Go to the middle of the map were the plane has crashed.... Find the Wing that is on a tilt... Go to the left of it and get a distance back, run and jump on the left part...

Run to the top of the wing on the right side and jump of the wing and press a when you hit these drains... The drains are at the end of the wing and above the cave intrance...

I recommend using a silenced gun and mainly do this spot in hardcore.. Best if you use a sniper rifle.