I am working with a small airfield at the moment with an entry point to the fairly short runway about 3/4 of the way down its length, aircraft will only line up and take off from this entry point meaning they overun the runway, how can you get them to backtrack to the end and then take off? any help appreciated.

If you have AFCAD use it to change your starting points for the runways involved.
Just click on the view icon  then select show starting points then all you have to do is to click on the circle and drag it to where you want. How ever I am not sure if that will change where aircraft land at if that is a problem as well. If they are landing to deep you might have to use some other method to change that.
Starting points effect where you start from when you select a airport.

thanks for all replies, I am working on Bourn (EGSN) and the holding point & entry point for runway 19 is about halfway along the 633m length - seems i will either have to add a taxiway at the 19 end (not real)or close the midpoint taxiway forcing aircraft to enter at the 01 end, they then happily backtrack the full length for a 19 departure.

Hi
  Ran across This while searching  for taxispeed gauge
Thought It might Help
                 Rich
http://forums.avsim.net/dcboard.php?az=show_topic&forum=126&topic_id=4471&mode=f...

Ok I've got this sorted now thanks to the above link, if you add a 'loop' of taxiway toward the end of the runway ai traffic will enter and backtrack then take the loop to line up at the runway end. Particularly for GA airfields that do not actually have this loop you can keep it fairly discreet ie. as small as possible with narrow taxiway dimensions etc. have tested this at UK airfields Bourn & Fenland and works ok.