Log on with the name Viper to enable cheat mode. 
In addition to being able to select any mission, 
enter one of the following codes during game play 

Result                            Code 
--------------------------------------
Invincibility                   - [Alt] + I  
Full ammo                       - [Alt] + L  
No damage                       - [Alt] + R  
View all targets on radar       - [Alt] + A  
Crash Viper                     - [Alt] + X  
Toggle real/arcade flight model - [Alt] + Q  
No smoke trails                 - [Alt] + K  
Display framerate               - [Alt] + J