Unlock more Planes
Complete the game and then save. Using that saved game, continue a new game. Now complete the Blue Wing Knights mission and then choose another mission. Press Square at the aircraft selection screen to choose from the Vic-Viper and other bonus planes. The Vic Viper has a long range machine gun that can take out another plane with one hit. Other bonus planes like the TwinBee, MX5000, Falsion, Blue Thunder, Manbou-J, Flint-Rock, and the Aura-Wing will also be unlocked if you did well enough in your cleared saved game. Complete the game on the same file repeatedly to unlock more planes.
Play as Ellen McNichol
Playing as Ken, shoot Albert down in every mission. Done correctly, Ellen will be unlocked for the space missions. Ellen has the Bird Of Prey plane available.
Complete the game, save and then continue a new game with that save game. You will begin with all airplanes (including bonus planes), medals, money, stats, FMV sequences, and replay levels that were previously unlocked.
Get Easy Money
Deliver the final hit to your enemies using your machine guns and you will get double money (or more).
Suicide in the Vic Viper
Press pause then press Up, Up, Down, Down, Left, Right, Left, Right, X, Circle. Un-pause and you will explode.