Engine Surge

[PIGDOG]

Could anyone please tell me what the flight manual of different aircraft says to do in the event of continuing engine surge.

Would I be correct in saying that the best thing to do is to retatd the throttle and slowly increase it again?

[bizi2675]

Well, I quess it's my turn to help you. :ok:
I don't know about different airplane manuals and their procedures for engine surge, but in general it is said that when surge occurs, it doesn't matter whether it's continous or not, the trottle of the affected engine must be closed slowly.

Surge is most commonly caused by either fuel system malfunction or mishandling of the power controls. Anyways in extrem cases could cause large bending stresses on the compressor rotor blades that they could contact stator blades and that could cause some serious trouble.
To prevent surge engines are equiped with one or more of the following:
- variable inlet guide vanes,
- variable stator vanes,
- compressor bleeds,
- multi-spool compressors,
- active control clearance.

Therefore on my opinion the best thing to do would be just to slowly close the trottle and probably reignite if the problem was mishandling. If surge occured because of the fuel system malfunction, I think the best would be just to slowly close the throttle.
Hope I helped at least a little bit.

Bizi

[Old Smokey]

Slowly retard the thrust lever until the surging ceases, all of the way to idle if necessary. If the surging continues, unload the compresser by increasing bleed (as long as TAT allows Anti-Ice to be turned ON). Then -

(1) If the surging ceases, gently increase the thrust until re-occurance, and then operate just below the surge threshold, OR

(2) If the surging will not cease, shut down the engine.

Regards,

Old Smokey

[PIGDOG]

Thanks you have both been very helpful:ok: :D

Bizi i'm doing an assignment on aircraft stall and surge. Mutually helpful.

[John Farley]

Pigdog

You talk of continuing engine surge. Presumably you have in mind a series of self clearing surges that will not stop. Bang, Bang, Bang etc. If so throttling back will almost certainly restore normal running.

BUT (and it is a big but) if by continuing engine surge you mean a locked in surge which is not self clearing then you may have only a few seconds to close the HP cock before the JPT/EGT/TET goes off the scale and the turbine is too damaged to get a relight even if the compressor starts operating normally again.

The temperature is everything, if it continues up at idle you must get the fuel off. At once.

JF

[bizi2675]

Pigdog,

if you need more help on your assignment, let me know. I don't have much practical experience, but have some books, so maybe I could help you with theory.

Bizi

[G-ANDY]

Apologies for butting into this thread, but flying only a piston engine aircraft at the moment, I don't know anything about engine surges - but am quite curious!

1). What exactly is an engine surge?
2). How do you know when an engine is surging (do you hear it, or is it discovered by monitoring your instruments)?
3). Can a flight continue after an engine surge, or must a diversion be performed? I guess if you have four engines and you loose one - it's okay? Probably not on take-off though?
4). Do they happen often?
5). How long's a piece of string?!!

Thanks,

G-ANDY

[bizi2675]

G-Andy!

There's nothing to apologize for. I'm glad you're interested in expanding your aircraft knowledge.
So, here goes abbreviated Engine surge theory.

First, you need to know that surge occurs on jet engines because they have, among others, compressors.

This compressors have blades (if you look at the front of the jet engine that's what you're looking at). Anyways, these blades are two kinds: rotor blades (which rotate) and stator blades (not rotating). These blades are twisted just like the aircraft wing is, from root to tip. Therefore, they have some sort of angle of attack, just like the wing. And just like the wing, they can stall.
The angle of attack of a compressor blade is the result of the axial velocity of the air and the rotational speed of the blade.
These two velocities combine a vector which gives the actual angle of attack of the airflow over the blade.

A compressor stall is basically imbalance of these two velocities.
When compressor stall ancompasses the entire engine, then this is called SURGE. So, basically, SURGE is COMPRESSOR STALL.
You can sense the surge by loud bang or series of bangs and by EGT going well up.

If you don't slowly retard throttle, surge could damage blades by large bending moments. These rotot blades could come in contact with stator blades and that would mean some serious trouble or in another words, close fuel on that engine with "supersonic" speed or you could experience engine blow out.
Another way of detecting engine surge apart from loud bang and EGT increasing rapidly, is yawing of the airplane beacuse of the loss of thrust on affected engine.

To prevent surge, there are some devices build in the engine. You can read them in my previous post at the top of this page.

Conclusion: Surge is compressor blade stall which happens because of abrupt decrease of airflow through the engine.

Hope, this was of help.

Bizi

P.S.: To jet pilots: I tried to be simple, so please forgive me on some expressions.

Sorry, forgot to answer on other questions.

If surge occurs, you need to retard the throttle, sometimes all the way o idle. If it still continous, you need to shut down the engine. You can fly with one engine out, but I would turn to alternate.

Surge to my knowledge doesn\'t happen so often.

If it happens on T/O, it depends on which part of the take off. If it\'s before V1, you have to stop. If it happens after that you still have to take off and return for landing.

