Para bellum is spot on. The troops in the field nearly always use smoke for the incoming slicks and other reasons. A couple of reason: Where they are located for pickup and the spot where the LZ was chosen. The smoke gives the wind direction for landing. Lastly, Smoke was or is used as last minute I.D. to prevent ambush. For example, The radioman on the ground would say smokes out and the incoming slick would reply with what color of smoke.