Ace Combat 5: The Unsung War

It sounds like you are asking about unlocking the Falken.

The Falken is unlocked by destroying the Falken hangars in missions 12A, 12B, 16A, 16B, and 27, and then successfully completing those missions. This can be done on any difficulty level and using any combination of Campaign and Free Mission.

You have to get close to most of the Falken hangars before they will show up on your radar, but this is typically only an issue on Mission 12A. The Falken hangars are labeled "Hangar" on the HUD and have a reddish-orange band on their outer walls. Shortly after they are destroyed there will be some radio chatter about "mechanical parts", and if it is the first time a particular hangar has been destroyed there will be an additional message in the debriefing that says "Acquired part of enemy prototype aircraft."

Once you get all of the Falken hangars destroyed, you will also get the "New aircraft is available" message in the debriefing, and the Falken will be available for purchase.

Note that this is the mechanism for unlocking the Falken as a complete aircraft. There is no way to add or alter parts of an aircraft in Ace Combat 5.