From 1986, the CIA began supplying the Afghan rebels with newer Stinger shoulder-launched, heat-seeking SAMs. These were a marked improvement over earlier weapons, but they shot down 43 aircraft of all types. While their actual military impact was not significant, their real value was as a deterrent against air power and for propaganda to groups opposed to the Soviet occupation. The Stinger missiles locked on to infrared emissions from the aircraft, particularly engine exhaust, and was resistant to interference from decoy flares. Countermeasure flares and missile warning systems would be installed in all Soviet Mil Mi-2, Mi-8, and Mi-24 helicopters, giving pilots a chance to evade missiles fired at them. Heat dissipation devices were also fitted to exhausts to decrease the Mi-24's heat signature. Tactical and doctrinal changes were introduced to make it harder for the enemy to deploy these weapons effectively. These reduced the Stinger threat, but did not eliminate it.