I was thinking about accurate criteria of selecting en-route alternate (ERA) weather from ground (planning stage) especially for ERA 3% contingency flight plan.
As we all know we should care about one hour before to one hour after of arrival time into destination hence the main question here how we should know what is the estimate time of arrival into destination i.e. 1400Z.
Personally what I am doing as a airline pilot is crosschecking the nearest waypoint to that "ERA" thence I can tell inaccurate estimate.
Is there any idea how we can get accurate estimate of en-route destination during planning stage (on ground) that can give us exact ETA of destination?