Instrument approach minimums

It's definitely possible. There is a difference between 1800 RVR and 2400 RVR, and you can tell. Now, some CAT I approaches will have 1800 required and some will have 2400 yet both still are labeled as 1/2mi vis. It is possible to see one of the FAR required items yet not have the req'd flight visibility, thereby technically requiring a missed per the FARs. If you are directly over the approach lights at mins you are 200' away from them. Just because you can see them 200' below you doesn't mean you can see anything 1800' ahead of you.

I think there is even something in the AIM that tells you how far apart each light is, thus supposedly allowing you to determine flight vis at mins. Realistically you'll have more to do at that point than count lights.

Now as far as how you would get 'caught', well I really have no idea... like a lot of things in flying it is honor system mostly, and I imagine you'd have to be having a pretty bad day to be questioned after a successful landing.
 
deek said:
Where did you come up with 2886'? neat math trick that seems to be evading me i'm guessing.
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I just realized I had a typo in the original calculation, and the real answer is 2890. Forgive me :o

The calculation is a little complex if it's been a while since trig, but here's how it goes:

Lets call the point directly above the threshold at the TCH the "threshold crossing point" When you reach DH, you're 200 ft above the runway and 141 ft above this point. Since the glide slope is 2.8 degrees, horizontal distance from this point is (141 / tan(2.8) ) = 2882.9 ft. Slant-range distance from the threshold is then sqrt(200^2 + 2882.9^2) = 2890

milski said:
200' / tan (3 degrees)

Comes as 3816' for me. :insane:
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Keep in mind that the glide slope antenna is not co-located with the threshold. This should be the horizontal distance from the ILS aim point.
 
bhp said:
I'm getting ready to take the instrument knowledge exam in the next 2 weeks and I have a question that hopefully someone here will be able to clear up.

So here's the scenario: You're en route to an airport with an ILS approach with minimums of 300AGL and 1/2 mile visibility. ATIS reports OVC002 and RVR of 2000 ft.

My understanding is that under part 91, it is legal to "go and take a look" and shoot the approach anyway, of course still observing the DH. Suppose that conditions are indeed slightly better than the ATIS indicates and you break out just at minimums and can see the MALSR ahead. The question is, what about the visibility? If it looks like the 2000 ft. visibility is still true (below minimums) do you have to go missed even though you have the "runway environment" in sight? Is it safe to assume that being able to see the lights at the DH with needles centered necessarily means that the required visibility exists?

If you couldn't tell, I'm new here, so If there's another section where this question would be appropriate, please advise.

Thanks,
bhp
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This is a good question.
I am no expert but like you said in part 91 there is no restrictions to attempt an approach (I am assuming an ILS) even with the conditions reported well bellow min. Once you are at your DH and have the Runway environment in sight (this can actually be only one of many things like the RWY numbers or edge lights, center lights, TDZ markings, VASI, approach lights, the rabbit, or your friend waving his hands franticly...........OK maybe not that one) then you can descent bellow the DH.


You could still loose that visual at anytime so be prepared to go miss (Keep your hands on the trottles). YOU determine that the forward visibility is enough for landing.


As a GA only pilot I believe in only one assumption:
Every landing is a go around and every approach is a miss........if you make it then that is a bonus. :)

BTW, good luck with your test
 
bhp said:
Keep in mind that the glide slope antenna is not co-located with the threshold. This should be the horizontal distance from the ILS aim point.
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Yep, I figured that out after your first edition. ;) And I went for generic 3 degrees vs. 2.8
 
bhp said:
Just to throw this out there:

On the ILS 34 at AVL (http://naco.faa.gov/d-tpp/0911/05061IL34.PDF), flying to the 200ft DH leaves you 2886 feet from the threshold, but almost right on top of the MALSR rabbit. At least in principle it would be possible to break out and see the lights but not have the charted min 2400 RVR visibility.

Is there something I'm missing?
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Slightly different circumstances than I was referring to:

SteveC said:
I'm drawing a blank trying to think of a time where I would be at MDA on an ILS approach, can see the runway, yet don't have the required flight visibility???
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tonyw said:
My question as well. If you can see the runway, then don't you by definition have the required flight visibility?
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Ground fog bellow the low ceiling hanging halfway down the runway enough to make you loose dirrectional control after touch down........I have seen them.

An examiner told me once long time ago that you can descent bellow DH per regs but not bellow 50 FT if you do not have the required flight visibility to land.
 
I would think that the published minimum visibility probably agrees fairly well with the pilots "does this look safe?" visibility, and since there's not much of a way to empirically determine vis, I guess it really boils down to pilots discretion either way. Still, I'm not eager to go shooting approaches reporting below minimums.
 
I've seen situations where you can see the runway from altitude, but there's fog hanging in over the ASOS. 135 you can't even shoot the approach. Cancel, make a pirep and land. If you're part 91, if you've met the criteria for descent below MDA/DH then you can land.
 
ppragman said:
I've seen situations where you can see the runway from altitude, but there's fog hanging in over the ASOS. 135 you can't even shoot the approach.
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I too have seen airports solid IMC over the tower and ASOS, yet clear on the runway you are landing (such is Florida weather).

As a thought question, it is a great one. In theory, you should make sure you have the required visibility (you are supposed to). Figure out something that would be that far away and make sure you see it. How realistic is that?

Real world - you see the runway and the numbers at DH and know you can make it, it's your call. There isn't really a black and white answer on this one, it is purely your call as PIC. If you have the lights and the numbers, no one is going to second guess you after the fact. If it doesn't look good, don't do it.

At the same time, don't be stupid. Also keep in mind your fuel situation - if you have already gone to your alternate and it looks this bad, the prudent move is probably going to be to go below DH even if it isn't pretty. Still safer than running out of gas in the soup. (you can easily survive flying an ILS down to the numbers blind, even if you aren't cat III)
 
MikeD said:
Only problem would be, if you're at the MAP when you see the runway, would you be in any normal position to land, since you'd be past the VDP?
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Agreed, this is the problem I thought of as well, however, if you have the runway environment at VDP you will also have the required flight visibility (I'll leave myself open here and say 99.99% of the time as I haven't seen every approach).

This situation we are getting into is a bit unrealistic. The bottom line is that when we have the runway environment we have the visibility necessary to conduct a safe landing. If you have the MALSR's we're going down to 100' above TDZE. If you get the approach lights you will pick up the runway environment shortly after with the exception of real thick (think London) fog or some nasty blowing snow.

We can really only ballpark flight visibility.
 
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