I searched for this but found almost 2800 matches, far too many to wade thruough to find a direct answer.
I would like my aircraft to always start from St. Louis, KSTL, Active Runway, or 12L or R, which allows me to see the arch as I take off over St. Louis.
No offense against Redmond or anything, but, I just don't want to have to change from the Washington airport everytime I fly. I looked in the FS9 directory for a .cfg file, but, I couldn't find one that seemed to be anything.
Start the Sim, free flight, choose aircraft, airport then click on save just below the spinning aircraft.
There will be a window that you can enter information and tick the box save as default flight.
That's one way.
The other would be to save it via the toolbar on whatever runway-gate you wish.