I didn\'t quite understand you last question.

Bizi

[Tonic Please]

....and the length of a piece of string is 'the same distance to each end as it is from the middle'

:8

[bizi2675]

ha, ha, ha.... I agree...

[G-ANDY]

Bizi2675 - thank you for in-depth reply! I THINK I understand it now! I didn't realise a surge was acutally a compressor stall. I always thought a surge was something to do with too much fuel going into the engine and it was making too much power - yet I couldn't have been more wrong!

So do the compressor blades actually change pitch or are they a fixed pitch?

Although you said it's an imbalance in the velocities which causes the the surging - how do you get imbalances in velocity - turbulence?

Cheers,

G-ANDY

[lomapaseo]

I think that Old Smokey and John Farley have summed it up KISS style.

A few additional comments based on all the posts so far. It isn't important usually to a pilot to distinguish between a stall and a surge, just follow the bold print in the manual.

There're a lots of reasons that either may occur and on some machines they have been way too common and a pain in the a$$.

If you don't retard the throttle in a few seconds then you can break the innards in the engine and after that there is no hope of a restart.

Temperature rise goes hand in hand with a surge and thats what distinguishes it from a flameout. If the surges are repetitive, bangs, pops or booms, then the temperature will take a liitle less than a minute to do big damage before you must retard the throttle and regain control of the engine. However, if the engine runsdown on its own with the temperature going off scale than you've only got a few seconds to shut off the fuel.

So in some cases retarding the throttle is enough, while in other cases where this is no response to throttle, you must turn off the fuel in order to even have a chance of restarting in flight.

When in doubt follow the manual.

[gas path]

Slowly retard the thrust lever until the surging ceases,
Some engines do not take too lightly to the T/L being 'milked' during a surge, the P & W Jt9d being a good example. In the case of that engine a 'slam' decel would ensure that the bleeds would all open by killing the fuel pressure in the JFC. and offloading the compressor. It would also help prevent the EGT from exceeding the limit........ (but one could always press the 'max indicator reset;) )

[Milt]

Surges at very high altitudes become a function of compressor RPM divided by the square root of the relative temperature.

So the colder the intake air temperature and the higher the RPM the closer will the most critical section of the compressor rotating blades be to a stall (as for a wing).

When the stall occurs the whole compressor usually unloads with a loud bang. At a high altitude this usually puts the fires out. This in turn becomes very embarrassing for a fighter if it is fighting.

Military flight testing carefully explores the surge boundaries and the effects of disturbances to the intake air flow. Typical disturbances can arise from gun firing, missile firing, wake turbulence or clear air turbulence etc. Such flight testing is usually performed in either of this planet's coldest upper air regions close to the equator where -90C is not unusual. One area is off the west coast of South America. The other is NE of Darwin, Australia.

Many aircraft with engines that may operate close to the surge boundaries will be fitted with "fuel dippers" which automatically reduce fuel flow during gun firing/missile firing and for several seconds after, followed by a controlled spool up to the pre condition power.

[bizi2675]

G-Andy!

Sorry, It took so long to reply. Have been nusy lately.
Anyways to answer you questions:

Compressor blades are fixed which means they don't change their angle of attack. Stator blades are completely fixed and don't move while rotor blades are fixed, but they are also a little bit loose. If you hear an airplane on the ground with engines windmilling, you will hear something like shacking bags of nails which is the sound of rotor blades windmilling. But as you increase rotational speed of the blades they become rigid beacause of the forces acting on them.

As far as the axial velocity and rotational velocity and their balancing goes:
Axial velocity is basically the speed of airflow entering the compressor and later, turbine. As the engine speeds up the volume of air entering decreases because of the higher air pressure at the high pressure end of the compressor.

When you slow down the engine, rotation speed of the compressor slows down too. This causes larger volume of air entering the engine. At the high pressure end of the compressor, this increased volume of air has difficulties passing through a small place that is available. Therefore it this air slows down. This reduction in axial velocity happens throughout the compressor and can cause STALL and if not checked and appropriately reacted, SURGE. Surge can, in the worst case, reverse direction of the airflow through the engine, which causes large bending stresses on the rotor blades and that could lead to collision of rotor and stator blades and could rapture the engine.

Therefore axial airflow velocity and rotor blade velocity need to be balanced. So, be careful how you handle your power controls.

Most common causes for compressor stall and engine surge are:

- excessive fuel flow due to abrupt engine acceleration (axial velocity decreases because because of the increased cimbustion chamber back pressure)

- engine operation above or below the engine design RPM parameters (increases or decreases rotational velocity of the compressor blade)

- turbulent or disrupt airflow to the engine intake - you were right about that :ok: (this reduces axial velocity)

- contaminated or damaged compressor components (axial velocity decreases because of decreased compression ratio)

- contaminated or damaged turbine (loss of power decreases axial velocity due to decreased compression ratio)

- excessively lean fuel/air mixture cause by abrupt engine deceleration (axial velocity is increased by decreasing combustion chamber back pressure)

Any of this could cause breakdown of airflow through the engine.

