ILS category, separation and protection area

[astonmartin]

Any thoughts, opinions and experiences on the following matter is appreciated:

Recently I made an ILS approach to a runway at my homebase (AMS Schiphol) with visibility deteriorating.

The ATIS mentioned "LOW VIS" procedures in operation. Also vertical vis 100'. This is why we prepared for a cat IIIb approach, because this was the lowest published category for that runway (18R). To be sure, we asked ATC for confirmation that the ILS was cat III. ATC replied that they were using cat II separation. Therefore we changed our setup to cat II, using 100' decision height. To see what would happen.

After hearing this, other collegues in the air insisted on cat III, because of the vertical vis. ATC replied that they primarily take the RVR (now 450m) into consideration when determining level of separation/ILS category.

To my knowledge, the ILS itself may be cat III and so may the aircraft en crew, but when the protection area is not free, because ATC is using separation criteria for cat II, the best ATC can offer, is an ILS cat II approach.

When a pilot hears 'LOW VIS procedures is operation' this does not imply that you can use cat III minima. ATC should communicate clearly about the ILS category in relation to separation used and pilots should actively ask for the ILS category after hearing 'LOW VIS procedures'.

I suspect this is not only relevant to Schiphol. Some people state that ATC separation is not the same as ILS category and that it is no problem to shoot an ILS cat III while ATC uses cat I separation. I believe that is not true.

[RAT 5]

I can not remember if I have ever seen a holding point that is only CAT II and a separate one for CAT III. They are normally combined. Quite what ATC means by "using CAT II separation." I don't know.
Some years I was in Germany and had this problem where the arrival radar was not at the airfield. It too was 450m and low cloud base. They told us it was CAT II. My opinion is why muck about with CAT II if the crew/aircraft/ILS is CAT III qualified. WE asked arrival if we could have CAT III and they said ask the local tower when we switched over, but for them 450m = CAT II. I thought this is not useful to use as operating crew. What is ATC doing to decide such a thing. When we swapped over to tower and asked for CAT II they said the sterile area was CAT II/III and yes we could have CAT III. I can't remember if there is any difference in the ILS installation e.g. electrical switch over time etc, between CAT II & III. I thought most of the CAT II restrictions were due to ground features on approach and perhaps the runway.
So wat is ATC doing making such a decision. Does their 'separation' mean something about spacing on finals and runway vacating time? I can't see how it can be technical about the installation.
I've never heard this issue in UK. Perhaps someone from ATC in any country can enlighten us to exactly what they mean & why.

[peekay4]

Does their 'separation' mean something about spacing on finals and runway vacating time?

Yes, in this case Schiphol ATC is referring to increased in-flight separation on final, as required to protect the ILS signal. When you're shooting a Cat II/III approach, you don't want a big A380 in front of you on the same localizer potentially blocking the signal! This signal masking is one of the weaknesses of ILS when compared to GPS based approaches.

In ICAO land as weather deteriorates, different LVP phases come into effect at the airport (e.g., Phases A, B, C and D).

For example, LVP Phase A might equate to CAT I conditions with few operational restrictions. The airport might stop some maintenance activities, restrict ground vehicle movements, etc.

If conditions worsen, the airport might invoke LVP Phase B procedures, which might equate to CAT II standards, increased in-flight separation, slowing down the rate of arrivals & departures, restrictions on which runways to use, etc.

With very poor RVR the airport might declare LVP Phase D, with only one CAT III runway for landings, max in-flight separation on final, huge caps on arrivals & departure rates, mandatory use of the follow-me truck or towing for taxi, etc.

(These phases and associated restrictions will differ for each airport).

So back to the original topic, ATC can't offer CAT III service if they are applying in-flight separation and associated flow-control for CAT II.

[Skornogr4phy]

I went into AMS a few months ago and RVR's reported on 18C (our landing runway) were reported as 300m or less. We asked if the airport was in LVP's and we were told "nah we can easily see the runway from the tower and the previous aircraft was visual from 10 miles out".

Now I can appreciate that the transmissometers often don't tell the whole picture, but surely once they are below Cat 1 minima they should be in LVP's. In the end we elected to land as the runway was visible quite far out as they stated, but if we were to have had a hard landing etc, I'm sure the board of enquiry would ask why we continued without LVP's in force.

