A lot of it is from the default spawn points in the airfields. Some of them obviously weren't tested in development, as you can spawn on a hill, or inside a hangar too small for you aircraft, or slightly offset from the actual ground level. With time and effort, you can fix these in the maps, but the sheer number of airfields + the number of spawn points on each airfield makes this a very daunting task. (1 airfield in the Biggin Hill area, can't remember if it is actually Biggin Hill has 128+ spawn points to have to check).