Trying to look for the exact rule (if any- FAA, JAR) regarding the destination alternate minimums once the flight has departed.Not the 1 Navaid/ 2 Navaid rule which applies for dispatching, but rather what happens when enroute the weather goes to actual landing minimums in the Alt airport. Can still shoot an approach but below the alt. mins that existed during planning.Can I press on to my original destination leaving my alternate as is, or should I choose a different alternate? Not considering ETOPS enroute alternates.
Enroute to our destination, it's well above mins, another hour or two for arrival.The alternate weather gets to minimums.Can we still use it as an alternate or have to reissue a new one? It's a matter of planning an alternate vs. using it as one while airborne (actual vs. planning).
FAA. Whilst airbourne, the originally planned alternate can still be used, so long as it remains above your lowest landing minima, according to your ops specs...which vary by carrier. However, it would be good practise to plan for another alternate, but you are not required to do so...whilst airbourne.
Alternate minimums are planning minimums, used to determine fuel load for dispatch. Once en route you can plan to land at destination, filed alternate or any other suitable field, as long as the approach you intend to fly remains at or above published minimums. If the Wx at your filed alternate deteriorates below the planning minimums while en route there is no regulatory requirement to chose another alternate, though obviously it would make good sense to do so.
There are two different parts to your question. One has to do with the paperwork requirements between the PIC and the dispatcher assigned to the flight prior to commencing the flight. This part of the question ends when the aircraft gets airborne from the departure airport. The required weather minimums at the destination alternate are spelled out in the FARs and may be amended by your companies Operations Specifications. These are preflight dispatching requirements only. They will be affected by runway options, navaid availability, possibly MELs, aircraft and crew limitations and probably several other things I have not mentioned. The main point is that they apply before the aircraft takes off.
Once you take off now you are in a completely different realm. As long as you have approach minimums you can fly to your alternate and fly the published instrument approach. If the weather goes below approach minimums you can still declare an emergency and fly the approach down to 0-0 and land if you need to. As others have mentioned you should be paying attention to your destination and alternate weather trends and if they deteriorate prepare a different plan long before you get to the point of declaring an emergency and landing at 0-0.
Once airborne your published approach minimums amended by aircraft/flight crew specific limitations apply at both your destination and alternate(s). There are no separate “alternate weather requirements” you need to meet once you are in the air.
For example: two flights from the same company are departing airport A for airport B within 3 hours of each other, the planned alternate is listed as airport C. The required alternate weather minimums at airport C are 2 miles visibility and 600’ ceiling (for example)(has a 1/2 mile 200' ILS). At the time of departure for the first flight the forecasted weather at airport C is 3 miles visibility and 1,000’. The first flight is dispatched from airport A to airport B listing airport C as the alternate.
An hour and half later a new forecast comes out for airport C calling for ¾ mile visibility 400’ ceiling. The first flight is airborne the change has no effect. The first flight can plan on using airport C as its alternate as it was legal when they departed. The second flight from airport A to airport B (still in the flight planning phase) will have to come up with a different alternate prior to departure as airport C no longer meets the preflight alternate weather minimums requirement.
In EU OPS you can dispatch with no alternate, provided certain conditions prevail, such as VMC at destination, less than 6 hour flight, two runways with independent approach procedures... Most MOs make mandatory to name an alternate anyway, but some airlines make use of that rule and take off with no alternate fuel reserve. Normally you will name one alternate, except if the destination airport is below minima or you don't have destination WX forecast for ETA +-1 hour. In these cases you have to name two alternates, and the alternate fuel reserve is the greater of both.
Standard alternate minimums requirement is based on the forecast conditions +-1 hour of the ETA at the airport. Visibility at least 2 miles and ceiling 600' (For precision approach). Visibility at least 2 miles and 800' ceiling (For non-precision approach). However, every airport may differ from the standard requirements and you have to make sure before departure