Select the A-10 attack plane and take off. Wait until the "Good Luck" message disappears, then keep flying in the same direction and speed. Shoot just one round from the air-to-ground cannon, then hold X. The phrase "Super A-10" will appear to confirm correct code entry.
Pause the game, then on control pad 2 press A B A B select A B and you will have all damage healed, full fuel, and double the amount of missiles WARNING do not do this so your missiles would go over 99, if you do you have 0 missiles and cannot get any more for the mission.