Are alternate minimums always higher?

[Martymccasland]

Are alternate published minimums always higher?

'Asking what I think is a simple question: Are alternate approach published minimums always higher than the otherwise standard minimums. i.e. instead of an ILS alternate min being 600-2, will it ever have a published alternate of 500-2 -- or will it always be higher than the 600-2 standard?

It makes sense for the published alt minimums to always be higher, but I can't find any text definitely saying that... Also, in the 10-12 airports I checked, every one had published alt minimums higher than standard.

Just curious if that's the case 100% of the time -- or if there's strange cases where it doesn't hold-up. i.e. Alaska with huge potential distances to the alternate.

 

[flyingron]

Yes, chapter 3 of the terps says to pretty much to compute the highest of all possible minimums and unless it's less than the standard alternate minimums to proceed through the rules for establishing the higher non-standard alternate minimums (or mark them NA).

 

[gismo]

Yes, chapter 3 of the terps says to pretty much to compute the highest of all possible minimums and unless it's less than the standard alternate minimums to proceed through the rules for establishing the higher non-standard alternate minimums (or mark them NA).

It always seem to me that alternate minimum should be a specific percentage or offset from the published mins rather than a fixed number for all approaches. IOW if the required offset for precision approaches was 400 ft, the alternate mins ought to be whatever the published mins plus 400 ft rather than 600 or the published mins whichever is higher. It makes no sense that a non precision approach with published mins of 1000 HAT can be used as an alternate when the forecast is for 1000 ft bases but a different approach with 400 ft published mins requires an 800 ft or higher ceiling to be used as an alternate.

Something else that was lacking in my IR training is the fact that you must consider whether an approach is viable given the wind forecast. Using an alternate that only has one approach and it's to runway 36 when the wind is forecast to be out of the south at 20 Kt isn't a very good idea unless the bases and visibility are well above circling mins.

 

[flyingron]

The derivation of minimums beyond the standard ones is NOT as straight forward as just raising minimums. If you really understand, go download and read the referenced section of the TERPS and then come back and ask your question. At that point we (especially aterpster) can discuss the issues.

 

[John Collins]

The concept of planning for an alternate is that the weather at the alternate will be forecast to be substantially better than the actual approach minimums required. If the approach in question has both precision and non precision, and may be used as an alternate, the precision alternate minimums of 600-2 must allow for a failure of any and all components of the approach and still be able to accomplish the approach. For example, it the approach is an ILS or LOC approach, one must be able to fly either set of minimums. In most cases, if the ILS is 200 DH, and the MDH for the localizer is below 600-2, the standard values will be adopted. If however, the visibility or MDH is over the standard values of 600-2 under any failure condition where the approach may still be performed, then non standard values will be increased to permit the approach to be accomplished in the worst failure condition. Things that can fail and yet the approach can still be used include GS, approach lights, local altimeter setting not available, lower DME step down not available, ...

This is all done to make the alternate as dependable as feasible.