Turbine Surge

[GravityHater]

What procedure do some have for a persistent, strong surge in a typical turbine engine where engine T, oil p and such plus external indicators (no fire and smoke) are all normal and no other attendant problems such as in cruise at altitude?

Just read a short blurb on one and realized it is not in the a/c manual (Williams FJ 44) so wanted to check with the vast, untapped wisdom available on FI.com!

I suppose any engine problem with two or more turbines is pretty easy to manage, but I just havent seen an official, "you should do .........." I assume not all turbine failures are "BANG" I see smoke! Shut it down! sorta thing.

 

[Yellow Snow]

Sems to me if you are feeling a surge wih no temp, oil press, or other indications of a problem you may be seeing a prop governor problem. The only indications I have ever seen of a failure in a turbine involve a definate loss of power followed by temp and pressure indications. If you are seeing a power surge I would be suspicious of the prop.

 

[sky37d]

Sems to me if you are feeling a surge wih no temp, oil press, or other indications of a problem you may be seeing a prop governor problem.... If you are seeing a power surge I would be suspicious of the prop.

No prop

Just read a short blurb on one and realized it is not in the a/c manual (Williams FJ 44) so wanted to check with the vast, untapped wisdom available on FI.com!

The FJ 44 is turbofan. Not turboprop.

 

[avbug]

I don't believe any body is using the Williams FJ44 to drive a propeller...so checking the propeller governor on a turbojet engine may not help much.

Surging in the engine can be one of several things, but it's always going to be accompanied by other indications, depending on what powerplant it is to which you refer.

The most common surging will be a compressor stall or something leading up to it. Most commonly when this occurs (and occurance isn't common), it's either an air inlet restriction or internal damage (ammounting to the same thing), or an internal failure such as a bleed failure. An excessive angle of attack restricting or altering airflow into the engine intake can cause a compressor stall or surge. Damage to compressor blades can do the same thing. An acceleration bleed, used on some engines to prevent compressor surges or stalls at lower engine operating speeds and pressures, can stick open at higher operating speeds or pressures and cause surges, stalls, and in some cases, even a flame-out (had that happen on a T-56).

A bad fuel control, or improper pilot action can also lead to surging or a stall, as well as torching. A hot start in flight or on the ground, with fuel applied too early, ignition too late, inadequate airflow or engine rotation, excessive residual fuel from a previous start or shutdown, etc, can also cause these situations. In such cases, however, generally you'll see excessive torching out the back of the engine, and in cases involving inadequate rotation and airflow on start, you can also see it out the front in some cases...potentially like some compressor stall situations.

In each case, surging occurs when airflow through the engine is inadequate, or uneven. Pressure builds through the compressor section of the engine, which feeds the diffuser and then the combustor. This pressure needs to be high for several reasons, but when it suddenly drops (for reasons previously here described), combustor pressure can back up, and reverse airflow forward through the compressor, and there you have a compressor stall, surge, banging, and forward torching. When this is occuring, you're going to be seeing rapid temperature fluctuations. If you're using EPR, you're going to see large EPR excursions.

Some engines use engine pressure ratio (EPR) but don't display it to the pilot, being a fuel control matter internal to the engine. As rapid pressures change throughout the engine, fuel input varies, causing further fluctuations in both temperature, and speed.

Oil pressure fluctuations are usually another matter entirely, and signify a oil pump failure or blockage. If you see a complete loss of oil pressure, you may be looking at an engine that's been shut down, a gauge failure, or line or sensor blockage. Many engines use the same shaft to drive both the oil pump and something else, such as a tach generator. To verify that the pump really isn't turning, know your engine, and visit the secondary indications that will tell you if you're looking at a gauge problem, or a real problem.

Oil is exceptionally critical to any engine, but particularly to a turbine engine. Any oil pressure fluctuations should be taken seriously. Very seriously.

Oil pressure fluctuations shouldn't normally occur, even with changes in engine speed, as the pressure is regulated. Only an engine failure, pump failure, or regulator failure should permit these fluctuations. Otherwise, you're looking at a serious problem, or a gauge or indication system problem. A serious problem includes a break in a scavenge return line or a pressure line (fire hazard, oil loss, lubrication loss, etc). It may merely be loss of lubrication...either way, you may not have time to see a secondary indication such as an increase in oil temperature, before catastrauphic damage occurs to the engine...weather it maniests itself or not.

You described an engine that's surging but producing no additional indications. That would mean no temperature fluctuations, no gas generator or engine or fan speed changes, no oil pressure fluctuations, no fuel pressure or flow fluctuations, etc. What exactly are the circumstances to which you refer, and where did you see this happen?

 

[mar]

Cheap gas

What procedure do some have for a persistent, strong surge in a typical turbine engine where engine T, oil p and such plus external indicators (no fire and smoke) are all normal and no other attendant problems such as in cruise at altitude?

Not to casually brush pass the comprehensive post by Avbug but I'd add water to the list.

