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How to destinguish b/w ENG Stall & Surge inside the cockpit

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How to destinguish b/w ENG Stall & Surge inside the cockpit

Old 9th Jan 2012, 10:13
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How to destinguish b/w ENG Stall & Surge inside the cockpit

Hello, just wanted to get feed backs as to how would a pilot inside a 737NG cockpit recognize and distinguish between an engine stall, surge, separation and severe damage
What would the Eng indications be like ( N1/N2 fluctuations etc) and any other physical characteristics like sounds, yaw.

Thank you.
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Old 9th Jan 2012, 12:56
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Stall and surge are synonymous, for all practical purposes; GE/CFMI like to use "stall", PW & RR "surge". Very loud bang, increasing EGT, maybe N1/N2 rollback. May or may not incur severe damage, which would be indicated by an inability to recover.

(Note for the record: I was once called to respond to a pilot's writeup of "flameout with increasing EGT". It was, of course, a stall) (I am not making this up...)

Separation would be indicated by a bunch of zeroes, and an intercom call from the F/A.
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Old 9th Jan 2012, 13:14
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... and (in a Boeing) a lot of crossed control (as you have lost a ton or so of metal from one wing, but are getting all of your thrust from the other - left aileron/right rudder, or right aileron/left rudder required).


Stall/surge may also be a series of bangs, rather than a single one.
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Old 9th Jan 2012, 13:37
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I would be very interested to read the comments from any pilot who has had the misfortune to suffer a 'real' compressor stall/surge, particularly their recollections of the sound of the initial malfunction. As an engineer responsible for creating the sound heard in simulators, getting first- or even second-hand information on the nature of such failures is pretty rare. As noted in the comments above, there are two implementations - currently we have sound effects available for a single loud 'bang' (akin to a shotgun blast), or a somewhat less dramatic series of repeating 'bangs' (more like someone smacking a metal filing cabinet with a baseball bat).

Obviously the vast majority of pilots only ever experience this type of failure in a simulator, so please don't tell me what your simulator sounds like... I may well have created the sound you are describing. What I'd love to hear is any comment from those of you that have had this happen outside of the sim - yes, real life! And, I'd be happy to get input from both military and commercial aircraft types.
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Old 9th Jan 2012, 13:53
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Only the one, on a business jet with tail mounted engines - even then it was very loud. I thought a baggage door had separated! I think a series would be just as loud -

One surge:


A series of surges:


Note the second video - the surges are easily loud enough to be heard as large bangs on the ground.
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Old 9th Jan 2012, 13:54
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Mishandling of the Nozzles in a Harrier gave me a surge once - several loud sharp 'bangs'. That is often described as a 'pop surge as that is a good description of the sharpness and duration of the sound.

As above, 'severe damage' or 'separation' will be unmistakable too

"Excuse me, Sir, my engine is missing"
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Old 9th Jan 2012, 14:18
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I believe that the simulator programs do distinguish between the cues from an engine stall/surge alone and "severe damage"

What's not severe damage is an engine that recovers immdiately from a stall/surge with only a bobble in the N2 and EGT (N2 drops and recovers and EGT goes up slightly and then drops)

Most severe damage cues include N1 not responding to throttle and EGT through the roof with perceptible increased vibration and/or fire warning bell

Some SOPS suggest you should pull back on the engine throttle (assuming you have correctly identified the single engine), after confirming that the EGT has not sky rocketed go ahead and advance the throttle.

Do not grab both throttles and do take your time and fly the aircraft first

The problem with the simulators is that they can not replicate the magnitude of the sound or vibation without damaging the simulator (they are fixed to a floor) as repeated test runs are as bad to simulators as they are to engines.

Couple that with the shock load to the pilots seat as the yaw develops as fast as hitting a telephone pole. I was in F class once B747 AI Bom-Del and I sprayed my drink across the cabin.
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Old 9th Jan 2012, 14:32
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here is what i got from a 737 pilot.

stall: loud bang and high egt
surge: large fluctuations in power (N1), high egt. accompanied by a loud bang
severe damage: main thing to look out for is unusually high vibrations and loss of N1.
seperation: loss of both N1 and N2 indication. Both these indications will be blank.

Any one care to correct or add anything else?
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Old 9th Jan 2012, 14:52
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I had several compressor stalls on the 727-200 with its engines located far away on the tail. Not uncommon for the #2 engine with its S-duct. All of them were during ground ops. I don't know if it is always the case, but I found most of them difficult to hear and could be easily missed. I do remember one where it was felt more than heard, as if we had momentarily taxied over something.

