Stand by a computer terminal and pull down and press fire. Now point the cursor in all four corners of the options screen. The cheat mode will now be active. This enables these keys:
'F'-----------------------Extra fuel
'H'----------Refills your life meter
'A'-----------------Extra ammunition
'RETURN'--Take off and land anywhere
The code to gain access to level three is: "2314". Use the joystick to enter the code at the end of level two.