Adrian, Do you mean the vertical lines that run from the aircraft to the ground - if so, then always show as far as I'm aware.
If you mean the aircraft trails, you can have these just on the aircraft you have 'selected' or they can stay on all the aircraft you have selected. See the options in the 3D View Settings window.
- Keep Previous Trails Visible after New Selection
- Default Number of Aircraft Trails
- Clear Selection Trails Now