sovietjet
Posts: 2549
Joined: Sat Mar 15, 2003 12:32 am

What Would Make A Jet Engine Get High EGTs?

Thu Dec 03, 2009 7:16 am

Someone I know is experiencing some high EGTs on his L-39 jet. I was wondering what might cause this. I figure there is a variety of reasons. He has cleaned the turbine blades and that helped a little but still needs to get the temps down more. Anyone here have any experience or recommendations?
 
crjfixer
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Joined: Tue Mar 18, 2008 5:15 am

RE: What Would Make A Jet Engine Get High EGTs?

Thu Dec 03, 2009 9:49 am

I have only worked on GE engines but the first thing i would check would be the probes themselves for false indications...More than likely it has 2 or more on each engine and you can isolate them and find a bad probe/rake.

Are all other indications normal? are the temps high at all power settings?
 
Tristarsteve
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RE: What Would Make A Jet Engine Get High EGTs?

Thu Dec 03, 2009 12:40 pm

Yes, I agree. Check out the EGT system first. An easy start is what does it read now. Should be the same as OAT!. Check connections and ballast resistors. They can be critical. On the RB211, even the stacking of the connectors on the lugs was important.
Then, if that is Ok, high EGT is due to high fuel flow. As an engine gets older the fuel flow and EGT will increase as the compressors and turbines get worn and have to work harder to do the job. I have seen over 50degC different EGT between two engines on the same aircraft, due to age. So you need to boroscope the engine to check the complete gas path for damage, not only blades and NGVs, but also the inside of the combuster for damage.. Take out the burners and check they are clean, they easily get clogged with carbon.

p.s., how do you clean the turbine blades? Is there only one stage?
 
CFMTurboFan
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RE: What Would Make A Jet Engine Get High EGTs?

Thu Dec 03, 2009 4:52 pm

Yes Borescope the engine. Pay particular attention to the compressor section. if this is worn or dirty, then it will not be as efficient as it should be, which mean that the engine fuel system may be putting in more fuel to get the same performance out of the engine.

If the gas path, and all the blades look good, consider doing a compressor wash.
 
avt007
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RE: What Would Make A Jet Engine Get High EGTs?

Fri Dec 04, 2009 12:30 am

Bleed air leaks will cause high EGT as well. Have any of the other engine parameters changed? Is he seeing higher fuel flow, or lower rpm, or is it temp alone?
 
sovietjet
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Joined: Sat Mar 15, 2003 12:32 am

RE: What Would Make A Jet Engine Get High EGTs?

Sat Dec 05, 2009 5:05 am

Thanks for the responses! We'll see what happens
 
AA737-823
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RE: What Would Make A Jet Engine Get High EGTs?

Wed Dec 09, 2009 2:58 pm

The EGT system is driven by thermocouple-type sensors. As such, things things RARELY, IF EVER LIE.
When they do, they lie low, not high.
And EGT indication that is above desired/expected is typically a true indication.

So, where do you go from here? Engine health. You say he's cleaned blades... good.
What about, as asked above, pneumatic leaks?
From there, you're really going to look at the FCU, but only if there is an associated error in rotational speed of the N2.
Not knowing which type of engine is on the L-39 (if he can't figure this problem out, he's more than welcome to donate the airplane to me... I promise I'll take good care of it), I can't get into specifics because I simply don't know them.

But, generally speaking, a high EGT indication is a TRUE indication, and is telling you that turbine blades, compressor blades, or FCU are having health problems or other issues.

And the FCU is a stretch, at that; the only thing an FCU (hydromechanical, which I'm assuming we're dealing with here, not digital) is sensing is RPM... not pressures, not temps, not what you had for breakfast, JUST RPM. So, unless there is also an RPM issue, stay out of the FCU.

He does realize that EGT rises continually as the engine ages... right? If it's gone past a limit, then... he's looking at a three-bolt fix, as we used to say on JT-8D's.
 
boeingfixer
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RE: What Would Make A Jet Engine Get High EGTs?

Wed Dec 09, 2009 11:38 pm



Quoting AA737-823 (Reply 6):
And the FCU is a stretch, at that; the only thing an FCU (hydromechanical, which I'm assuming we're dealing with here, not digital) is sensing is RPM... not pressures, not temps, not what you had for breakfast, JUST RPM. So, unless there is also an RPM issue, stay out of the FCU.

Actually most hydromechanical FCU's sense engine speed, compressor inlet pressure, compressor inlet temp, compressor discharge pressure and burner pressure with variations depending on the engine. It's slightly more complicated than sensing just engine speed.

For the OP, you should also check and make sure that the sensing lines going into the FCU are clear and open as well.

Cheers,

John
Cheers, John YYC
 
ex52tech
Posts: 553
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RE: What Would Make A Jet Engine Get High EGTs?

Fri Dec 11, 2009 4:08 am



Quoting Avt007 (Reply 4):
Bleed air leaks will cause high EGT as well. Have any of the other engine parameters changed? Is he seeing higher fuel flow, or lower rpm, or is it temp alone?

I would be looking for air leaks, but if the leaks are bad enough he can get a fire light. Wrap duct connections and flex bellows in tin foil and run the engine.

If you suspect the Fuel Control, or a sensing line leak like burner pressure, or discharge pressure, then check the acceleration of the engine to see if it is slower than recommended.

As already stated Borescope, will tell a lot. There may be some compressor blade tip clearance problems or turbine blade tip clearance issues. Compressor washes help, but usually not when the EGT is exceptionally high. Is this a high time engine? If at all possible try and borrow a fuel control from another operator and see what happens, just for process of elimination sake.

Quoting AA737-823 (Reply 6):
The EGT system is driven by thermocouple-type sensors. As such, things things RARELY, IF EVER LIE.
When they do, they lie low, not high.

That has always been my experience with the EGT system, they are very reliable.

High EGT is almost always F.O.D. or a wear issue.
"Saddest thing I ever witnessed....an airplane being scrapped"

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