Airmen from the 91st Security Forces Group use an M18 smoke grenade to signal a UH-1N Iroquois assigned to the 54th Helicopter Squadron during a winter survival field training exercise in Turtle Mountain State Forest, N.D., Feb. 14, 2018. The signal fire’s heat lifts the M18 smoke grenade’s green smoke for better signaling. (U.S. Air Force photo by Airman 1st Class Alyssa M. Akers)