Under the Bridge is one of two Aerial Challenges in the game, which requires the player to fly under 50 bridges location all throughout the game. Only 25 of the 50 bridges are required for 100% completion in the game, although all 50 are needed to contribute to the "Close Shave" achievement/trophy (along with the Knife Flights).
Note that you do not need to use a plane to do these, as a helicopter will work just fine. Maxing out the Flying stat will help in controlling the helicopter, which will definitely help in getting under some the tougher bridges.
