600-2, 800-2, 0ne hour before, 200' & 1/2, 400' abv mins, 2 aprch ends - - Oh my
ranklein,
In a generalized big picture view the regulations are trying to avoid a situation where you and I arrive after a flight and cannot land because the weather hads deteriorated and we run out of options and fuel at the same time.
To help preclude that unhappy state we have weather minimums that need to be met. We have destination, alternate and sometimes takeoff alternate minimums. [For the sake of this discussion the example under consideration is a passenger carrying Part 121 flight {Part 91 and flight into uncontrolled airspace is ignored.}
The alternate minimums have to be much “better” (greater visibility and sometimes ceilings as well) than your destination minimums. This is because we do not want to get into a position where we are unable to land at either the destination or alternate. So, to keep this from happening the alternate minimums are set higher. The system is designed to give you and me (and the paying passengers) a safety net.
During the preflight planning or the actual enroute phase your alternate minimums must meet and continue to meet the stated regulatory minimums for alternates. If during flight the forecast for the alternate changes and the alternate no longer meets the required alternate minimums, your dispatcher should/must find you a new alternate within the confines of your actual fuel load. If you don’t have a dispatcher then the entire requirement to monitor your weather at your alternate falls upon you. Remember you want to give yourself a margin of safety and somewhere to go.
If you cannot land at your destination because of poor weather then you proceed to your alternate. (You are not obligated to go to your filed alternate) The intention is to be able to get the airplane safely on the ground. Your alternate now becomes your “new” destination. When you get to the alternate airport the minimums published on the approach chart apply for the approach. Remember the intention is to land the airplane not to force some new artificial higher weather requirement on the airport.
Of course if you arrive at your alternate and the actual weather is below published approach minimums (that should not happen with accurate weather forecasting, tracking, in flight monitoring and proper dispatching) and you are limited on fuel; now you have another problem and it is time to use your emergency authority determine the safest course of action and do whatever you need to and get the airplane safely on a runway before fuel exhaustion.
I hope that is helpful.
Respectfully,
Northbeach