Even the scheduled Transatlantic airlines vary routes from day to day and what goes out over Dinard and Ushant today, might be up over York and Northern Ireland tomorrow.
North Atlantic Tracks.......the short(

) version
Not so important now as they used to be with the improvement in navigation systems. If we go back to when I was an Air Traffic Control Assistant at Scottish and Oceanic ATCC (1979-1982, long time ago, but the principals remain the same), it all started with a blank 250mb (approx FL350) wind chart for the North Atlantic. Calculated and drawn on to this, was the minimum time track from London to New York, this would vary from day to day dependent on the upper winds. Once this was set, the major operators (BA, LH, AF, PA, TW etc) would be overlayed and the whole thing would the be "straightened out" to cross each 10' of longitude at nearest 1' of latitude giving for example, 55N010W 56N020W 57N030W 55N040W 54N050W. This would become the middle track, say Track C, and 2 tracks at 1' of latitude seperation (60nm) to the north and 2 to the south.
In the example above, the entry point for Track C is at 55N010W so it would be...
Entry Points
Track A,,,,,57N010W etc
Track B.....56N010W etc
Track C.....55N010W etc
Track D.....54N015W etc Shanwick FIR began at 015W to the west of Ireland as opposed to 010W to the west of Scotland.
Track E.....53N015W etc
At this time, the primary navigation system for these routes was INS. The average crossing error was 3nm. I did once see a SAS 747 exit the Atlantic 30nm off track, supposed to be at 58N010W and was at 5830N010W. These days, there is a lot more random tracking because of the increased accuracy of navigation systems which allow routes more to the operators requirements
Tracks were drawn up twice a day, from a UK point of view, the day tracks which was a westbound flow and the night tracks which was predominately eastbound.
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