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Flying School

The Flying School is located inside the Abandoned Air Control tower at the Verdant Meadows airfield in the Desert. It is unlocked after you complete the mission "Verdant Meadows" for the Mike Toreno (which also involves buying the place for $80,000) and you will be required to complete it before being allowed to do any more missions.

The Flying School requires 10 small objectives to be completed as best as possible. You will need over 70% on every objective in order to complete the school, however getting a better rating will improve your flying skill and allow you to unlock special vehicles. This guide will try to explain the best techniques to get Gold on all of the objectives, because everyone knows that if you can get gold, you can get bronze easier!

Completing all of the objectives with at least Bronze will unlock the Rustler.
Completing all of the objectives with at least Silver will unlock the Stuntplane.
Completing all of the objectives with at least Gold will unlock the Hunter.

..... http://gta-sanandreas.com/guides/flyingschool/index.php