What exactly happens during a compressor stall, and what causes it?  I remember being told that is is caused by an improper air-fuel ratio, but not 100% sure if this is true, or if there is a lot more to it (?)

The blades/vanes of the compressor are airfoils, just like any other wing.



If the airflow over them is disturbed sufficiently,  those compressor blades go into a stall condition,

unable to generate lift (thrust) and thus the engine loses power until such time as the stall condition clears,

which might not even happen if the aircraft's speed and attitude isn't rapidly corrected.



Another consequence of a compressor stall is a loss of cooling airflow,  causing heat in the engine that SHOULD

be vented out to stay in the engine and cause an overheating condition.   It can rapidly become an overheating

condition that is serious enough, and hot enough, that the tips of the compressor blades start to melt and blow off.

Interesting, thanks for the info.

So what could cause a compressor stall during an otherwise perfectly normal take-off, for example, in #1 engine on this 767:

I had one caused by a bird at KHOU in B737 runway 22 at 125 knots. Fire came out the front. I thought the front fell off.
The brakes work really good!!

On a normal takeoff, such as in your picture, it is usually ingestion of a bird.  I was on a 767 (as a passenger a day before 9/11) that had several compressor stalls.   We returned back to ATL to get another plane.  Apparently there was a problem with some 767 engines losing stator vanes which in turn fragged the engine.  The 727 had such a problem with compressor stall (while taxiing in high winds) that Delta had a cue card for the crew to read on the PA.

I'd love to see that cue card!  "Uhhh, folks, the burst of flaming, super-heated air that you may see and hear exploding out the front of the 'ol engine there is perfectly normal.  Thank you for flying Delta, we hope to see you again real soon."

I thought I saw a training video that recommended not RTO for compressor stalls.. That being said, gut instinct says abort when you see flames out the front :)

Sometimes when IGV actuators go bad compressor stall is a symptom

A long time back, before CRM, EEOC, EPA, and sexual harassment lawsuits, etc. were invented, when flight attendants were all single young female "stewardesses" in hot pants and could carry on a good four letter word conversation with a sailor whilst he stared longingly at her firm thigh and supple buxom begging to be released from its tight enclosure, I had a trusty old P&W JT8-9 on a "ketchup & mustard rocket" that commenced to start multiple compressor stalls when the power was pushed up over the old Scurry VOR on descent into DAL.

It banged about four or five times before I could get the power back. Within five seconds the senior was dinging the call like a retard playing a tambourine and beating on the cockpit door.  When I let her in the cockpit she exclaimed, "There's fire coming out of both ends of that mother-fucker"!!!! I thanked her for that valuable and insightful bit of information, assuring her that it was not our day to die. I chose to leave it at idle for the remainder of the flight.

Maintenance made a few adjustments and sent it back out. It banged again on takeoff with the next crew and the engine got changed.

It happened before the training film was made
We didn't have buttons to push back then, so we looked outside a lot.
Steam gauges + 2 VOR's and an ADF and a really worn out "clacker"!!!

Not a stall, but still a cool video.

Not an engine aero guy, but as I recall stall margin and performance were at the opposite end the engine aero-design spectrum.  If you sought absolute optimum performance (thrust, sfc, etc), you got it at the expense of stall margin, and the engine would not tolerate inlet flow perturbations, or undisciplined throttle movements.

Go for stall margin (aka resistance to compressor stall), and the thrust and sfc were less than optimum for the engine frame size. Seem to recall reading that the TF-30 used in the F-14A were worked very hard for engine performance optimization, the result being that the pilots had to "fly the engine" to avoid compressor stalls and the resulting negative effect on airplane performance.

The Russians on the other hand, wouldn't or weren't capable of optimizing engine performance, with the result being all kinds of surge margin and the ability to do some of the amazing air show maneuvers that were (and maybe still are) their hallmark.

I suspect that advancements in FADECs and aero component desighn have allowed Western gas turbines to retain their advantages in thrust and sfc. AND retain adequate surge margin.

What is a compressor stall?

Hmmm...

An engine compressor stall is normally recognized by a noticeable bang or popping noise and possible aircraft yaw.  These responses are normally accompanied by the rapid increase in TGT TEMP and fluctuations in Ng SPEED, % TRQ, and % RPM reading for the affected engine.  In the event of a compressor stall:

   1. Collective Reduce

If Stall Condition Persists:

    2.ENG POWER CONT lever (affected engine) - Retard (TGT TEMP should decrease)
    3. ENG POWER CONT lever (affected engine) - FLY

If Stall Condition Recurs:

    4. Establish Single Engine Airspeed.
    5. EMER ENG SHUTDOWN (affected engine).
    6. Refer to single-engine failure emergency procedure.


Where's my IP when I need him, did I pass?

What's the first thing you inspect on a UH-1B-C-LMNOV after a compressor stall? H60ADriver touched on it.

Or, if you fly a superior aircraft, the Apache:

(A) Collective - Reduce

If condition persists;

(B) Power lever (affected engine) - Retard

If TGT decreases and there is no further evidence of a stall:

(C) Power lever (affected engine) - Fly

If stall condition recurs:

(D) Power lever (affected engine) - Idle

(E) Land as soon as Practicable.


Boom.

I would hazard a guess that it's either the IGV / Compressor blades for condition, or the casing for discoloration.  Am I on the right track at least?

A compressor stall may cause felt vibration or low rumble, dependant upon what engine. If a compressor stalls in a commercial type application, there is a mechanical or control system problem. High performance engines have a lesser stall margin and can have problems with disruption of airflow like can be experienced in a spin.

The bang and / or flames out the front or bleed ports (surprisingly called surge valves or surge port) are the result of a surge, where the pressure front moves forward and outside the confines of the combustor. This is where secondary airflow is located which is used to cool the turbine hardware. If the engine locks into a stall/surge cycle the engine will melt.
A compressor stall may result in a surge, but a surge doesn't always require a compressor stall.

We'll take more stewardess stories any time...

We used to get compressor stalls on a semi-regular basis in the 727 on the #2 engine taking off out of St. Vincent.  Low weight, max power, high deck angle all made the #2 engine more susceptible to compressor stalls.  Those were always fun.  Look up Amerijet 727 St. Vincent on youtube to relive my happy memories.

At altitude, a cure for the #2 engine compressor stalling was to swap the bleeds on either #1 or #3 and bleed #2.

The centrifugal flow jet motors on the "Tweet" would chug like a cow pulling its foot out of the mud if you yawed or advanced power too fast!!