If you find anything "special" under there or if you find this tutorial useful please mention my name (Sakis25) for helping you out.
1. Go to this location:
2. Get closer to the parking door
3. You can jump over the door through that part:
3. Use the Super Jump cheat or jump on a truck. The floor is invisible but you can walk on it.
4. Use the Spawn Buzzard cheat to drop the helicopter in front of you
5. Carefully fly to the right and go under the visible areas
That's it! Now you can fly anywhere you want.
Do not fly too low or else you will be respawn on the roads again.
I will suggest you to create a new quick save file so you will not have to repeat all this again.
What do you think?