How to get to the airport in San Fierro? (Grand Theft Auto: San Andreas)

I just quietly passed GTA: San Andreas, and then I got on this mission.
The mission with the helicopter to knock out an RPG while he flies away from us. (SAN Fierro)
The easiest and most real way to pass this mission heat rocket launcher. The nearest is at the airport, which we are not allowed.
Are there any ways to get there without cheats?