To get the second hidden pilot (Spanky) you must find and complete all the bonus stages in the game.
Hidden pilot code #1
On the main menu screen press C left, C down, C right, C up, C left, C right, C down. This will allow you to fly an F15J.
Change the four basic aircrafts colors
How to Enter: When selecting an aircraft in Main Game, Boss Attack or Practice modes, use Left/Right on the Control Stick to select an aircraft. Tap the R Button to toggle between the aircraft's color schemes. Press the A Button or Start to confirm aircraft selection.
On the screen after the Video Systems/Copyright information, while planes are flying, but before the title appears, enter the following code: Up on the Control Pad, C-Down, Left on the Control Pad, C-Right, Down on the Control Pad, C-Up, Right on the Control Pad, C-Left, L Button, R Button, Z Button. If the code has been entered correctly, you will hear a chime.
Fly enemy planes in deathmatch mode
A new enemy plane will be available in deathmatch mode for each level completed during game play. A total of six enemy planes may be enabled.
Fire the special weapon followed by a missile and then chaff while flying the F-14. Five Tomahawks and four heat-seekers will then be fired.
Complete the following tasks to unlock each bonus mission:
Air Docking: Accumulate a very high score.
Shuttle Defense: Quickly complete all the levels before the "Desert" level.
Goliath Defense: Successfully complete the first two bonus levels.
Space: Successfully complete all regular missions and "Air Docking", "Shuttle Defense", and "Goliath Defense" before reaching the "Ice Cave" level. Successfully complete the "Ice Cave" level to unlock the "Space" level.