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!

Cheat Codes
To bring up the cheat screen press and hold: ctrl, alt, x. Now, type in these codes:

Gort = shows alien dialog of when you beat the game.
Shields up = makes you invincible.
Radioactivity = makes a reactor blow up.
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.
Superpowermultiply = when you push shift it makes you go faster.
Been there, done that = career level skip.
Gas does grow on trees = full tank of gas.
Out for a sunday drive = gives you the view out of a car.
I'm the CEO of McDonnell Douglas = Get any chopper you want