but if I remember correctly there are two countries in Europe where you cannot continue (even if you are) below 1000 AGL if reported RVR falls below minima. UK and Germany.

Or it used to be, maybe things have changed?

I'm familiar with the "Outer Marker or equivalent position" wording, but there are now so few Markers and no clarity about what an "equivalent position" is. Furthermore, the "Approach Ban" AIC only refers to "1000' aal".
As a result we now use 1000' aal and ignore all reference to Markers and "equivalent positions".
Any comments on that?

keithl,

Well, normally, the marker is used for GS check or as FAF - so the "equivalent position" would be your DME range used for that purpose.

The advantage of using that position (operationally speaking) - is that it's a position already in use - if you've done your GS check, you continue, if GS check not performed yet, you discontinue the approach. So simple even uncle Empty can do it :O

If you choose to operate using 1000 ft. AAL - you have to figure outr a new altitude on the altimeter - altough I don't know if some operators performed their "stabilised approach´-cut-off at 1000 ft. AAL - most use either 1000 ft. RadAlt or baro minima +1000 ft. (to keep it simple). If your operator uses 1000 AAL for that purpose, well, might as well use that...

But - you delay your cut-off point (at most airports), thereby increasing the chance that you'll have to discontinue the approach i.s.o. going down to DA/H to take a look ;) - especially relevant in the circumstances described above.

To remain on thread - if you are carrying out an approach (JAR-OPS 1 operators, that is) and the RVR drops below approach minima (visual or otherwise) - your ops manual will prohibit you from continuing the approach. ATC (and through them, possibly, the CAA) will only be a circumstantial factor.

I think the reason why people focus on the ATC bit is that it seems much more powerfull when somebody speaks the words "You can't do that!" as opposed to knowing that the same words are written by your DFO in a book somewhere in the ships library :(

My 2 eurocents worth - brgds fm
Empty

RVR passed after the 4 miles point are advisory only. At least in the UK.

I know this as I am just back from the sim and this point featured several times.

Guys
At the risk of sounding a bit like "it's God here" can we be a bit less authoritative about what "we" do in the UK - I suspect some of what "we" do is governed by Company Ops manuals.

FWIW, in the UK, for precision approaches my operator uses 1000' agl as the control height ( no mention of 4 miles/ markers), and RVR deterioration or improvements after passing that point must be ignored.......( so if it drops below minima you do not have to go around.

Standing by to be flamed.........

Heathrow Director,
To pick up on one of your earlier points, all landings below Cat 1 minima are reported to the CAA, who then wade through them checking against the list of approved Cat II/III operators.
A friend of mine had an interesting time a few years back when the CAA reported him to the FAA for landing below minima, because they didn't check which runway at LHR he had landed on. The "in use" runway was below 550m, but the departure runway was above and ATC were quite happy for him to approach and land on that. Sadly all that the CAA picked up on was that the landing runway was below minima. Wonderful people:mad:

<<To pick up on one of your earlier points, all landings below Cat 1 minima are reported to the CAA,>>

OK BizJetJock, perhaps you can tell me how it's done. Having worked as a Heathrow controller most of my life I have to say that I was not aware such action was taken. Just the number of Cat II and Cat III landings which take place at Heathrow and other large airports during bad weather suggests that the paperwork involved would be mountainous.

With the usual proviso that things might have changed since I retired, I am not aware of any ATC involvement in such arrangements - we would have been too busy! At this time of the year Heathrow is below Cat 1 quite frequently and hundreds upon hundreds of flights operate below Cat 1.

Hi Heathrow Director,
Whilst not having any direct knowledge of the system, I would guess that since there is a record of all landings (for BAA to charge their ever so tiny fees!) and presumably a log of the RVR's then it is relatively easy for BAA to have a clerk or a computer correlate these and send them off. This would not require any action by the controllers, and would also explain how my friend came to be pulled up because a clerk/computer assumed that all landings took place on the "landing" runway:rolleyes:

Back to the original post, the question of patchy shallow fog should be considered on a practical basis.
I`ve landed at LGW on 26R with reported RVRs below minima for CAT 1 ILS to 26L. On landing the fog was very low and very patchy, had 26L been in use LVP`s would have been in force.
Why ban approaches in such a scenario just because the auto RVR meter indicates a low value! why can`t the tower controller be given some discretion.
In the USA the rules are even tighter, you may continue passed the marker but if at decision height the RVR is below limits you must go around what`s happened to common sense here?
Fog is often patchy and the best person to decide if a landing can be safely achieved is the Captain at decision height.
( Airbus may beg to differ)

Not strictly true as I recall. It was all landings carried out whilst RVRs were reported (commonly in vis conditions <1500m). It was something that the airport authority had to do so at a big airport like LHR ATC may not have had any active involvement beyond recording the RVR (which they would probably do anyway).

But it hasn't been done for several years to my knowledge.


