Weather help (RVR-VIS-CEILING-DA)

[Michael Moe 493]

Hi,

 

ILS CAT 1

 

I use ASN and i always get VIS (etc.  4000-10000) in the RAW data but i sometimes confuses this with RVR which i know is another instrument along the runway.

 

My problem is therefor when i for -instance see OVC001 in the METAR and no VIS-RVR measurement i dont know what i should do if the DA is 210 feet ?

 

I read that normally the RVR in same condition is higher than ceiling and therefor allow pilots to land below ceiling.

 

Can METAR be OVC001 and still VIS 4000?

 

Thanks Michael

 

 

 

 

 

 

[downscc 1,327]

Michael, please just one post per topic. Thanks.

 

Yes, it can be overcast ceiling very low and visibility unlimited.  Not something that you see very often but it is possible.  In general, RVR is used for CAT II-III approaches so for CAT I your interest is the reported visibility.  The FAA regulations allow you to fly the approach regardless of reported visibility, it is up to you to land or go around based on your observations at DA were you can land if you see the runway environment.  Reported visibility is usually subjective and many weather conditions vary a lot along the length of an airport.  RVR is a different beast altogether. The transmissometer is installed at the touchdown zone and it's measurements are precise, if the RVR is reporting below your minimums then you cannot attempt the approach.

 

You should read the FAA Instrument Pilot Handbook available free here: http://www.faa.gov/regulations_policies/handbooks_manuals/

[A_Pilot 5]

Micheal, if i understand correctly you've tumbled on an issue that I ran into in my flight school. The Ceiling and the Vis measure separate things, but both can impede your ability to see a runway at the DA/DH.

 

With the weather posted above - OVC 001, VIS 4000, on an ILS, there's a definite possibility that you won't see the runway environment. The visibility is definitely good enough to continue an approach, as Dan mentioned (from a legal perspective), but with the ceiling at 100 feet, and the DA/DH at 210 feet, you're probably not out of the cloud yet to see the 4000+ Vis anyways (your plane would still be approx 110 feet into the cloud, measure from the bottom)

 

This is usually more common on Non-Precision approaches where there is more room between your (higher) MDA and the ground for the cloud ceiling to lie in. And if you're descending to Cirling minima, you're even more likely to run into this issue.

[Driver170 42]

My understanding is regardless of ceiling you can continue to DA from the outermarker or 1000agl even reported vis. BUT at DA you need to atleast see the runway app lighting then you can continue.

I've read pilots speaking of ceiling that low and thick, they have still seen the APP lighting system beaming through the clouds because its high intensity lighting 

[Michael Moe 493]

Thanks,

 

this was my Q and i actually saw something like this in VC in real in EDDF 25R but i was very suspect . 

 

OVC at 001 ? and still DA and not DH. Strange if you ask me

 

 

Thanks

 

Michael Moe

[Michael Moe 493]

Just a heads up ,

 

This is a strange feeling having OVC at 001 and a DA at 210 on a ILS CAT 1 if you ask me .

 

What do you do if there is no VIS information reported in the METAR above? next stage CAT II or 3 ?

 

Michael Moe

[A_Pilot 5]

VIS is very rarely not reported. At that point as a PIC I'd reference the hourly VIS and make a decision based on that as to CAT I II or III. OVC 001 shouldn't prevent a CAT II or III landing, and as mentionned above may or may not prevent a CAT I. as Vernon said - the runway lights are very very bright and could pierce through thinner coulds

[Michael Moe 493]

Micheal, if i understand correctly you've tumbled on an issue that I ran into in my flight school. The Ceiling and the Vis measure separate things, but both can impede your ability to see a runway at the DA/DH.

 

With the weather posted above - OVC 001, VIS 4000, on an ILS, there's a definite possibility that you won't see the runway environment. The visibility is definitely good enough to continue an approach, as Dan mentioned (from a legal perspective), but with the ceiling at 100 feet, and the DA/DH at 210 feet, you're probably not out of the cloud yet to see the 4000+ Vis anyways (your plane would still be approx 110 feet into the cloud, measure from the bottom)

 

This is usually more common on Non-Precision approaches where there is more room between your (higher) MDA and the ground for the cloud ceiling to lie in. And if you're descending to Cirling minima, you're even more likely to run into this issue.