I suspect they were still quite loud because with engines running and the window open the engine sound is loud enough to be uncomfortable yet when the window is closed, virtually all engine noise is blocked out.
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Old 9th Jan 2012, 15:15
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Not any important differences (to a pilot) between a stall and surge.

The stall comes first (breakdown of flow across an airfoil) and the surge may come a second or two later if enough airfoils break down to allow the higher pressure air behind them to rush forward out the inlet creating the noise.

The corrective action is to reduce the workload on the airfoils (if still present)
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Old 9th Jan 2012, 15:19
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To add for 'garage' - getting true sound definition is not really important - an approximation is fine since no two real events will probably sound the same anyway. Far more important that the panel indications are correct and any performance changes reasonably realistic.
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Old 9th Jan 2012, 15:45
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Stall/Surge

From a technical point of view:

Stall is, like a stalled wing, separation of airflow on the compressorblade(s).
e.g. axial flow too low in relation to hi rotational speed, mostly on 1 or limited stage(s).
a stall can develop in a surge, meaning the airflow thru the engine can completely stop
or even reverse its direction....flames coming from the intake.

In GE/CFM the VBVs protects the Boosterstages from stall - by increasing the axial flow through the booster -
and the VSVs protects the core compressor by adjusting the 'angle of attack' of the vanes.
(P&W and RR by bleed valves to increase the axial flow and fewer stages of HPC/IP variable)

Pre FADEC generation engines are more sensitive to stall/surge,
with the modern FADEC engines there is a stall recovery scheduled in the ECU/EEC.

P&W and RR are more sensitive for stall/surge but can withstand it better than a GE/CFM,
while the latter is less sensitive.

During ground run, you will notice an unstable and roaring N1 (Fan Stall) before a compressor stall will develop,
a compressor stall comes with a loud bang and with or without rising EGT.
(depends on the reaction time of the operator)
A surge (never experienced from cockpit but from outside) with load bang(s), flames from the intake,
unstable parameters N1/N2, rise of EGT (as being told) and eventually blow out.
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Old 9th Jan 2012, 15:59
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The sounds and instrument indications will vary with engine and aircraft type.
A Spey at ground level will get everyone’s attention, similarly with an A330 at takeoff.
Alternatively a small corporate engine or regional jet, especially with mixed compressor systems may only be a slight ‘bump’ noise.

Most of the problems with simulation are that everyone has their own view of fidelity, yet few question the training point in having the failure option.

It is most unlikely that a crew will be able to identify the cause of a bang, or any other loud noise in isolation. Engine pop surges are often over before supporting instrument evidence can be seen. Longer duration ‘locked in’ stalls normally provide adequate supportive indications, possibly with some vibration.


Thus the point of a drill is to know what the combined symptoms are, how to confirm the event, and to have knowledge or guidance of what action to take.
Many modern engines self-recover from stalls/surges, and can be used without further fault. However, without knowing the cause of the event or if any damage has resulted there will always be a judgment of risk in continuing.

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Old 9th Jan 2012, 16:11
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@ BOAC : Thanks, that's really the tack I've been taking. The little input I have received directly has basically been the chance of any one pilot experiencing more than one engine stall in a career is low and that each stall occurrence will likely be quite different (single-bang, repeating, etc), such that doing much more than ensuring the volume/direction of the cue sound is appropriate would be a waste of time.

However, I am always looking for aspects that I can improve, so this seemed a reasonable opportunity. I am still wondering whether certain engine types are more likely to generate a specific sound cue, versus other engine fitments (CFM vs IAE on the 320 for example), but again, I may be barking up the wrong tree.
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Old 9th Jan 2012, 17:02
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SmartCockpit - Airline training guides, Aviation, Operations, Safety

Theres a nice Table of summaries starting at page 17 of this article which summarizes the differences.
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Old 9th Jan 2012, 17:24
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Stall verses Surge

To expand on A33Zab's post: A stall (rotating stall) is a flow breakdown at one or more compressor blades. A rotating stall is a stagnated region of air which moves in the circumferential direction of rotor rotation, but at a fraction of the rotor speed. At a given throttle setting it does not move axially in either direction although pressure waves may be created that move upstream (compression waves) or downstream (rarefaction waves). There are two types of stalls. Blade stall is a two-dimensional type of stall where a significant portion of the blade has large wakes due to separation of the suction surface boundary layer (convex side of the blade). The other stall is an end-wall boundary layer separation. In either case the sound would be a single "bang".

