Turbo fan anomaly... driving me nuts and I don't have many left to lose

[fdr]

TFE731, a blender.

Has a bunch of stages for sqeezing stuff, both axial and centrifugal... great for ears.
Has a bunch of bunsen burners, the compact kind...
has a set of HPT and LPT, that drives the blender at the front through a RGB. The very first (maybe) geared turbo fan. Yaay.
Shows N1, N2, ITT, F/F etc,
Has levers to the FCU & a DEEC that controls N1, which has some limited authority

Issue is...

Matched engines....
N1/N2/FF and thrust lever, all match, with DEEC on or off, and plane drives in a straight line (disregarding my version of IF)
ITT, 25C higher on one engine.
match ITTs, got a stagger of everything, N1/N2/FF and Thrust lever are all low on the high ITT side. Add rudder trim to keep face off the side windows.

Swap ITT gauges, issue stays with engine.
Swap engine positions, issue follows engine.

Borescope the inny bit, clean, burners same. Engine to engine, summa summa.
Surge bleed valves... through shop with engine and apparently OK.... (but not verified independently)

So...

I've had bad thermocouples on jets before, most of the time they gave low EGT/T5/TOT/ITT.... Is there a mode of degrading of the thermocouples that causes an increase in indicated temp? Sitting on a cold plane, they are within 1 degree.... the issue arises at idle, and remains almost constant differential up to full thrust.

My confusion arises from ever bothering to own aircraft. Except for the Pitts, and maybe the T28B, one cuz everyone should fly a Pitts particularly single-seat ones, and the other as starting an 1820 is a lot of fun.

A degradation of aero path in an engine should exhibit an increase in ITT, FF and also change N1/N2 relationship (well almost always for the last one)

I passed this issue past the OEM, and got blank stares for my efforts. The response is that it is within limits, which is only true to a certain point, as we are affecting thrust output when we use an ITT setting rather than an N1.

Anyone with an answer to the observations shall not go unpunished.

[FlightDetent]

I don't think it is the real case, but otherwise all things being equal (just being imaginative, and surely you considered)
- the physical location of the sensor is not the same (something mounted 180 degrees around)
- the power supplied to the sensor is not identical (RF / supply issue / grounding / loose currents / frequency).

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[Sword_2]

Bit different to jet, but the PT6's that I'm familiar with (purely from a pilot perspective), have had issues before with over-temp, or approaching limits, report to maintenance and they change out the' ITT trim stick'.
In very lay terms they explained it as applying a correction to the ITT gauges etc, to keep the pilots happy, but nothing (engine-wise) really changed.
Does the Garrett have anything similar?
I can't post the link but Google 'PT6 trim', and the top article from Kingair Magazine explains it in more detail.

[sycamore]

There was a similar problem on Rotorheads about different TITs when matching TQs(different engines ) a couple of weeks ago.Lots of suggestions,but not sure if the guy in Malaysia found the answer...My suggestion was to swap all thermocouples/check if bent,burned ,aligned/incorrect length,ie washers/spacers,between engines....

ed.Rotorheads,page2 -AS355 F2 Engine problems; I know power is measured differently,but it might give you ideas..another one,comp.wash..

[Vessbot]

Is one engine older than the other?

[Sig229]

I'm not trying to be a jerk, but you're really freaking out over 25 degrees? I've never heard of any requirement for engine parameters to match up exactly, and in almost no case will they unless both engines are identical in every way. The split you're seeing could easily be explained by a minor difference in wear, bleed valve position, temperature probe condition, etc. I've seen a 100C difference in the 737 during cruise, as you can imagine airlines swap engines all the time so it would be easy to have a mix of old and new. Now if you were seeing that much difference (or more) in new(er) engines, or if you suddenly went from a small split to a large one with no explanation it might be worth looking into. But if it's only 25 degrees and doesn't change wildly then there shouldn't be any concern.

And again no offense, but trying to match ITT and therefore throwing N1 and N2 out of whack is just comical. Besides making the airplane out of trim as you mentioned, having the engines out of sync and the resulting wah wah wah had to be quite annoying!

[fdr]

Originally Posted by Sig229

I'm not trying to be a jerk, but you're really freaking out over 25 degrees? I've never heard of any requirement for engine parameters to match up exactly, and in almost no case will they unless both engines are identical in every way. The split you're seeing could easily be explained by a minor difference in wear, bleed valve position, temperature probe condition, etc. I've seen a 100C difference in the 737 during cruise, as you can imagine airlines swap engines all the time so it would be easy to have a mix of old and new. Now if you were seeing that much difference (or more) in new(er) engines, or if you suddenly went from a small split to a large one with no explanation it might be worth looking into. But if it's only 25 degrees and doesn't change wildly then there shouldn't be any concern.

And again no offense, but trying to match ITT and therefore throwing N1 and N2 out of whack is just comical. Besides making the airplane out of trim as you mentioned, having the engines out of sync and the resulting wah wah wah had to be quite annoying!

Actually, yeah, it is frustrating... Many power settings are to a temp, and that means either accepting a difference or not. These two engines have common history, the other plane with common history matches remarkably, as in pretty much exactly.

In the end we respect the ITT limits but set equal N1s with one engine as a master, this clunker having no sync for the reasons you note, but look for reasons that the display is as it is.

