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In aviation a flameout refers to the run-down of a jet engine caused by the extinction of the flame in the combustion chamber. It can be caused by a number of factors, including fuel starvation, compressor stall, insufficient oxygen (at high altitudes), foreign object damage (such as caused by birds, hailorvolcanic ash), severe inclement weather, mechanical failure and very cold ambient temperatures.[1][2]
A flameout is believed to be most likely to occur when flying through certain weather conditions at a low power setting such as flight idle (e.g. during the descent). These conditions include flight through moderate to heavy turbulence, rain, hail or sleet. The potentially hazardous circumstances are highlighted in the flight manual with a requirement to select continuous ignition. Alternatively, the FADEC engine controller will select ignition automatically if it detects specific changes in engine parameters. It will also perform a relight if necessary. A manual re-light attempt is made by following the procedure in the Flight Operations Manual.[3]
Early jet engines were prone to flameout due to disturbances of inlet air, caused from gas ingestion. Fast acceleration or inappropriate throttle settings could also impoverish the fuel/air mixture causing a flameout or an excessive jet pipe temperature. However, if the throttle movements are made smoothly and not too rapidly, there was little or no risk of flame extinction. If this happened at low altitude, it would often lead to the total loss of the aircraft. However, jet engines were developed to be controlled with a governor, a centrifugal feedback valve for controlling the speed of the engine - sometimes referred to as FCU (Fuel Control Unit) or HMU (Hydro-Mechanical Unit) - allowing controlled fuel metering proportional to the engine's rotational speed. Current modern engines are engineered to a higher degree of technical quality controlled by digital electronic systems (FADEC) that constantly fine-tune their performance; thus such flameouts are not as common as they were in the early days of jet-powered aviation.
It is common practice for pilots to be taught and to practice flameout landings.[4]
In-flight restarts are designated as either windmill or starter-assisted, depending where in the flight envelope the restart is attempted. The re-light envelope occupies the lower part of the flight envelope below about 30,000 ft (28,000 ft in the case of a Boeing 747, which experienced flameouts at 37,000 ft[5]). When an appropriate initial altitude and forward speed are available, the mass airflow through the compressor blades can maintain sufficient rotational velocity for enough time to permit restart. If the aircraft's operating conditions are not sufficient for a windmill start, the engine's starter motor must be operated to increase the turbine velocity to the minimum required speed.[6]
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