Hope, this was helpful. ;)

Bizi

I apologize for few spelling mistakes. :uhoh:

[bizi2675]

Milt,

I am just being curious, but what are requirements to become Test pilot?

Bizi

[G-ANDY]

Many thanks for your explanations into surging - very interesting stuff. I'll know doubt come accross it shortly in my ATPL study, and now I already understand the general principle of whats going on.

At least when I here two jet pilots talking about "jet engine stuff" I can now throw in my own two pence!!

Cheers,

G-ANDY

[bizi2675]

No problem at all, G-Andy. I'm glad I was helpful. That's why we're here for. Some day, you will maybe help me with something... You never know... :ok:

Take care,
Bizi

[PIGDOG]

it has happened before, after all!!;)

[Milt]

bizi2675

You may care to lodge the question re TPs as a thread on the Flight Testing Forum if a search of the topic does not produce enough answers.

[coolape]

Rotor Blades = aerofoil section and twisted along their length in a similar manner to that of a propeller. The twist ensures that each part of the blade meets the airflow at the correct angle of attack.

The twist measured from the horizontal is called the Stagger Angle or the Angle of Incidence.

Stator Blades = an aerofoil section. The airflow through the stator blades has its pressure increased and its velocity decreased. The stator blades also directs airflow onto the next set of rotor blades.


Stall = Localised breakdown of airflow within the compressor

Surge = Complete breakdown of airflow in the compressor.

Surge is recognised by loud banging coming from the engine with engine instrument fluctuation and a rise in the turbine gas temperature.

Stall always occurs before Surge.

GTE (Gas Turbine Engines) are designed to run at high rpms, at the upper rpms of 80% plus very few problems are encountered.

However below 80% that airflow problems through the compressor are encountered.

Rotor Blades stall for a number of reasons:
1) the angle of attack is incorrect (too high or too low)

2) airflow velocity through the compressor has stagnated.

At low rpms the airflow quickly loses its velocity energy, and before it leaves the compressor to go to the combustion chamber causes the compressor to stall and then surge.

also at Low rpms the compressor is supplying more air than the engine actually needs for combustion.

[enicalyth]

Excellent reading this. I would just like to add a small slant. Taxing my memory back to university and air squadron days when I studied mechanical engineering.

A compressor stage adds a tangential component to the axial component of airspeed velocity.

Consider the ratio of axial-to-blade velocity for three stages - front, middle and back and factor them so that middle is unity. (Agreed, which part of the blade, which part of the annulus?).

At the front this ratio is much smaller and at the back much greater than unity. Calculate the angle whose tangent is this ratio and subtract from it the blade angle. This is the angle of incidence.

The smaller the ratio of axial-to-blade velocity (sometimes written v/u) then the more steeply positive is the angle of incidence. Blades are just cambered airfoils and they’ll stall at some angle of incidence, both positive and negative. If the impinging air is directed against the convex surface of the cambered blade it is positively incident and the machine is a compressor, doing work on the air.

However as the ratio of axial-to-blade speed v/u gets bigger the angle of incidence becomes less steep, may become neutral and go on to become negative, the air is now impinging on the concave side of the cambered blade so the machine is a turbine. Instead of the shaft doing work on the air it is the other way about and pressure decreases rather than increases.

If you try for too much compression on a single shaft you can have the first stage stalled and the last stage turbining. Not nice.

But return to the front end. If the first stage suffers positive incidence stall the pressure drops off and the front-to-back pressure gradient across the stage is reversed and so is the airflow. Air is trying to come out the front and a shock wave (the bang) propagates within the engine and may put out the flame.

Once this contrary pressure is dissipated the airflow reverts to normal but if nothing has changed the compressor promptly stalls again. If the interval between stalls is the natural resonant frequency you have bang Bang BANG. The opposite is when the oscillation does not die away quick enough with corrective action, BANG, Bang, bang being hysteresis.

In some engines the compressor stage may be permitted to bleed air and reducing the pressure increases the axial velocity giving a better axial-to-blade velocity ratio v/u, lessening the airflow angle of incidence and unstalling the stage.

Injecting more fuel to try to accelerate out of difficulty does not help, it only drives the compressor into deeper stall and overheats the turbine.

Moveable inlet guide vanes, moveable stator blades, inter-stage bleed and multi-shaft engines are all tools that modify axial-to-blade speed ratio v/u and guard against compressor stall.

I think.

Hope I've helped too.

Best rgds

enicalyth

[gas path]

GTE (Gas Turbine Engines) are designed to run at high rpms, at the upper rpms of 80% plus very few problems are encountered.
Mr P&W's finest, the Jt9d, when ground running could let go big time and without any warning at any power setting above 1.35 epr (tandem bleed closure point) and boy did you know it. :ooh: If the person covering the start lever was of a nervous disposition they used to pull the fuel out when they jumped.:E