Before you begin with this one keep in mind You'll be changing a file so make a backup copy before You begin. Now, use Notepad to edit GameSettings.script file in InstallDir\ folder of the game folder.
set bool ConsoleEnabled = true;
Set this, and then, then in the game you'll be able to bring up the console and enter following cheats.
LoadFullAmmo - Full Ammo
SetUnlimitedAmmo - Unlimited Ammo
SetImmortalMode - God Mode
Type them in the console if the above ones don't work.
unlimited_ammo = true - Unlimited Ammo
enabled_missions = true - Unlock All Missions
fullammo - Full Ammo
immortal = true - Immortality
god_mode = true - God Mode
You can also change everything by editing Helicopter.script file in InstallDir\Common\ folder of the game folder.