How do YOU determine Vis at the minima?
Thread Starter
How do YOU determine Vis at the minima?
Just interested in hearing peoples views and techniques on judging the visibility when at the DA,DH or MDA.
Are there ways of judging it accurately or is it at best a guess?
Cheers,
DoD
Are there ways of judging it accurately or is it at best a guess?
Cheers,
DoD
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Viz
I always rely what the ATIS says about the viz..... otherwise the TAF.
If neither is available or you think TAF or ATIS is inaccurate, then an educated guess ( or estimate ).
DeltaSix
If neither is available or you think TAF or ATIS is inaccurate, then an educated guess ( or estimate ).
DeltaSix
Count the runway lights / approach lights.
Approach lights are 900m long.
If you can see the approach lights/ threshold lights from where you intercept the runway MDA with the normal glide path, then you have got the required visiblity - as that is how min. visibilty is determined for a runway approach !!
The higher the MDA, the further back from the threshold will be the point to get visual on a normal 3 degree glidepath. Therefore the higher the min. visibilty required.
Approach lights are 900m long.
If you can see the approach lights/ threshold lights from where you intercept the runway MDA with the normal glide path, then you have got the required visiblity - as that is how min. visibilty is determined for a runway approach !!
The higher the MDA, the further back from the threshold will be the point to get visual on a normal 3 degree glidepath. Therefore the higher the min. visibilty required.
Great if you have the luxury of approach lights!!! Or an ATIS, oh joy... or a TAF luxury!
Usually I judge the visibility as a proportion of the runway that is visible. As the runway length is obviously a given. Which begs the question how do you know how much of the runway you can see!! Easy with familiar airports more of a black art with unfamiliar airports.
In light GA twins it is unlikely that the required visibility will be less than the ground run required so if you can see enough of the runway to land and roll out then you will have at least the required visibility.
Usually I judge the visibility as a proportion of the runway that is visible. As the runway length is obviously a given. Which begs the question how do you know how much of the runway you can see!! Easy with familiar airports more of a black art with unfamiliar airports.
In light GA twins it is unlikely that the required visibility will be less than the ground run required so if you can see enough of the runway to land and roll out then you will have at least the required visibility.
Last edited by Icarus2001; 9th Feb 2005 at 01:51.
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John C
Taken for granted that the runway has approach lights, but what if he doesnt become visual at the MDA instead becomes visual near the MAPT ( assuming MAPT is before the threshold ) and has sighted the approach lights but can only see half of the runway, this doesnt mean he has the requried viz for the approach.
It makes it even worst if he is required to do a circling because the viz might have changed by the time he gets to the opposite side of the runway.
I would still rely on the TAF or anything I can use from flight watch and estimate the actual using the runway length as a reference.
D6
Taken for granted that the runway has approach lights, but what if he doesnt become visual at the MDA instead becomes visual near the MAPT ( assuming MAPT is before the threshold ) and has sighted the approach lights but can only see half of the runway, this doesnt mean he has the requried viz for the approach.
It makes it even worst if he is required to do a circling because the viz might have changed by the time he gets to the opposite side of the runway.
I would still rely on the TAF or anything I can use from flight watch and estimate the actual using the runway length as a reference.
D6
There was a thread a couple of years back that discussed how min viz for an approach was determined and it seemed to have some relationship to the length of the runway at the aerodrome in the case of non-precision approaches. Anyone recall ?
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I look out the window.
PPRuNeaholic
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Here you go slice, checkout the online version of the Manual of Operational Standards. When the page opens, click on the item in the left side pane titled "1.7 Visibility".
Grandpa Aerotart
Just keep it simple...ozex, dude...now I like formulas as much as anyone...but please
For circling NPAs the AIP says '...or something associated with the approach end of the runway'. Well you either can or cant see that...simple really.
For runway aligned or precision approaches a 3 deg slope = 300'/nm....so 100' = 1/3rd of a nm....so for every 100' the DH is up you need generally about 600m of vis in order to see the touchdown zone, the 1000' markers....or just the runway.
An example might be SY 07 ILS DME ...minimum 270' (254')/1.5km...note that 07 only has HIRL but 16 has HIALS as well and has a minimum of 220' (204')/800m, or 1200m with no HIRL and 1500m with no HIALS.
As someone has noted HIALS are about 900m long...so you will be able to see them straight under the nose with 800m vis...the MAPT is 600m from the threshold so you will be able to see, theoretically, that as well (something associated with the approach end of the runway) but you won't see the touchdown zone until crossing the threshold...but that's ok.
If you don't have the above you don't have the minimum vis required and must carry out a missed approach.
Simple really...what you need to be able to see is right in front of you...metaphorically speaking
For circling NPAs the AIP says '...or something associated with the approach end of the runway'. Well you either can or cant see that...simple really.
