The Lead Computing Gun Site is enabled. This will display where you need to aim your gun site in order to hit the selected target plane. To select an enemy target plane, simply look at the target plane and press the tab key. A box will appear around the selected plane, along with the green + to show the correct lead. If there are multiple targets in view, and the wrong target was selected initially, simply continue to press the tab key to cycle through all the targets within view. To select a friendly (green icon) plane as the target, first you must enable "friendly lock" by pressing Ctl-tab, then follow the same steps as for selecting an enemy target.