Quickly pull out a sniper rifle after the level loads, then put yourself in first person mode. A circle with a cross through it will appear. After that, press FOCUS and wait for the plane to come into your sights. Wait for the front wheel to come into the circle in the middle and shoot. This must be done several times to shoot the wheel. When you finally get it, you will see the bullet hitting the tire and popping it. You will then have to shoot the helicopter and get out of the tower.