If you're in a helicopter, and a SAM site locks on to your helicopter, the heat-seeking missiles will destroy your helicopter. The missiles fired by the SAM site have a different exhaust from your own missiles and the sound is different, with a military personnel saying over the radio ″Missile away" that lets you know there's one coming for you.
A common strategy is to stay out of range and attempt to hit it with a missile from aerial weapons such as The "U-10 Chippewa" helicopter.
Another method is to approach it by gliding the helicopter right above the ground, though you must stay below the height of the SAM site. Within roughly 150 to 250 meters of the SAM site it will stop firing. Slowly approach it without any heat, not even yellow. As long as you stay below the top of the SAM site's height, you can destroy it with any weapon you have.