To obtain this achievement you need to fly under all 14 main bridges in liberty city. So you must go through all of liberty city bridges and when go under the right bridge on your top left corner there will be a text saying "Only 13 left to fly under for achievement", and for a second, if you open the map you can see the bridges still left to do.
You can also call 359-555-0100 to spawn a helicopter at your location. Just be careful to spawn it in an open-ish space like a large intersection! (this is a cheat)
If you flew under all 14 bridges and the achievement didn't pop out then it's possible you didn't fly right under the first bridge (from the sea) in Alderney’s industrial park. It counts if you flew under it right from smokestack.