[Sig229]

Originally Posted by fdr

Actually, yeah, it is frustrating... Many power settings are to a temp, and that means either accepting a difference or not. These two engines have common history, the other plane with common history matches remarkably, as in pretty much exactly.

In the end we respect the ITT limits but set equal N1s with one engine as a master, this clunker having no sync for the reasons you note, but look for reasons that the display is as it is.

I used to fly the Lear 45 with TFE-731s, it was long enough ago I don't recall many of the finer details. We did have engine sync, could couple to either N1 or N2. I don't recall ever being temp limited in that one engine was operating hotter enough than the other to the point it would affect operation. Same in the 737, the EEC does all the work determining thrust setting based on ambient conditions. The only time I've ever been limited by ITT was flying turboprops, we would set power by torque unless we hit maximum ITT then that would be max power for that current condition. If the engine "temped out" before reaching a minimum torque value there was something wrong and maintenance would have to look into it.

[Private jet]

The high ITT engine is degraded or damaged in some way. In order to generate as much thrust as the "good" engine, it is burning more fuel/requires more heat and it's not being usefully used, hence the higher than normal turbine temp. Compressor damage (lower pressure ratio= less thermal efficiency), leaky surge bleeds, degraded turbine (pressure/temp drop across the turbine is in line with power extraction and is a known quantity at the design stage). Is there a mismatch between N1 & N2? More info required. Line up the fuel flows and tell us what the other parameters do.

[fdr]

Originally Posted by Private jet

The high ITT engine is degraded or damaged in some way. In order to generate as much thrust as the "good" engine, it is burning more fuel/requires more heat and it's not being usefully used, hence the higher than normal turbine temp. Compressor damage (lower pressure ratio= less thermal efficiency), leaky surge bleeds, degraded turbine (pressure/temp drop across the turbine is in ilne with power extraction and is a known quantity at the design stage).

quite true, in this case however there is no increase in fuel burn at all if N1s are matched with a variation in ITT. That is the oddity. Over 1000 tons of fuel burn there is no difference in fuel burns.

Sig229 I used to fly the Lear 45 with TFE-731s, it was long enough ago I don't recall many of the finer details. We did have engine sync, could couple to either N1 or N2. I don't recall ever being temp limited in that one engine was operating hotter enough than the other to the point it would affect operation. Same in the 737, the EEC does all the work determining thrust setting based on ambient conditions. The only time I've ever been limited by ITT was flying turboprops, we would set power by torque unless we hit maximum ITT then that would be max power for that current condition. If the engine "temped out" before reaching a minimum torque value there was something wrong and maintenance would have to look into it.

A number of power settings are specifically ITT, the LR45 has a heavily derated engine, the TFDE731-20AR which is around a 20% derate from the same configuration in the -40. For those engines, that's around a 50C difference in ITT margin.

[Private jet]

If you line up the fuel flows and the N1's are the same then it's an ITT measurement problem, & if as you say it follows the engine position it is obviously within the engine itself. Probably a wiring issue if the thermocouples are good

[fdr]

Originally Posted by Private jet

If you line up the fuel flows and the N1's are the same then it's an ITT measurement problem, & if as you say it follows the engine position it is obviously within the engine itself. Probably a wiring issue if the thermocouples are good

That's what we are gonna go do in the next week or so. I can understand a low reading, high reading is odd. there is always the possibility that the other engine is in fact reading low... If the probes get swapped, then the replacements probably have to cycle through the engine to prove the point. There had been a split between the indicated and DEEC recorded data of the same amount, then they elected to match, more or less.

[Private jet]

Originally Posted by fdr

there is always the possibility that the other engine is in fact reading low...

NO. IF a genuine fault exists it will always change more than one parameter on the affected engine. If only one indication is abnormal it is an instrumentation fault.

[tdracer]

It's probably worth noting that the thermocouples that are used to measure turbine gas temperatures are not the most accurate way to measure temperature.
RTD's - Resistive Temperature Device - are very accurate, but can't handle the temperature ranges in a turbine (they work great for things like ambient temperature and total air temp), and are really easy to fault detect. Thermocouples not so much.
It's quite possible that the thermocouple on one engine isn't reading accurately.

[Private jet]

Yes, my poor choice of the word "instrumentation". I should have said "measurement system".

[EXDAC]

Originally Posted by fdr

I've had bad thermocouples on jets before, most of the time they gave low EGT/T5/TOT/ITT.... Is there a mode of degrading of the thermocouples that causes an increase in indicated temp?

Yes, an open circuit ground reference lead can cause spurious voltage to be added to the thermocouple voltage. There may be small voltage differences between ground at the thermocouple and ground at the sensing system. Not a failure mode of the thermocouple itself but the thermocouple wiring.

No experience at all with turbo fan engines but I experienced exactly this problem on my Lycoming IO-360 with electronic engine indicating system. Only one of the 8 thermocouple inputs was impacted.

[tsumini]

i used to cover airline and Part 91 corporate operators with on site engine performance and support before retirement. I
can attest that a mismatched parameter will drive operators bonkers. Especially a pair of engines
with matching parameters Before OH thaa come back mismatched after OH.
From your information I would guess that one of the ITT harnesses is the culprit.
I'd suggest bench check harnesses if BC available or swap one or both to identify bad harness.

[Cough]

Just on the bleeds - Do you get the same temp rise when the bleeds are turned on? Do you get the same temp rise when the engine anti ice is turned on?