Slant Range vs RVR......NDB approach

[bohpilot]

NDB approach at AMS06 has the following minima:-

CAT C MDA570 RVR 1200m

If the weather was just above this minima lets say 1500m would I actually see the approach lights on a continous 3deg descent..?

I have been told that a good rule of thumb to see the approach lights is 6xMDA, so at AMS06 this would be 3420m less length approach lights 900m, = 2520m some 1000m too short.

I realise this is all to do with slant range vs actual RVR, but how do I explain why i dont see the approach lights..?

[Strepsils]

Remember that whilst you can't see the ground or any features beyond 1500m, you'll usually be able to see the lights shining through the cloud/fog/haze/newspaper.

You can see a candle in the dark from 14 miles etc.etc. :ok:

[bohpilot]

Yes, understand that theory.

However this question has popped up in our company captaincy exam, and I just want to get the exact argument correct.

:}

[Jambo Buana]

If you are at 644ft agl that equates to exactly 2 miles which is 3704 metres. How would you expect to see the lights if you need say 1200metres? Simple answer is you wont. I have no idea why such ridiculous mins are set on NPAs that sucker crews in that dont have the benefit of your command course. In fact what many TRI/Es do wrongly, is to set the vis higher than required as they think the sim is wrong! That in turn produces crews that dont appreciate that pans ops mins are cr@p.

[bohpilot]

Jambo Buana

Thanks for that I can now draw that on the board with confidence. The catch on that approach was that the DME was offset so it shows 4D at MDA.

What you explained makes sense, and yes our TREs know that with that wx you will not make it.

Ta:E :ok:

[BizJetJock]

I think you're missing the point of minima. No-one is saying that you will be able to see the lights at that RVR - but that you definitely won't if it is less than that. A distinct difference.

[bookworm]

I have no idea why such ridiculous mins are set on NPAs

They are non-precision approaches and don't have to be flown using a constant glidepath to normal touchdown point. The MAPt for the approach in question is at the threshold, and it's permissible to fly a level segment after reaching MDA. With 3500 m of runway in a small aircraft, it doesn't seem unreasonable to do so, or perhaps to set your intended touchdown point 1000 m into the runway so that you reach MDA about 1200 m from the beginning of the lights.

It's with that in mind that the AIP minima are set. If your company flies them using a constant glidepath to normal touchdown point, then of course a minimum RVR of 1200 m is pretty daft.

[Clandestino]

Simple answer: can't tell. If there's 1200m RVR in rain or snow then there's very, very slim chance of seeing anything, but If you have RVR1200 with SKC then it all depends on the thickness of mist layer. In second case, visibility above mist is often 9999 and as you descend towards the runway you can often clearly see full length of approach and runway lights, until you plunge into mist - sudden reduction in visibility leads to illusion of pitching up and if one doesn't expect it, pushing the nose down and firm arrival are guaranteed.

Even worse awaits those foolish enough to try IFR approach into fogbound field which is below RVR minima, just because they see the lights. With low lying thick fog, they might be able to see everything down to and even below minima, just to loose it all in the flare.

[boeingdream787]

Couldnt agree MORE with Jambo.........