1-2-3 rule

J

jeanguypataterub

Well-Known Member
Quick question about alternates. If the wx at your alternate is good but there is a prob 40 in the forecast of say 1000 ovc can you still use it? Couldn’t find the answer anywhere to this.

Thanks.
PV
 
1000FT overcast doesn't automatically make an alternate illegal. There's not enough info there.

Depends on your alternate minimums and how you derive alternate minimums (either the published mins on the charts or deriving based on 1-nav/2-nav). Best case scenario is your alternate mins are 400ft and 1sm if you have 2 good ILS approaches and allowed to use the 2-navaid rule.
 
Salkadi said:
1000FT overcast doesn't automatically make an alternate illegal. There's not enough info there.

Depends on your alternate minimums and how you derive alternate minimums (either the published mins on the charts or deriving based on 1-nav/2-nav). Best case scenario is your alternate mins are 400ft and 1sm if you have 2 good ILS approaches and allowed to use the 2-navaid rule.
Click to expand...
I’m sorry I meant to say a prob 30 of 100ovc. Would you still be able to use it?
 
I'm not aware of any alternate mins that go that low. You must take into account all conditional statements (tempo, prob30/prob40) in a forecast in determining whether or not it's legal.

For most 121 domestic/flag ops, generally, the lowest you can go is 400ft ceiling (overcast or broken) and 1sm vis. There are some circumstances that allow for 200ft ceiling and 1/2sm vis and maybe lower, but those aren't common.

I'm not familiar enough with other ops (91, 135, etc.) but I wouldn't think they would consider an alternate with a prob30 of 100OVC at ETA to be legal.
 
MidlifeFlyer said:
This.

The rule says an alternate airport is required unless the forecast shows that "the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles" (my emphasis). A forecast which says "they might not be" is not a forecast that they "will be."
Click to expand...
Ok. That's what I was wondering. Let's say you need an alternate. It has 1 non precision approach so you need 800-2 to use it. The forecast is 10 OVC with a prob 30% 3 BKN. Is it still legal to use it. In Canada it would be as the prob 30 of 300 broken is not limiting. I was wondering if it is the same in the US?
 
jeanguypataterub said:
Ok. That's what I was wondering. Let's say you need an alternate. It has 1 non precision approach so you need 800-2 to use it. The forecast is 10 OVC with a prob 30% 3 BKN. Is it still legal to use it. In Canada it would be as the prob 30 of 300 broken is not limiting. I was wondering if it is the same in the US?
Click to expand...
I've seen references to that as the Canadian rule. Never really looked unto whether the US Rule is different or similar.
 
It's written in to our Ops Spec, as to when we need to consider Prob30, Tempo & "Prob30Tempo" as controlling.
 
Salkadi said:
I'm not aware of any alternate mins that go that low. You must take into account all conditional statements (tempo, prob30/prob40) in a forecast in determining whether or not it's legal.

For most 121 domestic/flag ops, generally, the lowest you can go is 400ft ceiling (overcast or broken) and 1sm vis. There are some circumstances that allow for 200ft ceiling and 1/2sm vis and maybe lower, but those aren't common.

I'm not familiar enough with other ops (91, 135, etc.) but I wouldn't think they would consider an alternate with a prob30 of 100OVC at ETA to be legal.
Click to expand...
So it’s an interesting question. I can think of a scenario where that would be legal. At my airline we can derive alternate mins off a CAT III approach on an Airbus to make 200-1/2 alt mins. Now in the US if you had 100-1/2 in the tempo at your eta you could apply 3585 to your first alternate as long as you list a 2nd alternate and aren’t reducing any mins for your second alternate.
 
Luigi said:
So it’s an interesting question. I can think of a scenario where that would be legal. At my airline we can derive alternate mins off a CAT III approach on an Airbus to make 200-1/2 alt mins. Now in the US if you had 100-1/2 in the tempo at your eta you could apply 3585 to your first alternate as long as you list a 2nd alternate and aren’t reducing any mins for your second alternate.
Click to expand...

3585 can only be used for CAT I minimums. If your carrier allows you to derive mins from CAT2/3 approaches, you can't 3585 those mins.
 
Salkadi said:
3585 can only be used for CAT I minimums. If your carrier allows you to derive mins from CAT2/3 approaches, you can't 3585 those mins.
Click to expand...
You’re not correct. I have the FOM open right now and that is not a listed a restriction. Furthermore that was not a restriction at my previous airline either. I’m actually curious where you heard that, is CAT I only 3585 a requirement at your airline?
 
Luigi said:
You’re not correct. I have the FOM open right now and that is not a listed a restriction. Furthermore that was not a restriction at my previous airline either. I’m actually curious where you heard that, is CAT I only 3585 a requirement at your airline?
Click to expand...

I stand corrected.

I've only been familiar with 3585 as a Cat 1 only tool. Of the 3 airlines I've been at, every one has had that restriction, but all also had the restriction where we could not derive alt mins from cat 2/3 approaches.
 
You must log in or register to reply here.
Back
Top