PPRuNe Forums - View Single Post - Destination Alternate weather mins
View Single Post
Old 16th Aug 2009, 16:16
  #7 (permalink)  
Northbeach
 
Join Date: Aug 2008
Location: North America
Age: 64
Posts: 364
Likes: 0
Received 0 Likes on 0 Posts
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.

Respectfully,

Northbeach
Northbeach is offline