On the summer offensive mission at the end when you have 2 destroy the dam, on the other side of the lake, you fly strait into the dam and you go right through it(yes,EVEN in the heap!). Works best in instant action with the dove.
On the first level or any level of the game, find a bridge. Once you spot the bridge fly toward it. If the bridge is high enough off the ground or water, you should be able to fly under it. Once you fly under the bridge, it will tell you what you have earned.
Once you are in the air on mission mode complete a few objectives once you have done so a message will appear on the screen that says "BONUS MISSION", or something like that. Complete the bonus mission to receive an upgrade of some sort, it's usually an engine upgrade, or weapon upgrade.
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