Originally Posted by
titaniumwings
Just to clarify. The decision that I am talking about is not whether to land, rather it is whether to launch the flight on time or to carry extra fuel to wait over the tempo period of which by judgement the commander reckon the possibility of landing is possibly low.
So to add to the question:
Suppose TAF 3000m you don't carry fuel. Would you carry fuel for TAF rain 2000 or 1000m?
In Australia those "decisions" are covered by the regulations and company ops manuals. If the weather is worse than the alternate minima for the destination then you need extra fuel, either an alternate or holding depending on the forecast. The alternate minima include a significant buffer over the approach minima.
Judgement comes in to whether you want additional fuel over the legal minimum in order to give you a couple of chances before diverting. I don't have any formula to base this decision on, just a gut feel as to whether the weather seems marginal or not. If it's only just below the alternate minima and forecast to either stay the same or improve then I might carry legal minimum, if it's forecast to deteriorate or fluctuate between the alternate minima and approach minima I might take extra approach fuel. Sometimes, even though extra fuel is not required, I might take as much fuel as I can without affecting the payload so that I have as many options available till as late as possible.