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 12th Jan 2015, 09:05 #1 (permalink) Thread Starter   Join Date: Apr 2008 Location: Among the Clouds Posts: 70 Visibility for Minima-measurement n evaluation Dear Aviators, I wish to consult the learned ones for opinions on a few issues on the visibility criteria for landing minimum. Let's talk about typical Cat 1 minima for discussion; 200' and 800m vis. Many that I know take visibility as the number 1 weather criteria (with the deceleration accounted for) in their day to day operations. 1. I have heard that some aviators translate visibility to height above which you are expected to see the runway, by using the 1/60 rule and some conversions- ie you are expected to see the runway with 1km visibility at 160', 2km at 330', 3km at 500'. This probably assumes no significant cloud at low level. This being the case, would visibility of 1km still good to conduct a Cat 1 appr? 2. It is argued by some that measurements of visibility is of static nature. This supposedly creates complication for weather condition with precipitation for pilots. That is when it is raining the 2km reported is not the same as a 2km without precipitation. I have heard of aviators who use a factor of half for interpretation of moderate precipitation-ie when it is reported 2km and using the method in point 1 you are expected to see the runway at 330/2=165'. The minima predicated on approach charts do not take this precipitation into account? Any views or comments for the above mentioned techniques please? Many thanks for sharing.
12th Jan 2015, 09:18   #2 (permalink)

Join Date: Jan 2014
Location: N5109.2W10.5
Posts: 455
Dear titaniumwings,
Quote:
 This being the case would visibility of 1km still good to conduct a Cat 1 appr?
You won't see the runway initially, but you should see the approach lights and lateral elements. Since you have visual reference you may continue.
If there are no approach lights then you probably won't see anything and will have to GA.

 12th Jan 2015, 11:58 #3 (permalink) Thread Starter   Join Date: Apr 2008 Location: Among the Clouds Posts: 70 Dear Goldenrivett, So you also agree with Method 1. It is interesting that though the cloud base and visibility are above Cat 1 appr charts but in "reality" you actually do not see the runway without appr lights (which is not a requirement for Cat 1). It is believed many companies put it as above landing minima for dispatch purposes. Wonder why not put it as 1300m for the minima? Thanks again for your reply.
12th Jan 2015, 12:09   #4 (permalink)

Join Date: Feb 2003
Location: Cambridge
Posts: 414
Quote:
 Let's talk about typical Cat 1 minima for discussion; 200' and 800m vis.
Well, in EASA land it is more typically 200 ft and RVR of 550m. The correlation between visibility and RVR is not straightforward.

But what does matter is what your Ops Manual says. It will define what you need to see at DH to be allowed to continue; if you can't see that, a GA is mandatory. So your point is rather beside the point.

