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Compressor Stalls?

Mon Apr 26, 2004 7:26 am

Hello everyone!

After looking at today's most popular picture which shows N783UA resting at Yellowknife YZF after suffering a compressor stall, I wondered what exactly a compressor stall is and what causes it.

Having found this thread on the topic, I'm not quite sure if I got it right, so please allow me to ask for a clarification in (very) simple words:

Is a compressor stall essentially an engine flame-out that occurs when there is either
too much air entering the combustion chamber (extinguishing the flame a bit like people extinguish candles) or
when too little air enters the combustion chamber, "asphyxiating" the flame?

Kind regards,
Don't cry because it's over, smile because it happened.
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RE: Compressor Stalls?

Mon Apr 26, 2004 7:54 am

Try this thread

A compressor stall is a breakdown in the airflow within the compressor. On an axial flow compressor each vane is shaped like an aerofoil so just as an a/c wing stall's so will a compressor vane. In this case one vane will stall and the stall will spread round the compressor on the opposite direction to the way the engine is rotating.

It is generally caused by airflow disruption across the intake of a bleed valve problem

Normally the flame doesn't go out, the engine just coughs. If you are running an engine and it stalls, just quickly throttle back and it should recover.
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RE: Compressor Stalls?

Mon Apr 26, 2004 8:06 am

It occurs when the compressor isn't being fed the amount of air it needs to be able to maintain the higher pressure on the other side. A compressor can only maintain a certain pressure ratio and when that ratio is exceeded you'll get a compressor stall. It's basically air "leaking" back the wrong way through the compressor when the pressure difference can't be maintained.
Never seen it myself but it should give a loud bang. Some engines take it better than others.
JT8D for example, to avoid stalls it has springed valves that will release air into the bypass duct if pressure builds up too high after the compressor stage.

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RE: Compressor Stalls?

Mon Apr 26, 2004 8:08 am

The 727 is notorious for compressor stalls in the number 2 position due to the disrupted airflow caused y the fuselage. In fact there are some engines we have that are not able to go into the #2 position due to the fact that they will be subject to compressor stalls. The weird thing is they can come from Overhauls and in the paperwork it will state that is it eligible for a pod installation only. They determine this through the test cell run.

Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
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RE: Compressor Stalls?

Mon Apr 26, 2004 8:14 am

I´ve boroscoped an PW 2040 once which had suffered a serious stall when the pilots throttled back for take off power to climb power. The engine quit with a loud bang (acc. to the captain they thought they hiot a small plane), with excessive vibrations afterwards. They shut down the engine and turned back.
There were a few pieces missing on the first stage of the HPC, but thew 17th stage looked like somebody had fired a shotgun into it. The aerodynamic forces during the stall apperently caused a blade in a low stage of the compressor to disintegrate and the damage spread like an avalanche through the engine. It was probably caused by a faulty VBV actuator (variable bleed valve). When the pilots throttled back, the HPC couldn´t digest the amount of air anymore the LPC was delivering and since this air didn´t have a way to escape from the engine through the VBVs it went back forward against the flow through the LPC. Needless to say that we spent a few days in Treviso changing it, together with a BA crew. Nice blokes!  Smile

Je Suis Charlie et je suis Ahmet aussi
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RE: Compressor Stalls?

Mon Apr 26, 2004 3:03 pm

MD-11 Engineer is right about that. Stalls, especially at high throttle settings are usually the end of the engine's internals. VBV and VSV malfunctions are usual culprits. They Pireps are almost always the same, loud bang heard/felt, throttled back, vibrations continue to worsen, engine shut down. We find the larger blades with minor damage and the really small blades toasted!

I've heard many a compressor stall on the ground, they sound like a cannon going off. From inside, it can feel like a large truck as hit the aircraft. You fell a solid thud in your seat and the airframe!
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RE: Compressor Stalls?

Mon Apr 26, 2004 6:57 pm

Thanks everyone!

Let me confirm if I got the "throttle back compressor stall" right:

When the engine is throttled back, pressure between the LPC and the HPC raises - which is normal. Sometimes, that pressure can raise too high, and then air would normally escape through dedicated (bleed air) valves. In case those do not function normally, the air has "no way to go" except back out through the LPC.
Don't cry because it's over, smile because it happened.
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RE: Compressor Stalls?

