Obtain a helicopter by going to the helitours area, or any other means you feel most comfortable with and fly under each of the following bridges:
There are 14 Bridges that are needed for this achievement. When you pass under a required bridge, a message will pop onto your screen letting you know you successfully went under, and how many you have left. If you go under a bridge and do not see the message, go back under it again, because it didn't count.
Helicopter Control Tips:
-Ascend using the until at a reasonable height above the water and a few hundred metres from the bridge.
-Hold forward on the to start the helicopter moving forward, then release the
-Descend using the , you will still be travelling forwards
-Once under the bridge, you should have enough forward momentum to take you under, use to descend and to ascend as you move under the bridge to avoid hitting the water or the underside of the bridge.
Low Bridge tips:
This bridge only took me one attempt, but was hard nevertheless. The trick is to hover the helicopter's "feet" in the water as you move very slowly forward. Because this bridge is so low and there is not much of a run-up, you will need to control your speed using the BEFORE attempting to go under.
**Note** You do not need to do all in one flight, but it can be helpful to make sure you don't forget which you've done and which need to be done.
Here is a video of it being done: http://ve3tro.com/gta/under-the-radar.mp4
**Can be done after completing story mode**