ir1
Using your example of Essendon, I think you're confusing Alt Minima with MDA/DA.
The Alternate minima is
nearly always 500' ceiling and 2 km visibilty above the circling minima.
You state,
alternate minima is 910'
910' is the CAT A circling minima based on actual QNH. It is 628' AAL but it is not the alternate minima.
The CAT A
alternate minima at EN is found on the airport chart (Jepp 11-1) and is 1128' on actual QNH, which equates to 500' added to the circling minima of 628' (AAL). Additionally one would have to consult forecasts to determine which approach is likely, because the ALT Minima can vary with RWY and aircraft type/equipment.
So in summary, to determine whether one needs to carry an alternate, check the "For Filing as Alternate" boxes. To get an idea of the likelyhood of having to divert, compare forecast cloud base against instrument approach MDA/DA. The greater the difference, in general terms, the less likely you will have to divert.
ABOVE ALL REMEMBER weather forecasting is an inexact science.
- Don't be tempted to not carry an alternate because the wx is just below the alt minima and you think it'll be ok.
- Don't be tempted to not carry an alternate because every other time you did you didn't need it.
- If your study of the wx coupled with knowledge of where you're flying tell you an alternate would be prudent, carry it if you can, or at least have a second option.
As an example, some years ago, CS-DN direct. No requirements on DN yet it was the start of the wet season. Full fuel carried because we could. Half way across the Gulf (of Carpentaria) radio traffic indicated aircraft holding at DN or diverting because of unforecast T/S. Thankfully we weren't affected, but we easily could have been. The storm was so violent it blew shipping containers off the wharf into Darwin Harbour.