Chaff and flares are passive, defensive countermeasures deployed by military aircraft. Their purpose is to confuse and divert radar-guided or infrared-guided anti-aircraft missiles fired by other aircraft or from ground installations. Chaff and flares are just two of the countermeasures modern military jets employ. Transmitters that sense a missile or ground facility's radar tracking signals and jam them with competing RF energy and optical infrared devices that saturate the air with reflected infrared light all contribute to the mix of suppression and decoy that surrounds a plane in the air.
Radar-guided missiles broadcast radiation and track the reflected signal that bounces off the targeted aircraft. The amount of radar energy returned to the missile is relative to the plane's radar "signature" -- a measure of how well it reflects a radar signal. While stealth aircraft are designed to produce a minimal radar signature, conventional aircraft must use other means to counter the threat from radar-guided missiles. In addition to electronic jamming, chaff is often deployed as a decoy.
Modern chaff, known as rapid bloom chaff is composed of millions of tiny aluminum or zinc coated fibers stored on-board the aircraft in tubes. When an aircraft is threatened by radar tracking missiles, pyrotechnically driven pistons inside the tube eject the contents into the turbulent wake of air behind the plane. The chaff "blooms" into a decoy cloud of metallic material with a radar signature much larger than the aircraft itself. Distracted by the abrupt appearance of this more radar-reflective target, the tracking mechanism of the approaching anti-aircraft missile locks onto the chaff cloud, instead, and the missile is diverted from striking the aircraft.
Infrared-guided missiles seek heat and its associated infrared signature. Attracted by hot spots on a plane, such as the jet engine nozzles and inlets, as well as the exhaust plume, infrared-guided missiles relentlessly track invisible light emitted by the heat all the way up to its source. One countermeasure is to produce aircraft that emit less heat or disperse it in a way less visible to heat-seeking missiles. Stealth airplanes are designed to have tiny heat signatures compared to earlier generation jets. Some aircraft also have spinning devices that optically "jam" the missile's infrared detector by flooding the surrounding airspace with decoy infrared light in rapidly shifting spectrums. Another countermeasure is the deployment of flares.
Chemical flares are used to distract heat-seeking missiles. Most are magnesium pellets ejected from tubes to ignite in the wake behind the aircraft. These flares burn at temperatures above 2,000 degrees Fahrenheit, hotter than the jet engine nozzles or exhaust and exhibit large amounts of infrared light. Confronted by these more conspicuous bursts of infrared energy, infrared-seeking missiles are decoyed away from the targeted aircraft to pursue the flares instead. Some countermeasure flares incorporate a small propulsion system to fire the flare on a level path that more convincingly simulates the flight of an aircraft to confuse sophisticated heat-seeking systems. Countermeasure flares are designed to burn out before reaching the ground to minimize fire hazard.