Alternate Weather Minima Requirements

[3dflow]

I'm not quite understanding this. If there are 2 or more useable precision approaches each providing straight in minima to separate runways the weather requirements are 400 - 1 or 200 - 1/2 above height above touchdown (hat)

First HAT is the minimum descent altittude above the highest runway elevation in the touchdown zone right?
400 - 1 does this mean the ceiling has to be 400 ft above HAT with a vis of 1 nm? and if so where does the 200 - 1/2 come in?

Thanks

[Sidebar]

I'm sure someone will correct this if I've got it wrong.

If you're dealing with 2 precision approaches, then DH is applicable rather than MDA. You need wx of 400 ft ceiling and one mile viz, or, if it would be more restrictive, 200' above the DH HAT or 1/2 mile above the designated viz for the approach.

Example 1: DH HAT 200' and viz 1/2 mile - 400 & 1 would apply in this case.

Example 2: DH HAT 250' and viz 3/4 mile - add 200 & 1/2 and you get limits of 450' (rounded up to 500') & 1-1/4 miles

[200hr Wonder]

Hope this helps:

Most all approaches are designed around a "standard approach". IE an ILS has DH of 200', however there certain obstacles and conditions that make this not always practical. So some places the DH is 400' or 500'


Now what does that have to do with Alternate minimums? Well 400' and 1 is the same as 200' 1/2 on a standard runway. You end up with the same answer. Now if you have a place where the DH is 400' and 1 you would then use 600' 1 1/2 being the most restrictive.

All Minima are based on DH for precision and MDA for non precision. Hope that sorts out why the 2 numbers.

Another point not to forget is that the requirement is 2 usable precision approaches. If you have a wind of 250/40 knots and your runways with the ILS are 25 and 07, can you still take credit for both runways? Nope!

[Tim]

the sliding scale doesn't apply to 2 precision appchs though

[glorifiedtaxidriver]

Go to the CAP and look at the approach you're going to do. You should see something like this on the bottom half of the page.

ILS 432 (200) 1/2 RVR 26

This is your indicated altitude and your (height above ground) of your DH followed by advisory visibility. The first two numbers tell you how low you can go - period, while the advisory visibility is a number, which, if you have it should provide enough vis for a successful approach. Approach bans are predicated on this number.

So, you have 2 ILSs at your alternate which means you can use minimums of either 400' and 1sm or 200' and 1/2sm above HAT and vis. In the case above, you add 200' and 1/2sm to the DH and advisory visibility. When you do that you get 400' (200 +200) and 1 sm (1/2+1/2). This is standard. If you were to see something like this:

ILS 488 (248) 1 RVR 50

You would do the same. Compare the standard 400' and 1sm to the numbers you get when you add 200' and 1/2sm to your DH and advisory vis. In this case you get 448' (round to 500) and 11/2sm. The higher of the two is your minimum, so its 500' and 11/2sm.

Note that your DH and advisory vis are individual and one can be standard while the other isn't, like this:

ILS 432 (200) 1 RVR 50

In this case you add your 200 and 1/2 and come up with 400' and 11/2sm, so this is your minimum requirement.

Either way, compare the standard 400 and 1 with the number you get when you add the 200 and 1/2 and always take the highest number.

Hope that made sense.

[BTD]

Why does the question get answered so many times?

[glorifiedtaxidriver]

Because different people understand things better explained one way or another and everyone has a different way of explaining something. Also, why do you care.

[TopperHarley]

Also, why do you care.

Because he's bored and on a layover right now

[BTD]

Also, why do you care.

Because he's bored and on a layover right now

[glorifiedtaxidriver]

If you're that bored you could start up a thread about training bonds and see how long it takes for Doc to bite.

[3dflow]

Thanks glorifiedtaxidriver. It makes sense now

[loopa]

Also, why do you care.

Because he's bored and on a layover right now

look up FCOM 6 lol

[old_man]

Here is some more stuff to wrap your brain around.

These minimas are based on TAFs. So you have to remember how to apply BECMG, TEMPO, and PROB.

If you have a BECMG and it is forecasted to improve you can only only apply the improved weather at the end of the BECMG period. If it is forecasted to deteriorate then you apply the weather at the beginning of the period.

TEMPO shall not be below your alternate minima.

PROB shall not be below the landing minima for that airport.

What about if the airport doesn't have a TAF and you are using a GFA. In this case the weather must be 1000ft about the lowest usuable HAA/HAT, vis must be at last 3SM, and there must be no CB.

Rounding up and down. 20' and below can be rounded down but 21' or more is rounded up (520' = 500', 521' = 600')

This one I am not certain, maybe someone else can clear it up. If you have an ILS for rwy 25 and and ILS for rwy 07, is that considered 2 usable precisions approaches to two seperate runways or just 1 precision approach because it shares the same piece of tarmac. I am under the impression that it only counts for 1.

Lastly, there are also more restrictions if you are using GNSS approaches.

Isn't IFR fun?
-matt

[Spokes]

Counts for 1.

[Sidebar]

Isn't IFR fun?

Ouch. Makes my brain hurt!

[planett]

old_man

Right you are! I've been at two companies that took that to Ottawa, and the clarification was that 07/25 is one runway, not two. Your mileage may vary.

[200hr Wonder]

old_man as I posted way at the top, they have to be usable, a 40knot wind is not usable. If the runway is say 5000' I will say upto 10knot wind both are usable as I will operationally take that tail wind. Of course you would have to justify this to TC and good luck with that.

[TopperHarley]

What about if the airport doesn't have a TAF and you are using a GFA. In this case the weather must be 1000ft about the lowest usuable HAA/HAT, vis must be at last 3SM, and there must be no CB

The way it's worded in the AIM is that it says "no cloud lower than..." not "no ceiling lower than..." So I was always under the impression that even if it was a SCT or FEW layer, it was still restrictive. RAC 3.14.1.