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Old 10th Jul 2006, 10:13   #1 (permalink)
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Engine Surge/Stall

Couls someone please explain to me the difference between engine/compressor surge and stall? In particular how a pilot might recognise the difference in flight and what he/she might do about it?
sally is offline  
Old 10th Jul 2006, 11:25   #2 (permalink)
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Stalling (breakaway of airflow from the convex side of the compressor blade, just behind the leading edge) can also be caused by improper angle position of fixed blades, or more usually by a transient condition during run-up to full compressor load. The surge phenomenon is a momentary flowback from high-pressure (aft end) to low-pressure (forward end) lasting only a few milliseconds, disappearing and returning some milliseconds later, creating a vibration which puts extreme load on blades and shaft. Surge during transient conditions can be avoided by modifying inlet guide vane angle, compressor blow-off valves set to open to atmosphere above a certain pressure, or even cutting off the fuel flow for a split second allowing the compressor to 'jump' from one speed/pressure characteristic curve to another.

I found this elsewhere and can't take credit for the info
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Old 10th Jul 2006, 11:36   #3 (permalink)
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Don't take my word for it but I think a surge/stall is actually the same thing - that is a compressor stall. Sometimes it will happen so quickly then clear itself by the time you have realised something has happened. I believe the safest thing is to close the throttle (thrust lever) and this will clear the airflow disruption that cause the problem. The re-open the throttle again and it should be OK. I once observed a compressor stall take place in a B737-200. I was watching the aircraft start to taxi when there was a huge explosion and a whacking great sheet of flame came out of the tail pipe. With some compressor stalls you can also get a sheet of flame out of the front of the engine. There is a great training video available that shows an engine on a test stand having birds thrown at it with a mass of flame coming out both ends of the engine. The engine ran normally immediately afterwards and the point made by the author of the video you do not necessarily have to shut down the engine after a compressor stall unless there is obvious indications of severe damage.

For other readers browsing this thread, I wonder if yyou can provide advice on how to distinguish between severe damage (and of course bits flying from the engine) and not severe damage. This question frequently comes up in simulator training and it is left to the instructor to advise what is and what isn't severe damage. And where does he get his information from? Who knows - because it is difficult to find a reliable source of impeccable information.
For instance: Repeated compressor stalls that cannot be cleared and a decision made to shut down the engine in flight. Is it severe damage or not? If it is assessed by the crew as severe damage then the the fire handle is pulled but the extinguisher is not actuated. As the fire handle pull can eventually cause the A system engine driven pump to run dry (?) in the 737 that is, then this takes about 5 minutes I believe and a pump change is needed (expensive).

In the simulator (depending on the aircraft - 737-200/300 or NG), the engine seizure button will cause the engine to come to a grinding halt. In the NG simulator this does not manifest itself with lots of vibration and noise leaving the crew to decide how serious the problem is. The instructor may well disagree and it becomes a matter of personal opinion. No prize for guessing who wins...

In the 737-300 and 737-200 sims (at least the ones I have operated), the seizure is dramatic - lots of racket, the sim shudders and you are in no doubt the engine is cactus - so you pull the fire switch but don't fire the bottle because there is no fire. When I sought an opinion from a Seattle Boeing instructor why the NG had "silent" engine seizure and the other types had noisy seizures, his explanation was that some types of engine seizures could be quite subtle and it was left to the crews to sort it out. The "noisy" seizures were too easy to diagnose. I was no better off. So when is the crew expected to pull the fire handle (no fire) on an engine shut down
So my question is: What type of engine indication in the simulator would signify the crew must use a severe damage checklist or not?

The clue may be found in my reading of a Boeing 727 Flight Operations Symposium held in Seattle by Boeing that took place in 1979. One of the questions asked of Boeing was:
"How would you handle a rotor stoppage, if no fire and no severe damage exists?" Boeing answer: "This case should be handled as any other engine shut down in flight - without pulling the fire switches."

The Boeing reply suggests that a rotor seizure (is that the same as an engine seizure?) is by defintion, not a severe damage job. So back to the task of the simulator instructor to give advice to his students. But where does he get the good reliable gen from?

Sorry to hijack the original post but the title of the post seemed to introduce my own question nicely.
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Old 10th Jul 2006, 15:35   #4 (permalink)
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Go to


click on engine training and you will learn what you need to know from a pilots perspective

Engine terminology (stall/surge) begins at the designer's level and is then promulgated into the training level (Procedures/simulators etc.)

Unfortunately by the time it got to the simulator training sylabus the words were not understood, nor the engine failure model representative.

There can be many levels to this discussion (how engines fail and what are the symptoms) but the minimum that the crew probably needs to know is contained in the link above.
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Old 12th Jul 2006, 06:08   #5 (permalink)
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Thanks for the replies - very useful thread.
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