[RAT 5]

That's curious as I thought the RVR was governing. They are recorded. If the transmissometers are not trust worthy they should be switched off; surely. If the ATIS is giving 300m, and ATC says no it's not, I'm not sure they're on solid legal ground. I'm very glad ATC tapes are kept longer than my CVR.

[peekay4]

Similarly, I've never come across different separation standards applied by ATC for cat II and for cat III. The increased separations is driven primarily by the need for previous landers or departures to have cleared the sensitive/protected areas before the next landing aircraft reaches a particular point, commonly 2NM from touchdown.

My previous wording was a bit clumsy. Depending on the airport there may be different in-flight separation standards defined for different low visibility phases (example later below), which in turn also correspond to different ILS categories.

In-flight separation on final is not just about protected areas on the ground. ILS signal masking from a leading aircraft is the largest factor determining separation, after wake turbulence. Many pilots & ATC personnel may not realize this because we only need to follow procedures in place.

Here's from an AWST article published last year:

(Airbus ProSky CEO Paul-Franck Bijou) says in-trail spacing on an approach is dictated first by wake separation constraints and second by the ILS signal-masking effects of the lead aircraft on those behind it on the approach. If wind conditions allow for closer separation, masking can become the primary constraint [...]

Bijou says testing has shown that in-trail separation can be reduced by 2 nm for some aircraft pairs compared to the existing constraints for ILS masking. “This is massive,” he says. Airbus ProSky will test an “optimized operation” concept of reduced ILS spacing behind medium-sized aircraft in low visibility conditions in Zurich, followed by tests at London Heathrow and Peking."

I would say the next largest factor for separation would be the airport ground ops capacity constraints during (increasingly worsening) low visibility conditions.

So after we combine all the factors, a particular airport might (for example) impose a minimum separation of 6nm during LVP Phase A, 8nm during Phase B, 12nm during Phase C, etc.

I'm puzzled by peekay's comments. I have always thought the ICAO standards for AWOs and LVPs were rather vague and simply said 'protect whatever needs to be protected'. More recently the concept of visibility conditions 1, 2 and 3, to help to reduce restrictions on the ground where they were not necessary, were introduced - but I do not recall any phases A, B etc. in any documentation.

By nature low visibility ops will be specific to each airport and difficult to standardize. Also the different operating levels may not be called "phases" specifically -- depending on national practice -- however the term is commonly used.

Two examples LVP phases from different sides of the world:

Thailand / Bangkok Suvarnabhumi


Netherlands / Amsterdam Schiphol


(extracts from Jepp chart for EHAM, section 1.3 Low Visibility Procedures)

Associated capacity constraints for Schiphol under different LVP levels, enforced in part through increased separation:

from "Improved Low Visibility and Ceiling Forecasts at Schiphol Airport", KNMI, 2008

So for example when RVR is less than 200m at Schiphol, LVP phase D will be in effect, increased in-flight separation applied, only one landing runway available (CAT III), with arrival rate capped at 16 per hour.

[astonmartin]

Thanks a lot for your comments everyone.

In general, I think ATC should be clear to pilots up to which category the ILS can be garantueed and on the other hand pilots should actively ask for this.

'Low vis procedures in operation' does not automatically mean that you can use the ILS all the way up to cat III.

[flyburg]

Originally Posted by astonmartin

Thanks a lot for your comments everyone.

In general, I think ATC should be clear to pilots up to which category the ILS can be garantueed and on the other hand pilots should actively ask for this.

'Low vis procedures in operation' does not automatically mean that you can use the ILS all the way up to cat III.

Hi Astonmartin,

I was the one that send you the mail with the 737 publication.

I've taken the liberty and called LVNL and asked for clarification. Here is the explanation I received.

The ILS is always CAT III. There is no switch between CAT I or III. At the most the signal can be degraded but then this will be published by NOTAM.

The separation between CAT II and III is 8 miles versus 10 miles. The purpose of the difference is only to give the preceding aircraft under CAT III more time to vacate the ILS critical area. Where as when it is very low VIS aircraft would possibly take more time to taxi clear of the of the critical area( they may not but in any case, more time is provided). I can see where when visibility is reduced to 50( if memory serves me right, this is the minimum for CATIIB no DH) you would take more time to taxi clear.

However in both CAT II and III the same area needs to be clear, here there is no difference!

