With a fast car and with your special bar filled, go to the military base as Franklin and drive at the front gate at full speed. Activate his driving ability just before you break through the gate arm. Make your first right and then a quick left and drive up the runway to the jet with his special on as long as possible. Usually you will reach the jet before the alarm even goes off. This works 9/10 times once you memorize the location, and if you're not unlucky enough for a tank to happen to be right next to the jet. Additionally, if you land the plane in the hangar by Trevor's house, (not the one you buy), try to pull all the way into the back of the hangar and quick save. Now load the game again and you should be inside your jet plane AND a second perfectly undamaged one will be waiting for you at the entrance of the hangar as well.