ILS Minimums Question

turbomax97

turbomax97

What can brown do for you?
Ok so we all know the difference between CAT I,II, and III approaches and the appropriate rvr etc.

My questions is this, if you look at the ILS @ bigger airports, say DFW for example, the non cat II/III ils has CAT II/III min's listed, for example, rvr18 etc. even though a cat I min is 200/ DA, with 1/2 mile vis/ RVR24.

So why are those lower mins listed, if there's a CAT II/III approach plate for the same runway? It seems illogical to have it redundant like that?:crazy:
 
First, except for special circumstances, that 200 feet isn't a minimum. The visibility is - and to land under part 91 it is flight visibility that matters.

According to 91.175, 1800 RVR converts to 1/2 SM visibility. Plenty of CAT I approaches go down to 1800 RVR.

If an airport only had CAT II/III approaches listed - many of us wouldn't be able to get in because we are not allowed to execute those approaches.
 
Ian J said:
First, except for special circumstances, that 200 feet isn't a minimum. The visibility is - and to land under part 91 it is flight visibility that matters.

According to 91.175, 1800 RVR converts to 1/2 SM visibility. Plenty of CAT I approaches go down to 1800 RVR.

If an airport only had CAT II/III approaches listed - many of us wouldn't be able to get in because we are not allowed to execute those approaches.
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I may be misreading something so please correct me, but FAR 91.175(h)2 shows 1/2 mile vis as 2400 RVR?

Ok, so in AIM 1-1-9, i found that it reads " Category 1. Decision Height 200 feet and RVR 2400 feet(with touchdown zone and centerline lighting, RVR 1,800 feet)

I don't see how the 200 feet is NOT a minimum? The way I'm interpreting it is that to go lower, you've gotta be CAT II or III.... I guess I'm missing something here? I'm good on the visibility part now.
 
turbomax97 said:
I may be misreading something so please correct me, but FAR 91.175(h)2 shows 1/2 mile vis as 2400 RVR?
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Sorry about that - read AIM 5-4-19 a. - that talks about what to do when the value falls outside those listed in 91.175.

As you mention below - there is a reason some ILS cat Is are 1800 rvr.

Ok, so in AIM 1-1-9, i found that it reads " Category 1. Decision Height 200 feet and RVR 2400 feet(with touchdown zone and centerline lighting, RVR 1,800 feet)

I don't see how the 200 feet is NOT a minimum? The way I'm interpreting it is that to go lower, you've gotta be CAT II or III.... I guess I'm missing something here? I'm good on the visibility part now.
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Read ALL of 91.175 - you'll find the items required to operate below an MDA or DH while IFR. You'll find a mention of flight visibility - nothing about ceilings.

The 200 feet it talks about, in most cases, is just the DH. In the Army - that 200 feet (or whatever was listed in the parenthesis on the approach plate - it's not always 200 feet), WAS a minimum for us. But, like I said, in part 91 it is simply the flight visibility.

You'll also notice that no where in part 91 does it restrict you from beginning an approach under any weather circumstances (much UNLIKE part 135/121.) So if the weather is reporting OVC100, 1500RVR, you may begin the approach, and if at decision height you have the items listed in 91.175 and the required FLIGHT visibility (as determined by you the PIC), you may go below the DH. (In some cases just 100 feet below, but you can read up on that! ;))
 
Yeah I've got 91.175 down, it's just that the way it's worded - No aircraft may operate below MDA/ DA unless etc etc etc.... sounds like a limitation. The way I was taught last year was that in addition to being @ Min's, you must have the ALS to descend to 100/ TDZE, and then you need all the other jazz to land, but the first step in the process being, normal descent w/ normal maneuvers to the rwy of intended landing, then the flight visibility req'ts.

I kind of see the point that you're making - if you are in the clouds @ min's, as long as you have the visibility to see the visual cues, you can land... hmm i'm gonna have to think over this one some more.
 
I see the way you're going with this. I see "200 feet" and think "weather limitation," which it is not. You see "200 feet" and see "decision height - a point in space one cannot descend lower than without x, y and z," which is correct.

In your original thread your wrote:

even though a cat I min is 200/ DA, with 1/2 mile vis/ RVR24
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Which I lumped together in my head as "weather conditions." So let's clear this whole mess up:

DH (whatever the height AGL is) is a "minimum." Not a wx min, just a "can't go lower than unless min."

An actual weather ceiling is not a minimum.

The visibility IS a weather min. You may not land unless you have the visibility required for the approach.

Agreed?
 
turbomax97 said:
I kind of see the point that you're making - if you are in the clouds @ min's, as long as you have the visibility to see the visual cues, you can land... hmm i'm gonna have to think over this one some more.
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Absolutely! I did an approach like this the other day. The weather on the field was VV001 - RVR 1800. Everything from 400 Agl and up was clear and a million in all directions - there was just a blanket of fog on the ground.

At 400 AGL I descended into a cloud. At DH I had the required items to go 100 feet lower. At 100 feel lower, I had the rest of the items, and landed, even though I was in a cloud.
 
Ian J said:
Absolutely! I did an approach like this the other day. The weather on the field was VV001 - RVR 1800. Everything from 400 Agl and up was clear and a million in all directions - there was just a blanket of fog on the ground.

At 400 AGL I descended into a cloud. At DH I had the required items to go 100 feet lower. At 100 feel lower, I had the rest of the items, and landed, even though I was in a cloud.
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You freight dawg you!
<thread hijack>
Can I catch a ride tonight from TEB - BOS
</thread hijack>
 
I disagree. The DH is not a minimum, it is a point at which a decision must be made to continue the approach or not. I can't think of many airplanes that can instantly stop their descent the moment you decide to go missed. Hopefully when flying an approach without verticle guidance you have stopped your descent and reached your MDA before reaching the MAP. 91.175(c) has the subtle distinction between the MDA and DH. Any thoughts?

Ian J said:
I see the way you're going with this. I see "200 feet" and think "weather limitation," which it is not. You see "200 feet" and see "decision height - a point in space one cannot descend lower than without x, y and z," which is correct.

In your original thread your wrote:



Which I lumped together in my head as "weather conditions." So let's clear this whole mess up:

DH (whatever the height AGL is) is a "minimum." Not a wx min, just a "can't go lower than unless min."

An actual weather ceiling is not a minimum.

The visibility IS a weather min. You may not land unless you have the visibility required for the approach.

Agreed?
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Semantics, really.

But the regs do say "may not operate below an authorized MDA/DH..."

I see what you're saying though, but pitching for Vy and applying climb power right at DH will pretty much get you climbing right away. At least in the planes I fly.
 
It seems like splitting hairs, but the regs actually say "operate below MDA, OR continue an approach below DH. There is a difference. That's one of the reasons why when you are localizer only it switches to an MDA not DH.

Ian J said:
Semantics, really.

But the regs do say "may not operate below an authorized MDA/DH..."

I see what you're saying though, but pitching for Vy and applying climb power right at DH will pretty much get you climbing right away. At least in the planes I fly.
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Study up, Turbomax! I expect to have this matter fully clarified and properly explained during our next ground school session! :yar:
 
cfii2007 said:
So what are the required items to go below the DH/MDA?
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for category one, memorize 91.175(c)

start by saying there are 'three basic requirements':

1) be continuously in a position to land using normal rates of descent/maneuvering

2) have the required flight visibility (although i wouldn't challenge an actual rvr report below minimums for a specific runway)

3) 'the airport environment in sight' - then explain what that means..know that when you see the approach lights you're authorized to descend another 100' until you see...(memorize the next nine items that constitute the 'airport environment')

meet all three? you may descend below da/mda.
 
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