Surge is a response of the entire engine which is characterized by a flow stoppage or reversal in the compression system. Upon surge, a compression component will unload by permitting the compressed air in downstream stages to expand in the upstream direction forming a planar wave which at high rotational speeds leads to flow reversal. The compressor can recover and can begin to pump flow, however if the cause of the surge is not removed, the compressor will surge again and will continue to surge/recover until some relief is provided. A surge can be initiated by a rotating stall. In this case, the sound would be a series of "bangs" with noticeable flames coming out of the engine. Surges can be very damaging to the compressor system.

For a given compressor, one can construct a performance map given developed data. On the vertical axis would be total pressure ratio, and on the horizontal axis would be corrected compressor inlet airflow. A stall line is constructed above the engine operating line. At low inlet airflow, the lines are very close to each other, but diverge as total pressure ratio and inlet airflow increases, e.g., takeoff, max climb and max cruise. So there is more stall margin designed into a compressor at critical speed operational points verses typical ground operations.
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Old 9th Jan 2012, 20:05
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Originally Posted by GY
The little input I have received directly has basically been the chance of any one pilot experiencing more than one engine stall in a career is low and that each stall occurrence will likely be quite different (single-bang, repeating, etc)
Gee, I must be lucky! I've had at least 4, all in the F-4. Of course, on three of them I was asking a lot from the engines.
  • First was going into full AB at 30,000' at very high AOA, low speed. A single shot gun blast in the ear (intakes adjacent to cockpit) and I pulled the throttles out of AB to recover.
  • The second was in the mid-twenty thousand foot range going downhill fast at 700+ knots IAS (supersonic) while pulling the engines out of full AB back to idle. That was a bit more interesting. A loud bang, followed by 8 seconds of loud buzz and then the engine recovered.
  • The third was 33,000 feet Mach 2.165 accelerating, more or less level. The aircraft began to yaw a bit and we trimmed to move the ball back in center. There was a loud report, a thump on the airframe, the nose immediately sliced toward the problem engine and rolled a bit that way (even though it was a ~centerline thrust aircraft). Reducing power ended the problem immediately.
  • The fourth was a loud bang on takeoff around 130 knots that recovered by itself. Learned later that this was caused by FOD.
Can I claim some sort of record?
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Old 9th Jan 2012, 20:10
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My friend on a DH leg sat next to an engine the surged several times after T/O on a 73N7G.
His comments were: sounded exactly like the sim… and when he talking to the pilot after landing , he was told the aircraft behaved just like the sim with the rolling surges, and the ”tail kicking”… and inside the dials showing increasingly fluctuating N1 anf EGT(red).

I used to fly an old worn out straight jet, biz jet, where, when standing up the throttles for T/O it was easy t o “stall” the engines if you didn’t easy the throttles up very very carefully. In flight the engines behaved much better… But on T/O you could easily give your self a week of not flying... if you did not pay attention setting T/O thrust… the sound of bad thing was a very quick fluf fluf fluf fluf fluf…

Last edited by plain-plane; 9th Jan 2012 at 20:48.
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Old 9th Jan 2012, 20:26
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Machinbird:

Can I claim some sort of record?
Hahaha, thanks for the input, you mil/ex-mil guys always on the edge of crazy things

To be fair, I did say the chance of more than one was "low", not "nil"... even so, thanks for the input. The second incident does sound more than mildly concerning - the buzz is interesting. On our F-16 simulations we have an "inlet buzz" sound cue - but I always thought that was only triggered at high AOA? Probably not related.

- GY
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Old 9th Jan 2012, 20:49
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Hello, just wanted to get feed backs as to how would a pilot inside a 737NG cockpit recognize and distinguish between an engine stall, surge, separation and severe damage
What would the Eng indications be like ( N1/N2 fluctuations etc) and any other physical characteristics like sounds, yaw.
A stall will blow forward, so for a second you will not only have a loss of thrust but it be the opposite. In low light you may see a fireball on the stalling engine. I have been at the nose of a 742 with JT9's advancing to high power during a stall and I could feel and see this happen.

Having said that it was quite normal on older engines with loose tolerances and simple controlls. On the modern engines it is rare but more detrimental to the motor, normally cause for it to come off wing.

When there is severe damage it can result in siesure (you will feel this and obviously see it in your instrumentation). Seperation is the worst case scenario, Drastic EGT change is th first indicator followed by eng speed reduction and fire is the largest issue.

Hope this was the simple answer you seek.
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