The critical area is always monitored and a final determination made at 2 miles. If at that time the critical area is not clear they will tell you to go-around.

To summarize: in both cases(CAT II/III) the critical area will be clear, however under CAT III more time will be provided for the leading aircraft to taxi clear, if the critical area is not clear by the time you are at 2 miles the will send you around.

Makes sense to me now, hope for you too.

[astonmartin]

Originally Posted by flyburg

To summarize: in both cases(CAT II/III) the critical area will be clear, however under CAT III more time will be provided for the leading aircraft to taxi clear, if the critical area is not clear by the time you are at 2 miles the will send you around.

Makes sense to me now, hope for you too.

Thanks a lot flyburg! Makes sense indeed. As we agreed in the mail, ATC should not use ILS category to describe separation. This is confusing.

Best regards.

[RAT 5]

Is this a case where ATC & pilots who are on the same team, but are not singing from the same hymn sheet. The same complaint was said over many years when ATC would declare a runway wet when for the pilots it wasn't, but then had to operate as if it was, which could cause problems. i.e. the 2 team players were not using the same criteria, but ATC are the manager. Sometimes there is a change in role of who is the chief & who is the indian. That is also confusing.
It needs sorting out.

[Gonzo]

Agreed.

As far as flight crew are concerned, "ATC LVP in force" should mean that CAT III approaches are expected.

That phrase (or lack of it!) should override anything else. I say 'lack of' as at LHR we now ignore reported cloud for LVP, we just use RVR, so we can be out of LVP even if the cloud is OVC001 (providing aircraft are landing ok from CAT I approaches).

If ATC has a subset of procedures in terms of arrival spacing to account for different runway occupancy times then that should never be communicated to flight crew as it's confusing.

[Mikehotel152]

A few months ago I was operating into London Stansted with a low cloud base at 200' and RVR of well below 1000m. The ATIS was informing pilots that Cat III holds were in use but LVPs were not.

Pretty much everyone was setting up for a Cat III approach and the Approach/Director was unable to confirm whether Cat III approaches were allowed.

Tower was becoming rather terse when informing pilots on the ILS that only Cat I was available.

We couldn't quite understand why they were using Cat III holds with no LVPs, and only Cat I available.

[Goldenrivett]

We couldn't quite understand why they were using Cat III holds with no LVPs, and only Cat I available.

Probably in preparation for LVPs. It takes sometime to check all the protected areas are clear and air traffic separation standards are met before LVPs can be assured.

[RAT 5]

I'd always thought, perhaps from long ago, that UK went LVP's with cloud base <300' and/or vis <1000m. This of course assuming 200/550 for CAT 1.

I'd also thought that ATC was a service to the a/c offering the best suitable to ensure a successful approach & landing. I'd not heard of the flow rate differing between CAT 11 & CAT 111. One could say that keeping the highest flow rate possible, if so busy, is providing the best service. However, if there is no other traffic, and the protected area is the same for CAT 2 & CAT 3, why not allow the arriving crew to have what they want.

It was the case, at places like e.g. LTN RW08/26, where the cloud base was measured directly over the airfield, but due to the valleys west & east, the airflow would drag the cloud base lower on finals. It was not uncommon to have an ATC cloud base of 300-350' and be IMC at 250'. Having this local knowledge, and the if vis was also <1500m, we would ask for an LVP approach to have a lower DH. If there were no other awkward circumstances it was always allowed. Like I surmise, ATC is a service provider, is it not, with limitations at certain times; that is understood.

[Gonzo]

MikeHotel152,

The worst case scenario would be that if CAT I holds were used up until the RVR dropped to 550m, then a departing a/c holding at the CAT I hold would get 'trapped'.... just when you need the CAT III protection. The solution is to move to CAT III in advance of needing them.

Some airfields need to have the CAT III stop bars manually inspected to ensure they are operating correctly before LVP initiated.

We go to CAT III holds at about 1000m RVR.

Surely if ATC are telling you "LVP not in force" then you know you cant do a CAT III approach?

LVP in force, CAT I, II, III.

LVP not in force, CAT I only.

It has to be that simple.

[FlightDetent]

"low visibility procedures in force category three"
"category two only low visibility procedures"
"ay-el-es category one approach"

If I may suggest the wording. Hearing barriers, confirmation bias and such...