All Aircrafts 
Use the following steps to unlock all aircraft, including special colors. First, acquire an aircraft by destroying it in battle. You can acquire three different paint schemes for each aircraft. Two of the paint schemes can be acquired by destroying that color aircraft in battle. To acquire the special brightly colored third aircraft, you must find and kill the pilot flying it. On each level of the game there is a single hidden aircraft (eighteen total hidden aircraft). Each one of these aircraft has a specific pilot. When you see the aircraft in front of you and lock on, it will display the pilot’s name next to the aircraft name and lock on square. For example, "S-37A SMIRNOVA". The hidden aircraft will not show up on radar until you get nearby. On radar, the aircraft will be marked with a red triangle instead of yellow. Each hidden aircraft is always in the same place on their specific level. This is always in an empty spot on the map away from the main combat zone. The aircraft can usually be found directly behind the main combat zone. Last but not least are the three remaining aircraft. To acquire the Yellow Squadron fighter (SU-37) of your nemesis Yellow-13, you must kill all of the pilots with names flying the special colored aircraft (eighteen total). The method used to unlock the F-4E with tiger stripes is unknown at this time. To unlock the X-02 Stealth, get an "S" rank on all 18 missions on normal mode.
Cheat Submitted By: Ace McCloud - jervanb@sympatico.ca

Two Alternate Color Schemes For The X-02 Stealth
Get “S” rank on all 18 missions in Hard mode, which will unlock Expert mode, then get “S” rank on all 18 missions in Expert mode. Completing Expert mode will unlock Ace mode.
Cheat Submitted By: Ace McCloud - jervanb@sympatico.ca

Bonus Options
Successfully complete all 18 missions, then save the game. Load the saved game to unlock the following new options at the main menu: "Special Continue", "Free Mission", "Trial Mission", "Scene Viewer", "Music Player". The "Special Continue" option allows the entire game to be replayed with new fighters and weapons, plus the money earned in the first game. The "Free Mission" option allows any mission to be played. The "Trial Mission" option allows the game to be played in either score attack or time attack modes. The "Scene Viewer" allows FMV sequences to be played. The "Music Player" option allows any of the 36 songs in the game to be played.
Cheat Submitted By: Ace McCloud - jervanb@sympatico.ca

SP. Continue
Start over from mission one with all Aircraft, Weaponry and $$$ obtained previously.

Free mission mode
Play any mission from the game.

Trial mission mode
A new mode which let you play Score Attack or Time Attack mode.

Also, beat the game once and go to option screen, you'll now have new goodies like

Scene Viewer> let you watch any scene from the game.
Music Player> listen to all 36 tracks