In your underlying data, you'll have a start lat/long and an end lat/long. For displaying the middle of the route, you'll need the middle points. You can achieve this in ESRI by using ET Wizards polyline to point and choose centers.
But first, you'll need xy to line (management) which gives you great circle arcs (To make the polylines from your start x,y and end x,y).
If you don't have ESRI, or access to it, then you'll need to work out the centres, perhaps some calculated field might do the trick.
Thanks for your response! I dont have ESRI at the moment, but as I'm using straight lines as representation of the flight routes, establishing the mid-point for longitude and lattitude was fairly simple. I approached it via pre-processing however, not in Tableau, so I'm now loading the coordinates as two extra columns in my data extract.
I'm still not completely clear on how to apply these coordinates to position the labels however. If you have any ideas on that to help me a little into the right direction that would be very much appreciated
[CF - midpoint]
((x1+x2)/2, (y1+y2)/2) will get you there. kind of. It's not hugely accurate, but as you are using straight lines...