[Edit below]
And another thing, MANTHRUST, the reason that the rigid rules are in place in some parts of the world today is because when pilots were allowed to make the judgement they go it wrong more often than the authorities - and the passengers - could tolerate.

<<LHR ATC may not have had any active involvement beyond recording the RVR (which they would probably do anyway). >>

Yet again I have to hope that somebody in current practice can help, but during my time at Heathrow ATC certainly did not record the IRVR and I'm sure it wasn't automaticallly recorded. Heck, it would change by the second, let alone the minute!

Sorry, HD, but I'm pretty sure that the AGI IRVR system records data that it measures. And, yes, there can be loads of it! At the likes of LHR I guess this would all be within the ATE's domain.

OK many thanks Spitoon.. I learn something new every day!

It's always been my understanding that IRVR readings are automatically recorded. It was once explained to me, many moons ago, that this is a CAA requirement, and that the data had to be available for cross-checking by their 'AWOPS Department', should they so desire.


Yes, the AGIVIS 2000 website confirms this:-

"The information is also entered onto a magnetic cartridge recorder and page printer for archiving and legal record purposes."

MANTHRUST, “Why ban approaches in such a scenario just because the auto RVR meter indicates a low value!”
”Fog is often patchy and the best person to decide if a landing can be safely achieved is the Captain at decision height."

The reasons against change are because the scenarios are unpredictable, often with misleading visual cues. Furthermore human judgement is often flawed or the perceptual system confused. The RVR system was developed to overcome disparities in visibility measurement; I flew on many of the flight trials in various fog conditions. By far the most challenging were the periods of fog formation or dispersal, most often associated with Cat 1 becoming Cat 2, or Cat 3 improving to Cat 2.

When ‘shallow’ fog forms there are successive periods of layering and subduction; each layer associated with different visual ranges, vertical extent, water content, etc. The conventional view of shallow fog is a single layer, thus over flying at height you can see down through it to the runway, but during an approach the visibility reduces as the aircraft enters the layer. This effect alone has cause many accidents.

In reality a shallow fog layer may have many layers and the visual effects are similar to entering and exiting many stratus layers, which alternately reduce and brighten the approach lights and have similar effect on the slant visual range. Thus, the far point of visibility can oscillate towards or away from you; it is very disconcerting.

When fog begins to clear, often from a stable homogeneous deep layer (that is why Cat 3 conditions are more predictable), the clearance pattern is similar to the formation of small cumulus clouds. Even where the sun burns through the fog some areas remain thick and dense with greatly reduced visibilities. Flying through these ‘cumulus’ causes rapid changes in visual range, approach light intensity, and ambient light level.

Don’t mess with the unstable periods of fog formation or dispersal. Cat 2 (RVR) is one operation where you can make a valid decision to land and then find that you are wrong before you get to the threshold.
--------------------
Unless specifically authorized everything else is forbidden.

Would this approach ban speech be given if the RVR dropped below minima once established on the approach but before the 1000ft AGL check?

After passing 1000ft AGL if the RVR dropped below minima and the pilot continued to DA/MDA made visual contact. Would you still refuse the clearance to land and if so what RT phraseology would be used to indicate that it is safe to land?

My understanding was we could continue to land after passing 1000ft AGL assuming adequate visual reference is maintained at DA/MDA?


Thank You,

Johnny

Hi Wiggy,

>>FWIW, in the UK, for precision approaches my operator uses 1000' agl as the control height ( no mention of 4 miles/ markers), and RVR deterioration or improvements after passing that point must be ignored.......( so if it drops below minima you do not have to go around.<<

My operators Ops Manual only refers to RVR deteriororations at/after the relevant point (usually 1000' AGL) It doesn't really make sense (practically or commercially) to ignore improvements after that point.

Cheers,

THICKO

Hi Thicko

I take your point , but as far as "my" ops manual is concerned RVRs passed below 1000' are ignored, and since that's what the boss says......( I guess it's to prevent "a can we, can't we?" conversation at not a lot of feet if you get a subsequent equipment malfunction)
Rgds

Just a thought... all this talk about the 1,000 ft "point"... I presume that this has all come in during the last 2-3 years? I worked at Heathrow thousands of times in LVPs and I've seen hundreds of aircraft carrry out missed approaches from below 1,000 ft (and well inside 4 miles) when the RVR has dropped below their minima. Just recently I was told about the pilot of a large private jet asking ATC if he could "have a go" even though the RVR was below his minima. The controller, having already obtained the pilots' minima, emphatically responded with the RVR reading. The response was "roger" and the aircraft continued holding...

Surely, the various rules regarding starting or continuing approaches in poor visibility are framed with safety in mind? It would be a great shame if, under commercial presure, things went back to the "old days" when people were regularly "having a go".. and occasionally ending up strewn down the runway. If the final decision is left to the captain and not based on the RVR system there must always be the temptation to descend that little bit further...........