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135 and 121 alternate weather minimums


Well-Known Member
Ok here is a question. I am well versed in the derived alternate method of figuring out weather minimums for alternates. (one nav aid two nav aid)

One nav aid available, add 400' and 1mile
Two independent nav aids you add 200' and 1/2mile to the highest one

My question is how does non-standard alternate minimums affect deriving these alternate minimums? If OpSec CO55 ignores standard alternate minimums in the first place how does it interpret non-standard minimums?

For example the approach chat has an "A" in a triangle so you look up the non-standard alternate minimums. For the ILS it states Cat. A,B,C,D requires a minimum of 800-2 to be used as an alternate.

Does this mean you use that 800-2 or do you still add 400-1 to that figure for your derived alternate?



o- - - - - - -l
In the case of one Nav Aid, and one straight-in precision or non-precision approach to the only available runway you would add 400-1 to the MDA/DH.

Mike Massimini

Well-Known Member
Good answers above, but you actually need to refer to your own company op specs; that's the definitive answer. This case should be covered.

I'm working overseas and don't have access to my old airline's paperwork, else I'd look up that case.

EDIT: Ooops, I lust saw the rest of TwoTwoLeft's post; he nailed it. :)


Well-Known Member
You get to use your made-up alternate mins.
Thought I'd resurrect this thread since I'm studying for an interview and came across some scenarios where "get to" may be the wrong phrase here and should be "have to" instead. It's quite possible that the published non-standard alternate minimums are lower than the CO55 derived minimums.

Suppose there are two runways with separate navaids providing a straight in approach at the proposed alternate. The non-standard alternate minimums as published are 800-1 3/4 for both runways.

Approach A has the following minimums: 5018' DA (608' DH) 1 1/4 mi, Approach B has: 5043' DA (635' DH)/2 mi. Adding 200' and 1/2 to the highest DH and visibility, this gives us the derived minimums of 835' and 2 1/2 mi, i.e. 900' and 2 1/2 for what a forecast would show.

If the forecast ceiling and visibility is 800-2, you would have been fine with the non-standard minimums but now are not if you are following CO55.

As Mike and @TwoTwoLeft mentioned above, it comes down to your particular OpSpecs in the end.