 

[avbug]

With water, like all other potential explainations (don't forget birds and volcanic dust), other notable fluctuations in engine performance will take place. The mystery here is that the question regards surging without any other indications...which should never be the case.

 

[Yellow Snow]

Didn't see the williams part obviously not a prop problem.

 

[Torqued]

Not knowing the specifics of your engine and not being familiar with Williams engines in general, there are a few things that are common to all turbine engines.
Higher pressure compressor stages are less efficient at lower RPM's than the lower (front) stages. So the front stages tend to try to cram more air into the engine than the higher pressure end can handle. Result is lower air velocity in the lower pressure stages and that gives the blades a higher angle of attack. At some point, the blades stall = surge. To fix this either air is bled off from the lower pressure stages to maintain a higher air velocity or the non rotating stator vanes can pivot and can be continuously set to adjust the angle of attack on the next stage blade.
The former method is usually simpler and cheaper and the bleed valve (or valves) may be controlled by an internal diaphragm or by an external control. The latter is complicated but more efficient. It involves linking all the adjustable stators and then controlling them with a servo.
IF all this stuff is working properly, damage to the high pressure blades can reduce its efficiency further and the surge will happen anyway.
The first thing I would do is verify by borescope that the high pressure blades aren't damaged. If not then the next step is to check the bleed valve(s) or variable stators for proper operation.
You'll want to use a shop that is very familiar with your engine or you may end up paying for someones learning curve.

 

[stingray]

Just get the dam thing check out by a qualified technician (mechanic). It should not be surging

 

[avbug]

I believe the poster is asking a theoretical question. It's not his engine, not his scenario, not really his problem...he's asking for information. Telling him to go get it fixed isn't helping.

Yes, the engine shouldn't surge. That much has been established. Yes, mechanical failures or damage can cause surging, which has also been established. Perhaps more germain to the posters question, it shouldn't be able to surge without attendant indications other than the surge itself. Other indications will be in evidence.

The poster also questioned regarding shutdown on turbine engines, and the reasons for doing so. That part hasn't been addressed yet.

Shutdown on a turbine engine isn't always cut and dried, any more than it is on a piston engine. Sometimes problems can be solved by carrying partial power, simply by reducing the power setting on the engine.

Shutdown proceedures for turbine engines, just like piston engines, are clear, and are part of every checklist and every aircraft flight manual. In the case of surging or a compressor stall, retarding the power may stop the immediate problem. In an aircraft that has the capability to continue flight without that engine, then shutting it down may be a safe, conservative thing to do. In other cases, this may not be desirable or the best course of action.

I had a door in the inlet system an aircraft separate and partially block the intake. The first indication I had was a low vibration, and then a rumble and a hooting characteristic of a compressor stall. By retarding power slightly on that engine, the symptoms disappeared. Without further indication of the problem, and based upon the circumstances and needs of the flight at the time of occurance, I elected not to shut it down. During inspection on the ground, the door was discovered to have failed.

On another occasion, a fire light illuminated shortly after takeoff. We were heavy and performance limited, and on an assignment that made the nature of our delivery somewhat critical. We made several 90 degree turns while sending the flight engineer aft to check for secondary indications at the engine itself, before the fire light extinguished. That particular aircraft was known to have false indications occasionally from bright light reflections into the engine nacelle, hence our turns. If the fire indication went out after turning to eliminate the light in nacelle, we would, and did continue with our assigned mission.

In such cases, shutting down may not be the first or best choice. One must consider the situation on it's own merits before deciding to simply shut the engine down. Fast hands kill. I've seen several pilots make hasty attempts to shut down the wrong powerplant over the years. In some cases this might be an annoyance, and in others it might be a critical or fatal mistake.

Gravityhater, I'm guessing that you're asking based on an article you've read. If you're flying a turbojet airplane, you're invariably going to get simulator based training that will address most questions you'll have. You may have questions or not see scenarios that you wish addressed, and in many cases the sim operator or instructor will modify or add to the syllabus to answer that question for you. I once had a question in a two-engine airplane about a dual engine failure, which was not covered in the recurrent training I was undergoing. At the conclusion of the session, I was given one to a landing, at my request...if you're flying an aircraft which has available simulator based training and you have specific questions about performance or actions regarding any particular scenario, request that it be set up so you can experience it. Most of the time, the folks conducting the training are going to be quite willing to help you out.

 

[GravityHater]

Yes it was a theoretical question.. Some people I fly with call me Captain What-if because I ask a lot of questions to cover things I have never been trained in. My initial training in the CJ did not cover this, I do not see it anywhere in the manual nor the checklists, and I read somewhere about it happening to an engine. Hence I wondered what the procedure was. I tried to limit the question by offering only a surging engine with no other indications (although that may be unrealistic), I thought it would help focus the discussion.
My impression is that I should probably at least throttle back and see the result. If the engine is happy and no abnormal indications I might continue. If the engine continues to surge, or we have visible smoke/flames or panel indications of abnormalities I would follow the shut-down procedure.... and continue the flight to London on the remaining engine. JUST KIDDING!! Hey what did happen to those blokes, anyway??