 12th Jan 2015, 12:37 #5 (permalink) Thread Starter   Join Date: Apr 2008 Location: Among the Clouds Posts: 70 The operations for low vis approaches are often very thoroughly defined. To keep things simple let's discuss about Cat 1 and the visibility required to see the runway. The basic questions are: 1. Do you also use 1000m vis to relate to seeing runway at 160' agl. If not, why or any other method? 2. Do you discount the visibility reported if there is precipitation? If yes, how do you go about doing that? Trying to make it simple. Thanks a lot for your inputs fellow aviators. ps: Weather is probably one of the biggest enemy for pilots. The evaluation of it can be a matter of "art"
 12th Jan 2015, 13:00 #6 (permalink) Join Date: Apr 2012 Location: In Space Posts: 546 Titanium - as a rule of thumb we use 1nm = 300' 1nm = 1852m so therefore 1000m per 160' wouldn't exactly work. Another rule of thumb is RVR REQ = DA/MDA x 6 again this is a ball park figure. Its an interesting topic, but it wouldn't be so critical on an ILS where on a Non precision approach it would (taking cloud ceiling into account). If you take for example a VOR approach with MDA = 660'(50' added) and RVR 2km Using the above conversions, you will need a minimum of 4km to see the runway. Question is why is the RVR down to 2km?
 12th Jan 2015, 13:33 #7 (permalink) Join Date: Oct 2002 Location: In my seat Posts: 826 Guys! STOP right here. ICAO ans EASA minima prevail. STOP trying to invent rules by converting height to distances etc. Minima for EASA and most other authorities are for Cat1 : 550m. Rvr and Decision Altitude of 200 ft. But variable for each runway. So as long as you have more than 550'rvr ( or 800 m) ( taking into account approach ban obviously) and see the runway or approach lights at your DA, you are legal to land. There is NO CEILING limit! there is NO requirement to interpolate, calculate or anything else than to follow the bloody minima depicted on your charts.
 12th Jan 2015, 16:23 #8 (permalink) Join Date: Apr 2012 Location: In Space Posts: 546 Despegue - Im not making up new rules, I'm fully aware of what EU-ops and the FAA have to say. My post was regarding how they determine what the min RVR is required for the approach when the math doesn't exactly fit. I believe the OP was referring to it also. From a planning point of view Ceiling is taken into account for NPA. If you have a NPA and the minimums are as in my previous post, do you think you will see the runway? If you include a cloud base that will make it even more difficult. Would you consider taking extra fuel? But lets not go off topic.
 12th Jan 2015, 16:27 #9 (permalink) Join Date: Jun 2000 Location: last time I looked I was still here. Posts: 4,100 As Despegue says: KISS. Deciding to land is something you do at or before minima. Being legal to make an approach is decided earlier and is dictated only by vis/rvr. Some years ago, under PANS Ops, the visibility bore no sensible relationship to the MDA on NPA's. It was quite obvious that at MDA you might not seeing anything of the visual reference if the minimum visibility was a fact. This was to be forewarned and forearmed. I use 2000m = 320'. You can measure the approach lights, assuming 150m lateral bar spacing. For ILS minima you will usually find different values for different lighting systems. Thus there is now a better correlation between DA & visibility. I'm not sure if the same is now applied on NPA's. It should be as most NPA's are promulgated as continuous descents and a DA is published rather than MDA.
 13th Jan 2015, 04:04 #10 (permalink) Thread Starter   Join Date: Apr 2008 Location: Among the Clouds Posts: 70 I am sorry, perhaps I did not make it clear and created some misunderstanding. As I mentioned for dispatch purposes the weather is checked against the minimums on the approach charts. The legality issue is well-defined in this case. But as B737900er mentioned I was hoping some would share experience as to how as pilots would you interpret and relate the weather to the minimums on the charts, including uplift of additional fuel. This is the main point. The guide given can be quite useful. We are however not advocating for not conducting the appr with the rule of thumb. It is only an assessment of weather for contingency planning. Just to share, in the Asian part of the world where tropical thunderstorm and rain prevalent. It does seem to reduce the visibility considerably so much so that there seems to be a possible need of a reduction factor from the rule of thumb. Different regions with the different characteristics for weather. Thanks again for sharing.
 13th Jan 2015, 05:09 #11 (permalink) Join Date: May 2000 Location: Seattle Posts: 2,947 What is "a reduction factor from the rule of thumb" supposed to mean?!? Visibility and ceiling are NOT necessarily related, so there is NO NEED for any J factors of any description! If you have a 3000' ceiling but heavy rain, visibility may be less than 500m, and NO "reduction factor" will change it. Either you see the runway environment at the decision height, or you do not. You either land or go around, respectively. Why do you want to make it complicated?!? If there is a 200' ceiling, you will either see the runway environment at the decision height, or you will not, depending on the exact height and density of the cloud at that point. You will either land or go around, respectively. What else is there to understand?!? It makes NO DIFFERENCE if you are "in the Asian part of the world where tropical thunderstorm and rain prevalent" or in another part of the world where snow or fog or rain occasionally pass through. Uplift of fuel depends on MANY factors, including "an assessment of weather for contingency planning." If you think the weather will be near minimums, uplift more fuel if you think you may have to hold or want to make more than 1 approach before diverting.
 15th Jan 2015, 09:33 #12 (permalink) Thread Starter   Join Date: Apr 2008 Location: Among the Clouds Posts: 70 Hi Intruder, I try to put this as clearly as I can. What this idea is trying to illustrate is only on the visibility part of the minimums. Say weather is reported at 2km and this is measured by the standard equipment on the ground. What this theory is saying is from the point of view of a pilot in an aircraft which may be moving at 140kt this 2km visibility reported will yield different visibility with and without precipitation. Pilot actually "see" less in precipitation in 2km visibility with precipitation. This reduction in visibility is worse if the precipitation is heavier and the aircraft moves faster. Correlate this to driving or biking. Do you experience a drop in visibility with increased speed? Anyone can affirm or deny this theory with experience or technical explanation? Anyone ever come across the term dynamic visibility? Believe me, I am a true proponent to KISS though I never want to get on the wrong side of the biggest enemy-weather. We are now talking about the evaluation of weather.
 15th Jan 2015, 20:27 #13 (permalink) Join Date: May 2000 Location: Seattle Posts: 2,947 There is, to my knowledge, no practical means of measuring "dynamic visibility" along the glide path. Therefore we are stuck with the 3 measures in place: prevailing visibility, RVR, and Vertical Visibility. In low-visibility situations, RVR is the most important, since it is on the runway that your visibility is most critical. That is recognized already in the rules for Cat III approaches -- you need not see the runway before landing. In ALL cases for Cat I and II ILS approaches, the Decision Height is, in reality, a single point in the sky where the glide path intersects the Decision Height altitude. Either you see the runway at or before that point, or you do not. Respectively, you either continue to a landing from that point, or you initiate a go-around. Visibility depends on more than precipitation. Lighting conditions (e.g., contrast) are a significant factor. For example, it is MUCH easier to pick out approach lights at night because they are so much brighter than the background. Bottom line is, there is no need for any "reduction factor". Either you see the runway at the DH or you do not. If you want lower minimums, you get certified for a Cat II or Cat III approach. If there is no Cat II or Cat III approach for that runway, it is because approaches in lower visibility have not been deemed safe.
 16th Jan 2015, 06:25 #14 (permalink) Join Date: Nov 2001 Location: South Seas Posts: 3 Commencement and continuation of approach. Despegue is right. See CAT.OP.MPA.305 Commencement and continuation of approach. If the reported RVR is at or above your minima when passing 1000 ft above ground you just continue to DH/MDA and if you are happy with what you see just land. If not then go-around. Ceiling above DH/MDA is not a legal requirement to continue beyond 1000 ft/outer marker.
 16th Jan 2015, 12:54 #15 (permalink) Thread Starter   Join Date: Apr 2008 Location: Among the Clouds Posts: 70 Revelation is a trainer is advocating a factor of 0.5 in the evaluation of visibility in precipitation. This means in rain a 2km visibility can be interpreted as 1km. The dynamic term is also introduced by him. I search everywhere to no avail. I do discount the concept but the experience may be contrived from the simulation job. Thereafter the correlation interpretation of the visibility to contingency planning and fuel uplift is up to the individual. With particular reference to single runway stations without the luxury of low visibility operations, which could possibly due to cost and funding. Thanks again for sharing and the inputs.
 21st Apr 2017, 10:26 #16 (permalink) Thread Starter   Join Date: Apr 2008 Location: Among the Clouds Posts: 70 Do We Discount for Precipitation for Landing Minima? Just to re-frame to a simple question. The reported (MET) and forecast (TAF) visibility is based on static clear condition, so do we apply a multiplier or safety factor for conservatism for landing? Eg TAF heavy rain visibility 1000m. Would you reckon actually having a visibility of less than 800m for Cat 1 when you are flying in? Thank you very much guys n gals.
 21st Apr 2017, 11:37 #17 (permalink) Join Date: Jan 2004 Location: Australia Posts: 2,342 Over thinking to the max. Decision process: Can I see enough at the DA to continue the approach? Yes, continue. No, go-around. Very simple, no maths involved. I'm not going to sit there counting approach lights at the DA.
21st Apr 2017, 13:04   #18 (permalink)

