Following the police vehicle repair script, I decided to make a flying vehicle repair script. Just like law enforcement vehicles, helicopter planes can be repaired at specific locations. There are 3 places. As you probably already guessed, these are 3 airports. In Los Santos, San Fierro and Las Venutras.
When you are in a flying vehicle, 3 airplane icons appear on the map (although it looks more like a cross) Go to one of them in order to fix it. Unlike police vehicles, which are paid for by the department, repairing air vehicles will cost you 25000$