Helicopter Crash Sites will appear randomly on the map. While they can appear anywhere, they are typically found in inland fields and near mountains although occasionally they have been seen in forests.
These sites may spawn some of the rarer items such as the M249 SAW, the FN FAL AN/PVS4(night vision) or the Mk48.
As you would expect, the MI-17 wreck spawns mostly Russian/Eastern weapons and the UH-1Y and UH-60 wrecks spawn mostly US/NATO/Western weapons.
Helicopter crashes are the ONLY place to find Night Vision Goggles (NVG).