After crossing the bridge, the helis will come to give your position and the mortar rockets will start to hit your car. To avoid this, when near the bridge, stop and go very slow so you can get the turret gun in front of you out. Once done, you will see the chopper, but no rockets. You can blow it up and move undamaged. Make sure to have a TRT (turret) machine gun full repaired to make killing the choppers easier. Recommended equipment are the 50mm machine gun turret, or a 7.62 mm machine gun turret and nitrous oxide (use wisely).