Get the missing parts from missions 12a, 12b, 16a, 16b, and 27a to unlock the Falken. You must destroy the hangar of each mission. For mission 12a, follow the river north, then go east at the fork in the river. You must be close for the hangar to show appear on radar. For mission 12b, after taking out the four radar sites, destroy enough targets and the hangar will show up near the smokestacks. The one in mission 16a is between the field HQ and the airfield. Mission 16b has it west of the river, between the airfield and oil refinery (south of the north-most river). The part on mission 27 is very close to the gun tower, in front of the tunnel.