is there a cheat to unlock locked vehicles & planes

If you've just started the game, you don't need to cheat to get a plane. Just drive a car up to the (Los Santos) airport gates, jump onto the car and over the fence. There are two planes, a single engine light aircraft and an executive jet. These are both unlocked. Also at the west end of the runway, between two ramps, is an M4 assault rifle. Worth having. I say again, there is no cheat involved. You can do this from the very start of the game. If you fly around enough, your flying skill could go to the level where you get a pilot's licence, which would unlock all aircraft.