Press T for the message prompt and type:

Code                        Result 
you got what i need         Unlimited Ammo 
food goes here              Reload Plane 
big gulp                    Refuel Plane 
youre here forever          Invincible 
damn that corner            No Crashes 
chiliburger                 Repair Plane 
spindive                    Cannot be Hit 
upside down                 Auto Level Fly Upside Down 
paperairplane               Paper Airplane 
water boy                   Allows Plane to Skim on Water