you can set it so planes start out at 10k
after placing an airfield somewhere select "entrpt" from the object list, place it where you want to start off and when a pop up window comes up asking for object info, look for object type, change it to bmb entry. then look in the top right corner of the pop up window where it says X, y and z. change Y to 10000 and click ok. (also make sure the spawn point is attached to the field you want to launch from)
voila, you have a 10k spawn point