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EGT - How hot is too hot?


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1 hour ago, A64Pilot said:

Picking nits here, but I believe gauge markings are considered placards, and the AFM and placards both are mandatory.

I’m not disagreeing with you on ignoring the redline though, I do wonder why it’s there.

Looks like they simply used the same gauge for both Bravo and Ovation.

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17 hours ago, PT20J said:

That redline is suspicious. 1650F is a common TIT limit (turbos have close tolerances, the turbine spins at very high rpm so there is a lot of centrifugal force and the metal expands when heated. At some temperature, the turbine wheel will drag on the case). Perhaps that gauge marking is a holdover from a TIT gauge.

If there is no limit listed in the Limitations section of the AFM, then there is no limitation according to the FAA (the limitations section of the AFM is the only part of that document approved by the FAA and exceeding an operating limitation listed in the AFM is a violation of FAR 91.9).

In the picture posted, the Mauritz EGT gauge shows an EGT of 1552 F while the engine monitor is showing EGTs over 100 F  lower. I would just ignore the Mauritz EGT gauge.

Skip

Concur with all of the above. Will add that it’s highly unlikely that an IO550 is producing single cylinder EGT’s in the 1580° range much less 1650°. 

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3 hours ago, A64Pilot said:

Picking nits here, but I believe gauge markings are considered placards, and the AFM and placards both are mandatory.

I’m not disagreeing with you on ignoring the redline though, I do wonder why it’s there.

I'll add another nit for fun... since the JPI is an 800 it is not primary, and the Moritz cluster is primary - so it technically cannot be "ignored." However...

... on my JPI 900 there is no such redline on the EGT (like there is for CHT) - and those folks are absolutely manic about duplicating all appropriate gauge markings on their primary instrument replacements.

But then, maybe the digits turn red when the limit is reached (if it even could be) like they turn blue for temps in the blue arc.

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23 hours ago, Hank said:

Depends upon the alloy. I've heated some machined tool steel parts to 2200ºF in a furnace as part of the hardening process. They didn't melt . . . .

Good point.   2500F is probably a better number.

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For whatever it’s worth turbines can suffer from blade creep, that is if you heat them hot enough and spin them fast enough for long enough they will grow in length and then of course rub the case. They can also crack too.

Usually it’s both a temp and time limit, so it’s not usually just how hot, how long when that hot figures into it.

Carrying over a gauge from a turbo airplane makes the most sense, but I’m surprised the FAA didn’t catch the redline on a gauge that’s not in the POH, they normally look at those things pretty hard, perhaps that was a change after Certification?

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As others have mentioned, 1650f is not in the AFM, however, for some reason, I’ve always thought that 1,650f was the max.  Don’t know why though.  The attached document from lycoming recommends egts of less than 1450f for longevity.  Lee
 

https://www.lycoming.com/content/tips-extending-tbo#:~:text=A maximum 1%2C450˚ F,420˚ F is recommended.

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13 hours ago, laytonl said:

As others have mentioned, 1650f is not in the AFM, however, for some reason, I’ve always thought that 1,650f was the max.  Don’t know why though.  The attached document from lycoming recommends egts of less than 1450f for longevity.  Lee
 

https://www.lycoming.com/content/tips-extending-tbo#:~:text=A maximum 1%2C450˚ F,420˚ F is recommended.

I believe that article is for turbo motors, specifically the TIO-540J

Not saying there isn’t one, but I’ve never seen a recommended max for NA motors.

Piston temp and valve temps are much lower than EGT, I typically cruise around 1400F and peak about 1475F but the typical piston alloy will melt at roughly 1200F.

Forced induction can raise temps well beyond what a NA motor can get to, I don’t think my NA motor can get to 1650 under any conditions, so I suspect there is no need to spec a max EGT for a NA motor, because it just can’t hurt itself with EGT.

 

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My G1000 has a redline on the MFD. N/A engine. That said, I don't see much over 1550. Personally I would not trust the number an EGT gauge puts out unless I had calibrated it with a pyrometer recently. As EricJ points out they deteriorate over time and read higher. It is cheap and easy enough to pop in a new probe and see what's up if the current reading bothers you.

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On 5/30/2023 at 8:15 AM, GeeBee said:

My G1000 has a redline on the MFD. N/A engine. That said, I don't see much over 1550. Personally I would not trust the number an EGT gauge puts out unless I had calibrated it with a pyrometer recently. As EricJ points out they deteriorate over time and read higher. It is cheap and easy enough to pop in a new probe and see what's up if the current reading bothers you.

Even with a calibration it’s temporary, during Certification test flights I had to have everything calibrated within 90 days, even if not installed 90 days was the limit.

Then there are single point calibrations and multi point, FAA required multi point cals, which they should.

I got stung once with a Scale, we were Certifying the S2R-T660 and discovered one cell read incorrectly, it was within the 90 day cal but was reading incorrectly, so when did it go bad? That started a mess, some demanded we re-start the Cert flights from the beginning and repeat them all, we settled on just repeating the ones at the max fwd and rearward limits.