 

 

I am sorry but yes this is my Q. How do i approach then ?

 

EKCH ILS22L DA 210

 

Michael Moe

I am sorry but yes this is my Q. How do i approach then ?

 

EKCH ILS22L DA 210

 

OVC at 001 is not un usual

 

Michael Moe

[Driver170 42]

But remember the controlling factor for a precision approach is usually the RVR!

 

From Eu ops1

 

According to OPS 1.405: 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. If, after passing the outer marker or equivalent position, the reported RVR/visibility falls below the applicable minimum, the approach may be continued to DA/H or MDA/H.

[scandinavian13 5,192]

Whoa...hold up a sec...

 

I think we need to step back here and get a higher up view of the question to help Michael understand things a little better.

 

Visibility comes in a few varieties:

1. Vertical visibility: this is indirectly reported as a the height of the lowest layer of overcast or broken clouds (AGL) - this is referred to as "ceiling."
2. Horizontal visibility: this is reported directly as a metric followed by SM (for statute miles - e.g. 10SM).
3. Runway visibility: this is directly reported when it's relevant (when less than 6000'), and is another horizontal vis metric, but is being recorded right at the runway (which is usually more accurate than the ASOS/AWOS location).

Example:

METAR KIAD 220152Z 26005KT 10SM FEW070 SCT250 11/03 A2997 RMK AO2 SLP151 T01110028

1. Vertical visibility: unlimited (no ceiling - no broken or overcast layer)
2. Horizontal visibility: 10SM (or greater)
3. Runway visibility: not reported (note horizontal visibility is much greater than 6000')

When it comes to airline flying, both horizontal and vertical visibility are relevant to your ability to completing an approach. Horizontal visibility (more specifically RVR) is the only metric that can prohibit you from even attempting the approach, though (FAA).

 

It seems that Michael is confusing the two horizontal visibility metrics.

[downscc 1,327]

If it is a CAT I with a minimum of 200 and the reported ceiling is 100 then you have two options:  Go someplace else or try the approach but be prepared to go around at 200 because you are still in the cloud.  Don't confuse visibility with ceiling.  Visibility is the distance you can see once below the ceiling.

[Driver170 42]

At 200 you are more than likely going to see those HIALS beaming through those clouds check this out

 

[Michael Moe 493]

Great stuff guys , thanks B)

 

Michael Moe

[skelsey 765]

 

 

VIS is very rarely not reported. At that point as a PIC I'd reference the hourly VIS and make a decision based on that as to CAT I II or III. OVC 001 shouldn't prevent a CAT II or III landing, and as mentionned above may or may not prevent a CAT I.

 

Just to clarify (and I know that you'll know this, Luc, but for those who may not be clear): in order to carry out a CATII/CATIII approach, you also need the airfield to be in LVPs (Low Visibility Procedures). This entails amongst other things ensuring that stuff like the ILS backup generator is up and running, that the ILS critical areas are sterile (i.e. no aircraft or ground vehicles inside them) and that ATC are using CATII/III holding points, low visibility taxi procedures, the approach controllers are spacing inbound traffic further apart and so on and so forth.

 

This safeguarding can take some time -- commonly in the region of 15-20 minutes -- so the important thing to remember is that if the airfield is not in LVPs at the time you make your original approach, it's not just a case of flicking a switch so you can do a quick circuit and a CATII/III. Depending on how much fuel you've got, this might play in to your decision making about whether you decide to divert or have another go.

 

LVPs reduce the landing rate and generally make operations clunkier and less efficient, so many busier airfields are reluctant to enter full LVPs if they don't have to and they're extremely unlikely to do it for one aircraft. Having said that, a cloud base <200ft (BKN) is normally sufficient to trigger LVPs even if the met visibility/RVR is otherwise sufficient.

 

Of course, it also goes without saying that the ILS needs to be certified to CATII or CATIII standards and the aircraft also has to be certified and current as do the crew (every aircraft in the fleet, as well as the crew, have to complete a minimum number of autolands within the currency period - from memory each crew member needs have have completed at least one autoland in the preceding 90 days and I imagine similar for the aircraft, but I haven't got the relevant material to hand to confirm.)

[Driver170 42]

Simon well said. Don't think anyone else can add much to that.

 

You sir, know your stuff keep up the good work in this community !!!!