Heatseeker

When you are told you need to land your plane, make sure you look for some blue rings floating around close to where you need to land. Slow your plane down (by using your airbrakes) and fly through the blue rings. This will deploy your landing gear and you will then be able to land your plane successfully.

Successfully complete the Pilot bonus objective for the final mission (Mission 4.5: Final Count Down) to unlock the SR-71 Blackbird.

Successfully complete the Ace bonus objective for the final mission (Mission 4.5: Final Count Down) to unlock the SR-71 Blackbird's fourth weapon pack. This weapon pack includes the game's super weapon, the A-SLAMR. The A-SLAMR does not recharge once fired so use it wisely. Note: The SR-71 Blackbird and "The Big One" weapon pack can both be unlocked at the same time if playing on the Ace difficulty setting. Just complete both of the required bonus objectives to unlock the SR-71 and "The Big One" weapon pack together (instead of separately).

Heatseeker cheat codes enter these on the bonus codes page:

Enter QGWE2F6TRG297 - No Gun Overheat
Enter 43CXQGAVNEHD6 - IC Forces Plane Pack 1
Enter DQH25HT8YTVH3 - Enemy Plane Pack 1