PPRuNe Forums - View Single Post - Help with ILS Visabiltiy??
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Old 28th Jun 2010, 16:39
  #14 (permalink)  
Join Date: May 2008
Location: Somewhere
Posts: 286
A standard ILS brings you down at 320'/nm. At a typical DA of 220' (BN 19ILS) you will be 1270m from the touchdown zone - min vis, 800m, you MIGHT see the runway end lights but no way the VASI - you will see the the HIALS.
The HIALS is exactly what I'm talking about.

Correct me if I am wrong, but doesnt the ILS/PAPI/VASI bring you down to the 1000ft markers? 1000ft is approx 300m into the runway- so from your figures at the 220' point you are 970m from the end of the runway. If you can see that you have 5 (4 is not quite enough) of the crossbars, you have over 800m vis from your current position.
If you only see 1 or 2 crossbars, well you don't have the required vis.

The above probably comes down to what you say here:
If you see anything vaguely resembling what you expect to see you're visual - you don't and you're not. You make an instant decision.
CNS is 320ft (311AGL)and 1.2k vis.
Which off your figures again would put them 1nm from the TDZ.
At the DA (1nm/1800m from the 1000ft markers; 1500m from the end of the runway) you would plan to be able to not see the runway end lights but to know you had the required vis you should see 4 of the crossbars. If not then you dont have the vis.

When I heard the minima call I can make out the approach lighting. Because the video quality is crap and the eyball has better clarity, I later paused the video at this point (yes I know you will tell us all that pausing cannot be done for a real approach!) and can definitively count at least 3 crossbars. Only half a second after the minima call I can make out the 4th crossbar.
Surely they wouldn't have continued the approach if they didn't have the required vis! And how do you propose they worked out that they had the vis? Can you hear someone calling "can you see the approach lights?" during the approach before they get to the minima?

NEVER have I counted lights, never even occurred to me.
Got it suggested from guys driving jets.

Last edited by MyNameIsIs; 28th Jun 2010 at 17:20.
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