For runway aligned or precision approaches a 3 deg slope = 300'/nm....so 100' = 1/3rd of a nm....so for every 100' the DH is up you need generally about 600m of vis in order to see the touchdown zone, the 1000' markers....or just the runway.
An example might be SY 07 ILS DME ...minimum 270' (254')/1.5km...note that 07 only has HIRL but 16 has HIALS as well and has a minimum of 220' (204')/800m, or 1200m with no HIRL and 1500m with no HIALS.
As someone has noted HIALS are about 900m long...so you will be able to see them straight under the nose with 800m vis...the MAPT is 600m from the threshold so you will be able to see, theoretically, that as well (something associated with the approach end of the runway) but you won't see the touchdown zone until crossing the threshold...but that's ok.
If you don't have the above you don't have the minimum vis required and must carry out a missed approach.
Simple really...what you need to be able to see is right in front of you...metaphorically speaking
You need to be able to see the threshold or associated bits of airfield, and also have enough visual information to judge your flight path with respect to them, glide path and centreline, so you can set up and maintain a normal-ish landing profile.
That's done with, as said above, a look out the window...lengths of HIALs etc are well and good, but basically it's a judgement decision based on your experience and skill - am I happy to continue and land, or does it feel too dodgy - that's basically what we do, isn't it?
That's done with, as said above, a look out the window...lengths of HIALs etc are well and good, but basically it's a judgement decision based on your experience and skill - am I happy to continue and land, or does it feel too dodgy - that's basically what we do, isn't it?
Thread Starter
Many thanks for the informative replies.
I have to admit that I always went on "how good it looked" at the MAPT and whether I was happy or not would be the deciding factor.
I was asked the question last week and was stumped at that moment to come up with a logical answer. Don't you love the ATO's eyes looking over his glasses at you with a smirk on his face!
As always, Chimbu Chuckles you are a wealth of knowledge!
Again, thanks for the discussion people
I have to admit that I always went on "how good it looked" at the MAPT and whether I was happy or not would be the deciding factor.
I was asked the question last week and was stumped at that moment to come up with a logical answer. Don't you love the ATO's eyes looking over his glasses at you with a smirk on his face!
As always, Chimbu Chuckles you are a wealth of knowledge!
Again, thanks for the discussion people
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required visibility is one minima which is so subjective,In the heat of the moment the last bloody thing im going to do is count runway lights for god sake,but thats probably the only way your accuratly going to be able to measure the vis.
If you read the regs they state the term along intended flight path so: that would mean if there was say 600m vis which aint good you could fly a four leg circuit and still be legal.Vis minimas are there to promote saftey and interpretation of the regs does not always do that.
I do it everyday and fly pretty tight cicuits,about 1.5nm to 1.6nm and in that sort of vis you would be screwed,in cat b.
If you can see the runway use it...
RWS888
If you read the regs they state the term along intended flight path so: that would mean if there was say 600m vis which aint good you could fly a four leg circuit and still be legal.Vis minimas are there to promote saftey and interpretation of the regs does not always do that.
I do it everyday and fly pretty tight cicuits,about 1.5nm to 1.6nm and in that sort of vis you would be screwed,in cat b.
If you can see the runway use it...
RWS888
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This depends a lot on where you fly. Going into Africa......mostly procedural approaches..no atis RVR..etc......we use that well worn item "the Mark 1 eyeball"...a bit of local knowledge combined with experience is basically how we do it.
Europe and the US..Asia, Australasia well its all done using ATIS and ATC..if say there are vis reports, but no reported RVR, then the standard 2 x reported vis (night) or 1.5 x reported vis (day) is used if high intensity lighting is available. The whole Vis determination subject is a bit of a black art.
Europe and the US..Asia, Australasia well its all done using ATIS and ATC..if say there are vis reports, but no reported RVR, then the standard 2 x reported vis (night) or 1.5 x reported vis (day) is used if high intensity lighting is available. The whole Vis determination subject is a bit of a black art.
Hmm, you're coming out of the schmoo at the minima, transitioning from instruments to visual, and now you want to start counting lights 
I know I couldn't do it.
My common sense suggests to me: If you can see where you're gonna land, land. If you can't, don't.
I know I couldn't do it.My common sense suggests to me: If you can see where you're gonna land, land. If you can't, don't.
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Landing Visibility required is:-
1. Inversely proportional to the fuel remaining
2. Inversely proportional to your familiarity with the port
3. Inversely proportional to the quality of your planned social engagements
4. Proportional to the quality of accommodation and possible social engagements at your alternate
5. Proportional to the perceived number of persons who will bust your arse for appearing to be too dodgy
1. Inversely proportional to the fuel remaining
2. Inversely proportional to your familiarity with the port
3. Inversely proportional to the quality of your planned social engagements
4. Proportional to the quality of accommodation and possible social engagements at your alternate
5. Proportional to the perceived number of persons who will bust your arse for appearing to be too dodgy
Last edited by Spinnerhead; 14th Feb 2005 at 11:08.