AT-400
Find the large single hanger at the end of the main runway at the Las Venturas airport. Approach it and it will open. The AT-400 jumbo jet will be inside at certain times. It can climb to the highest altitude possible in the game. Go to the northeastern part of the Las Venturas airport, where a huge hanger with a green door is located. When you walk up to it, the door will open downward to reveal an AT-400 airliner. When you see it, there will be a platform to the left of the plane that you can go on. When you get onto the platform, go to the plane and press Y to enter it and fly around.