Once you have finished a level in the carrer mode, a message from HQ saying 
that you have completed the mission and should return to the Hangar now. At 
the end of the message it also tells you that you have recieved $500 for your 
troubles. At this point you should save the game, WITHOUT returning to the 
hangar. After you save the game you should leave the city, then load the 
game again (whenever you feel like it) and wait a few moments, then another 
message that you have completed the level will appear and give you another
$500! You can do this as many times as you wish!

To bring up the cheat screen press and hold: ctrl, alt, x
To get any chopper you want type in in the cheat screen:
'I'm the CEO of McDonnell Douglas'
Then go to where you buy the choppers and press 2-9.
And check out these codes...

* Gas does grow on trees - full tank of gas
* Shields up - makes you invincible
* Out for a sunday drive - gives you the view out of a car
* Radioactivity - makes a reactor blow up
* Superpowermultiply - when you push shift it makes you go faster.
* Gort - shows alien dialog of when you beat the game
* Been there, done that - career level skip
* Warp me to: xx - warp to career level. (xx stands for #level)
* The map, please - shows map outside of your chopper
* PAMCAREYGOLDMAN - shows picture of directors wife
* I love my copter - warps you to copter wherever you are

Make the man walk on his hands:
To do this enter the cheat window(ctrl-alt-x) type in superpowermultiply, 
find or cause a medevac, land and get out of the copter and pick up the victim. 
Then push shift and hold. Notice that you turn into a dog. Release the shift 
key. You should then see the man walking on his hands.