“Under the Radar” achievement in GTA4

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This is the key exerpt:

Find the smokestack in Alderney’s industrial park. If you have maneuvered your helicopter underneath all four of the bridges that cross the inlet above Alderney’s industrial park and the game gives you credit for only three of the four bridges, fly to the bridge located at the opening of the inlet. Open the map and find the smokestack, which is indicated as a gray circle. Step 3 Maneuver the helicopter south of the smokestack. Fly your helicopter south of the smokestack, then fly straight ahead under this section of the bridge. Flying under the section that crosses water will not give you credit. Once you have flown under all 14 bridges, you will obtain the Under the Radar achievement and receive 40 gamer points.