Interpretation of Alternate Minima
 

Was flying into Dusseldorf DUS/EDDL the other day using Dortmund DTM/EDLW as the alternate. All navaids at DUS were OK so the ILS Cat l min applied for planning, and it was OK. However at DTM the Glideslope was out leaving the best approach navaid as the LLZ/DME approach. As it was the alternate, the 'step down' approach was the VOR/DME approach and the weather was well above that minima too. On closer inspection the Minima for the VOR/DME was the same as the LLZ/DME so in effect, no stepdown. As the principle of the 'stepdown' usually raises the minima making the best letdown become visual above the minima and in the event of a failure of that best letdown aid the second best letdown should get you visual AT the minima, or, if the weather does deteriorate you have a margin before it reaches the lowest minima assuming the best aid remains serviceable, what happens when both minima are, as in this example, the same values. Seems to me that you no longer have a safety margin on that airport considering that you'll only be using it as a last resort anyway, leaving you in the unenviable worst case scenario position of possibly having to bust the minima.

Any thoughts ?

Or nominate another alternate that does give you the 'stepdown' margin of protection and, if required, take a bit more fuel?
If extra fuel is not possible for some reason then you may wish to consider an en-route alternate being a point at which you can re evaluate the actual weather at both destination and the preferred alternate and make a decision based on that.

Problem is that you didn't apply the stepdown.

Cat III to Cat I ILS to non-precision to visual. You simply went from one non-precision approach to another. You should have planned for visual weather minima.

If you need an alternate, you MUST have a usable one at the planning/dispatch stage. You can always revise that later when enroute in the light of more up-to-date weather reports, ILS's becoming serviceable, etc.

You didn't have any protection from weather at DTM becoming worse. There was some redundancy in the equipment, but that's only one reason for the way the rules are formulated. You missed the other.

Thanks C.S. the precise answer. By the way we did have Koln-Bonn as well, so we were covered, however it's not surprising to find differences between the Flt Planning dept's produced flight plan who dont always read the notams and our choice of alternate (Commercial alternate V's Legal requiremement) .

Are youmenat to refile if you use a different alt to the one ops has filed?

MJ

Jock, no. You don't have to file a new plan with ATC if you decide to choose another alternate while enroute. Once you've taken off, your alternate is whatever you say it is - within, of course, the bounds of legality.

Aye but before that if your all sitting round the fax waiting for the next batch to come through. Then suddenly some pax decide that after a 1 hr wait the meeting is a waste of time so cancel.

Hey presto you can load enough fuel to reach half way up the country again so can now legally launch. Can you just load and go, or should you really refile.

This is more regional TP thing I know.

MJ

Hi ooizcalling,

I do not know if you operate under JAR-OPS1, but if you do, you go "one step down" on a non-precision-approach by adding:

- 200 ft. to MDH
-1000 m. to RVR/VIS required

Strangely enough, if you plan on a circling approach on your destination alternate, your alternate minima are... circling minima. No additions :hmm:

Brgds from
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