Mon Apr 26, 2004 7:34 pm

There is a video of a Swiss A330 having a compressor stall. You can find it on this site:, but you'll have to registrate. It is FREE!
It's Scandinavian
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RE: Compressor Stalls?

Mon Apr 26, 2004 10:02 pm

There are essentialy two types of compressor stalls:

1) Stall on engine acceleration, often caused by a malfunctioning variable stator vane actuator. The LPC accelerates and the VSVs don´t open to let more air into the HPC, resulting in a pressure buildup between the LPC and HPC, causing a reversion of flow in the LPC.

2) Stall on engine descelaration, often caused by a VBV malfunction.
HPC spools down faster (it doesn´t have the inertia of the big fan attached to it) and can´t digest the air coming from the LPC. Normaly bleedvalves at the engine station 2.5 (between the LPC and HPC) will open to let the excess air escape. If this doesn´t happen, same result as above.
Stall might also be caused by a low pressure in the intake, e.g. during ground runs.
At CGN we have to use a closed run up hangar for power runs due to noise reasons. A 757 is about the limit what this hangar permits, because it limits the air going into the engine (not enough volume in front of the engines). Once we were testing a 767 with GE CF6-80 engines and had to stop it after some moderate stalls. Thisd is also the reason why you should turn an engine facing the wind for starting it in high winds.

Je Suis Charlie et je suis Ahmet aussi
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RE: Compressor Stalls?

Tue Apr 27, 2004 10:15 am

Why would this condition require an engine change?
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RE: Compressor Stalls?

Tue Apr 27, 2004 1:37 pm

The reason is that a severe stall will overstress the compressor blades and vanes and cause them to break off. A broken off metal part in a rotating engine wreaks havoc with it. You can literarely shake the inards out.

Je Suis Charlie et je suis Ahmet aussi
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RE: Compressor Stalls?

Tue Apr 27, 2004 1:51 pm

The high bypass engines,(CF-6, JT-9, RB-211) will NOT take any compressor stalls without requiring a borescope inspection for damage, Usually a stall on one of these engines is accompanied by an overtemp and part failure, thus requiring the engine change.

The JT-8 engine used on the DC-9 and 727 will just about take anything you throw at it. I've seen and heard them sound like a choo-choo.

Several years ago a major airline almost lost their ETOPS because of "top of descent" stalls on the 767 powered by JT-9 7R4D engines. When the crew pulled back power to descend, the bleeds didn't react properly and caused the stall. Turned out that during rebuild the tolerances in the HPC were not being held to tight enough clearances,
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RE: Compressor Stalls?

Wed Apr 28, 2004 6:42 am

A compressor stall, as several have pointed out, is a disruption of the airflow through the compressor. Compressor stalls are typified by the bang and flames out the inlet and tailpipe. Generally, the larger the engine, the louder the bang. Many times, the bang is so loud that the crews think that they may have hit something or even had an inflight explosion. Stalls can be caused a bleed or variable geometry system malfunction, throttle movement, extreme yaw, pitch, or roll maneuvers, cross winds, ingestion of foreign material, internal damage, engine deterioration, exhaust gas reingestion during thrust reverse. Stalls can occur just about anywhere the engine is operating: taxiing, power up for takeoff, top of climb, top of descent, cruise, landing roll. Different engine react differently when they have a compressor stall. As Tim T pointed out, the JT8s can stall a lot and not suffer any damage (with some exceptions). Others engines, especially the bigger high bypass fans, can have major turbine damage following a stall due to an overtemp. A compressor stall will rarely cause damage to the compressor other than doing minor damage if something like a rivet gets shaken loose. Usually, if the compressor is found damaged, it was that damage that lead to the compressor stall. The exception is if the engine is continuously stalling that can lead to a high cycle fatigue fracture of a compressor blade and the resultant downstream damage. Some engines are really easy to stall such as a JT8 in a 727-100 center position with the oval inlet and S-duct. Do a snap accel with the throttle and you could probably get a stall. Other engines have so much stall margin that you have to really work at it doing multiple Bodie excursions to get it to stall.
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RE: Compressor Stalls?

Wed Apr 28, 2004 12:03 pm

Compressor stalls on RR RB.211-524 engines (used on B747's and L1011's) are nearly unheard of, at least with the airlines that I have personally operated for.
The three shaft design (complete with slugger valve) was very tolerant of rough throttle handling.
Good thing too as some pilots don't have a clue.
There were however a few problems with the VIGV's, which produced pod oscillation, AKA 'pod nod'.

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