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Surge suggests that something speeds up, yet I read elsewhere that in fact in jet engines, an engine surge is actually a compressor stall.

Whether that is correct or not, what exactly is doing the surging in the case of an engine surge?

What kind of fault within the engine or conditions of its environment causes an engine surge?

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The surge is the increase in rpm of a stalled compressor. The compressor blades are airfoils not unlike propellers and wings. They will stall if airflow is not maintained at the proper angle of attack.

Interruption of airflow in jets can be caused by abrupt maneuvering, ingestion of foreign matter, or excessive application of throttle. Much has been done to eliminate this issue in modern jets, but the nature of the design still makes it possible. If air flow is interrupted to the compressor, the jet can literally backfire, creating a loud "bang". A large yellow flame may also be seen coming from the back of the jet as the fuel/air mixture is now excessively rich due to insufficient air.

If the engine is not damaged, it can be re-started the same way we unstall a wing, by re-establishing proper airflow angle of attack. This is done by lowering pressure BEHIND the compressor. Throttle is reduced. Provided the engine is not damaged, throttle can be carefully restored.

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    $\begingroup$ Sometimes the backfire is bad enough to get flashes of flame at the front. Also note you can also get surge with centrifugal compressors if you choke off enough flow although they are way less sensitive. Basically like blocking a vacuum cleaner. Moderate stalling and surging will produce an oscillating cycle that comes through the airframe as a "rumble" sensation. In the old days when suck-in doors were first introduced to provide extra air during flow disruptions, they were called "de-rumble doors". $\endgroup$ – John K Nov 26 '18 at 5:30
  • $\begingroup$ Modern FADECs usually implement the recovery procedure automatically. When they detect engine surge, they restrict fuel flow and gradually increase it back and only if the engine surges again generate a warning. $\endgroup$ – Jan Hudec Nov 26 '18 at 6:05
  • $\begingroup$ So basically, it's an aerodynamic stall within the engine, as opposed to a stalling of the engine like you'd see in a ground vehicle? $\endgroup$ – a CVn Nov 26 '18 at 7:26
  • $\begingroup$ Correct. Amazing how they got that right. Instruments would read out a "surge" in rpm as the compressor unloads "like blocking a vacuum cleaner (perfect)". Can also be considered a "surge" forward of higher pressure air (and flame) as stalled compressor can not maintain proper airflow. Jets have a continuous combustion process, which helps account for their huge amount of power for their weight compared with piston engines. $\endgroup$ – Robert DiGiovanni Nov 26 '18 at 7:57
  • $\begingroup$ Once I was riding in an Air France 747 that had a surge event at the start of the takeoff roll. As the thrust came up, there was a boop-boop-boop-boop-booop BOOM sound, with a flash of light (it was at night), then a spool down and we taxied back to the gate. $\endgroup$ – John K Nov 26 '18 at 13:58

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