Surge is an extremely unpleasant phenomenon, which can lead to the complete destruction of the engine and to the death of people. He entered our lives with the era of jet aircraft, which began in the 50-60s of the last century. Now this phenomenon no longer poses an absolutely fatal danger. Thanks to the accumulated experience and technical solutions, surging of the aircraft engine in many cases can be prevented, but this violation continues to occur today.
Passenger liner engines
Contrary to popular belief, modern passenger liners are only 20-30 percent jet jets. That is how much thrust the reactive component of a modern turboprop engine gives . The remaining 70-80 percent is obtained due to the traction of the good old screw rotated by the turbine. True, this screw is completely different from the classic aircraft propellers of previous generations of aircraft . To understand the phenomenon of surging aircraft engine, it is necessary to get at least a minimal idea of the structure of a modern aircraft engine. The engine diagram is shown in the picture.
The principle of operation of the aircraft engine
The modern engine, which usually hangs under the wing in a beautiful fairing-nacelle, is structurally a double-circuit system. The external circuit is a fan, which actually operates on the principle of a classic aircraft propeller and is a combination of many blades, which are perfectly visible in front of every modern aircraft. The second task of this fan is to take air for the internal circuit of the engine. The latter is much more complicated and consists of the following chain of units interacting with each other: the already mentioned fan pumps air into the compressor, which compresses it to the required values, then heated and compressed air enters the combustion chamber, where it is mixed with aviation kerosene and ignited.
Outgoing gases (combustion products) turn the blades of the turbine, which rotates the fan (they are located on the same shaft), after which the remainder of the exhaust gases is ejected through the nozzle and adds traction, acting already on the jet principle. If you remove the external circuit, you get a purely jet engine, and so it is a turboprop. Well, or turbofan - as anyone likes.
Of course, this description of the engine is extremely simplified. Aircraft engine is the most complicated technical system. If in the world there are many countries capable of designing and manufacturing airplanes, then countries with technologies for the development and production of turboprop engines are much smaller.
Surge phenomenon
The word pompage itself comes from the French language and means “pumping”, “pumping”. Its essence is in violation of the normal flow of air in the engine. Its longitudinal vibrations occur along the entire air path, causing an imbalance in the ratio of air and fuel in the combustion chamber. If the imbalance is in favor of fuel, the re-enriched mixture explodes, if the imbalance is in favor of air, combustion stops.
Outwardly, the surge of the aircraft engine looks like a chain of explosions, accompanied by strong vibration and the ejection of flame from the nozzle. The sequence of events in this process can be imagined as follows: lack of air, an excess of fuel in the combustion chamber, an explosion in the combustion chamber, a sharp acceleration of the turbine, a sharp jump in fan speed, an excess of air, cessation of combustion, a decrease in speed, again a lack of air. The cycle is closed.
If all these events are repeated more than once, vibration and local explosions can lead to very sad consequences, including a plane crash.
Causes
The causes of surging aircraft engine can be different. The most common are damage to the turbine blades due to wear or a foreign object entering the air intake. For example, a bird or stone from the runway. Also, the cause may be crew errors, the supply of excess fuel at start-up, engine operation in extreme conditions, as well as atmospheric phenomena such as the appearance of vortices or excessively low pressure in hot weather.
How to prevent and eliminate surging
On modern engines, sensors are installed throughout the airways. Based on the sensor readings, the built-in automation immediately makes changes to the fuel supply mode and compressor parameters. In the engine itself, instead of one, two or even three shafts are used, which ensure its stable operation when surge phenomena occur, separating the direct connection between the fan and the turbine.
Elimination of turbine surging, if it occurs in horizontal flight, is performed by simply temporarily turning off the engine or reducing its speed, after which it is possible to carry out the so-called “cold purge”, that is, to blow excess fuel from the combustion chamber by an incoming air stream.
The most dangerous thing is surge on take-off when the decision point is passed. However, modern airliners are able to continue to take off even with one failed engine.