 

[mar]

From theoretical to real life

Ok then. Here's a real life scenario for you.

We're about 1.5 hours into the flight when #4 "surges". Personally I observe, all four thrust levers moving slowly *towards* idle. I personally observe #4 N1 decreasing and I personally observed a slight roll to the right and the autopilot make a correction.

Naturally this all takes about one second.

The next thing I observe is the FE applying Max Continuous Thrust on all *four* engines and #4 N1 has recovered.

The FE thinks the auto throttles were making an uncommanded roll back towards flight idle. The Capt is asking for more information as he was checking a chart and just caught the roll back when the plane rolled.

I don't know what to think. I thought the autothrottles were making a normal adjustment. We were in level cruise so I wouldn't consider a compressor stall and certainly not a bird. Besides that the engine recovered and continued to operate normally for the next six hours.

The only thing we could come up with was water. As for Boeing procedure (maybe I should have posted this earlier):

----------------------------

ENGINE STALLS

Engine stalls may be indicated by fluctuating engine parameters, abnormal engine noises, sluggish or no thrust lever response and/or high or rapidly rising EGT. Record all stall incidents and include altitude, TAT, N1 and peak EGT observed in the Maintenance Logbook.

IF AN ENGINE STALL IS INDICATED

1.THRUST LEVER...CLOSE
Check EGT and N2.

IF ENGINE STALL CONTINUES

2. ENGINE FAILURE/SHUTDOWN CHECKLIST...ACCOMPLISH

IF EGT AND N2 STABILIZE AT IDLE

3. THRUST LEVER ... ADVANCE
Advance thrust levers slowly. Check N1, N2, and EGT follow thrust lever movement.

IF STALL DOES NOT RE-OCCUR AND THRUST LEVER MOVEMENT IS NORMAL

4. OPERATE ENGINE NORMALLY.

IF STALL RE-OCCURS

5. OPERATE ENGINE BELOW THE SETTING AT WHICH THE STALL RE-OCCURRED.

OR

6. ENGINE FAILURE/SHUTDOWN CHECKLIST...ACCOMPLISH
Report all stalls for maintenance action, including engine parameters and conditions at which stall occurred.
-------------------------------

And that's exactly what we did. We operated it normally and then wrote it up.

Who know's what evil lurks?

 

[atpcliff]

Hi!

Avbug: Thanx 4 the interesting explainations.

I've had about 5 fluctuating oil pressure indications, with no other indications present. Each time we had serious problems with oil loss-something was broken on the engine. The first time we thought it was just an indicator problem. Most of the other guys I've flown with have thought that also, but I tell them we need to check the engine out, and each time we did something was wrong.

Here's a situation I know about that just occured. The aircraft was passing through 10K feet, when the Rt. engine started to surge. The flight crew cross-fed the fuel system so the Left system was feeding both engines, and then both engines started surging. The flight crew landed successfully. I don't know yet what was wrong.

Any ideas???

Cliff
HEF

 

[avbug]

Cliff,

Did that take place on a KC-135?

 

[TonyC]

Ok then. Here's a real life scenario for you.

We're about 1.5 hours into the flight when #4 "surges". Personally I observe, all four thrust levers moving slowly *towards* idle. I personally observe #4 N1 decreasing and I personally observed a slight roll to the right and the autopilot make a correction.

Naturally this all takes about one second.

The next thing I observe is the FE applying Max Continuous Thrust on all *four* engines and #4 N1 has recovered.

The FE thinks the auto throttles were making an uncommanded roll back towards flight idle. The Capt is asking for more information as he was checking a chart and just caught the roll back when the plane rolled.

I don't know what to think. I thought the autothrottles were making a normal adjustment. We were in level cruise so I wouldn't consider a compressor stall and certainly not a bird. Besides that the engine recovered and continued to operate normally for the next six hours.

The only thing we could come up with was water.

Icing? Any chance you could have encountered an area of moisture and temperature that affected all four engines, but #4, perhaps being a bit more worn and out of tune than the other three, exhibited ill effects first? You didn't mention EPR (perhaps because you don't have EPRs), but icing would cause the EPR to rise. If the autothrottles are tied to EPR, there might be an initial throttle reduction in response to the rising EPR, the #4 may have stalled in the reduction, and you caught it in time to increase throttles and overcome the transient icing condition. Just a theory....


How about your airpseed? If a brief, transient malfunction in the airspeed indicating system was telling the autothrottles that the airpseed was increasing, the normal response would be to retard the throttles. Again, perhaps the #4 was just the first to exhibit untoward behavior as the throttles were retarded. Again, just a theory...



But, really, do you expect us to believe the FE was actually AWAKE!?!?

Cliff,

Did that take place on a KC-135?

My guess is no. He referred to "the Rt. engine" (singular) and "both engines", as in two. I don't think that description would fit the case of the KC-135. He also referred to "the left [fuel] system", another term inconsistent with the StratoTanker. It has (I'm stretching my memory a bit here) at least nine separate tanks from which to choose.




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