How to get an accurate time of arrival to the en-route alternate airport during flight planning? 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 &quot;ERA&quot; 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?
This is very much like the information mandated for EDTO/ETOPS.
Computerised flight planning systems should have the ability to calculate the ETP, equal time  point (aka CP) based upon ERA/ERAs selected. If 2 ERA are required due to the length of flight, depending on the geographical disposition of the ERA you'd get an ETP for each pair, ie ETP1 for DEP Stn/ERA1, then ETP2 for ERA1/ERA2, then ETP3 for ERA2/DEST Stn.
The earliest ETA for ERA1 would be a diversion from ETP1 and the latest would be from ETP2 and you would need to subtract/add the 1 hr buffer.
And so on . . .