Join Date: May 2000
Location: Seattle
Posts: 2,947
Quote:
 Just to re-frame to a simple question. The reported (MET) and forecast (TAF) visibility is based on static clear condition, so do we apply a multiplier or safety factor for conservatism for landing?
Just to re-frame to a simple answer.

NO!!!

21st Apr 2017, 14:21   #19 (permalink)

Join Date: Apr 2008
Location: Among the Clouds
Posts: 70
Just to clarify. The decision that I am talking about is not whether to land, rather it is whether to launch the flight on time or to carry extra fuel to wait over the tempo period of which by judgement the commander reckon the possibility of landing is possibly low.

So to add to the question:
Suppose TAF 3000m you don't carry fuel. Would you carry fuel for TAF rain 2000 or 1000m?

Quote:
 Originally Posted by AerocatS2A Over thinking to the max. Decision process: Can I see enough at the DA to continue the approach? Yes, continue. No, go-around. Very simple, no maths involved. I'm not going to sit there counting approach lights at the DA.

21st Apr 2017, 14:31   #20 (permalink)

Join Date: Apr 2008
Location: Among the Clouds
Posts: 70
Actually the last part of this reply is what I am partially verifying. Let's just take a CAT 1 for reference, vis required 800m 200'. Without precipitation, what TAF would you consider near minimums?

I am fairly sure that 3000m is comfortable for a Cat 1. By experience and not going too in depth into calculations, would you take fuel for TAF 2000m? 1000m? How about TAF with rain 3000m? or rain 2000m? Would you read TAF 2000m vis with and without rain to be the same?

Quote:
 Originally Posted by Intruder What is "a reduction factor from the rule of thumb" supposed to mean?!? Visibility and ceiling are NOT necessarily related, so there is NO NEED for any J factors of any description! If you have a 3000' ceiling but heavy rain, visibility may be less than 500m, and NO "reduction factor" will change it. Either you see the runway environment at the decision height, or you do not. You either land or go around, respectively. Why do you want to make it complicated?!? If there is a 200' ceiling, you will either see the runway environment at the decision height, or you will not, depending on the exact height and density of the cloud at that point. You will either land or go around, respectively. What else is there to understand?!? It makes NO DIFFERENCE if you are "in the Asian part of the world where tropical thunderstorm and rain prevalent" or in another part of the world where snow or fog or rain occasionally pass through. Uplift of fuel depends on MANY factors, including "an assessment of weather for contingency planning." If you think the weather will be near minimums, uplift more fuel if you think you may have to hold or want to make more than 1 approach before diverting.