I had the welder weld up a steel box and we put 600 lbs of lead shot in it, then every time before we weighed an aircraft we took the overhead crane and weighed that box with all three cells.

On edit, we “Jet-cal” the older aircraft in the Army OH-58, UH-1, OV-1’s etc  to calibrate the temp indicating system.

But the newer aircraft, AH-64’s, UH-60’s etc we didn’t. I always wondered why not, but we didn’t.

Having said that on my MVP-50 I installed on my Maule I had 14 thermocouples, 6, egt, 6 cyl, 1 oil and 1 OAT. If left in a hangar for more than 24 hours they would all read within one degree, which astonished me, maybe these newer ones are dead on balls accurate?

 

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You are correct. The only time an EGT probe is correct is when it is new. After that it is in continuous degradation. The reality is, most degradation is small enough to live with since EGT on a recip is a "relative" measurement. However if it pegs the case or goes over redline, it is time to look at it. Turbines is a whole other kettle of fish.

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27 minutes ago, GeeBee said:

You are correct. The only time an EGT probe is correct is when it is new. After that it is in continuous degradation. The reality is, most degradation is small enough to live with since EGT on a recip is a "relative" measurement. However if it pegs the case or goes over redline, it is time to look at it.

Not necessarily. Mine went over redline on one flight, turned out my left mag had died and still-burning fuel was going into the exhaust. 

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On 5/25/2023 at 11:11 AM, Hank said:

Depends upon the alloy. I've heated some machined tool steel parts to 2200ºF in a furnace as part of the hardening process. They didn't melt . . . .

Also, well before melting, material strength drops radically.  This is the common failure point.  Things will deform before it starts dripping into a puddle.  Not that you would see that under the cowling during flight, either way.  

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1 hour ago, Hank said:

Not necessarily. Mine went over redline on one flight, turned out my left mag had died and still-burning fuel was going into the exhaust. 

So the probe was correct. It was measuring the correct temperature as a result of your faulty engine.

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31 minutes ago, ilovecornfields said:

I guess my next questions should be if anyone knows the part number for the factory EGT probe in an Ovation 2. Seems easy enough to replace it and see what the readings are.

Sometimes cleaning the connections to the probe can correct errant readings

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4 hours ago, A64Pilot said:

Why would a turbine be different? Same type of thermocouple, just usually a turbine has several and an average measurement is taken.

30 min temp limit on what I used to fly was 867C which is right at 1600F, so even temps are close.

One, a N/A recip temp is relative. If you remember many EGT gauges have no numbers. Turbine EGT have absolute numbers 

Two, turbine engine EGT probes are faster reacting and more robust. Reaction time is more critical on FADEC engines.

Three, because thrust is often limited by maximum EGT on a turbine, accuracy of the absolute numbers is paramount. Which is why before test cell running or trimming a fuel control you calibrate the EGT. Failure to do so can result in underperformance or compressor stall if the engine over temps while showing some EGT headroom (depending on turbine and compressor condition)

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I haven't studied thermocouples since an undergraduate instrumentation class many years ago. But, I don't remember drift over time in service being a really big deal. I did a little prowling around the Internet. Type K thermocouples have a sensitivity of about 41 microvolts per deg C. I found many descriptions of aging and drift in Type-K thermocouples, but it seems that it's only an issue where precision measurements are required and a change of a couple of degrees makes a difference. An aging curve I found shows that the effect of running at 500 deg C for 1000 hours causes an increase of about 65 microvolts or 1.6 deg C (3.6 deg F). From: https://www.msm.cam.ac.uk/utc/thermocouple/pages/Drift.html

DRIFTbis.jpg.a10370569518351d7ca4de08fc150602.jpg

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12 hours ago, GeeBee said:

One, a N/A recip temp is relative. If you remember many EGT gauges have no numbers. Turbine EGT have absolute numbers 

Two, turbine engine EGT probes are faster reacting and more robust. Reaction time is more critical on FADEC engines.

Three, because thrust is often limited by maximum EGT on a turbine, accuracy of the absolute numbers is paramount. Which is why before test cell running or trimming a fuel control you calibrate the EGT. Failure to do so can result in underperformance or compressor stall if the engine over temps while showing some EGT headroom (depending on turbine and compressor condition)

I understand why ITT / EGT is important in a Turbine. I’ve got way more turbine time than recip, and ALL my test pilot time is in turbines. Majority of my mechanic experience is with turbines too. It’s important because most turbines are easily over-temped and over-temping is destructive and very expensive.

My point is that Turbine probes don’t deteriorate significantly as they age, if they did then there would be a life limit on them, and there isn’t, because of the importance of not over temping a turbine, on the Million dollar plus GE engines in Apaches and Blackhawks we don’t even calibrate them, and turbines live much longer lives than recips.

As far as I can tell the thermocouples for a turbine and a recip are nearly identical, so why do recip ones deteriorate so quickly?

I don’t think they do.

If an engine is slowly over time increasing EGT, my bet it’s from compression loss from normal wear, not degrading probes.

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Price a turbine probe, and a recip probe. Then you will understand. Also generally turbine probes are exposed tip and recipe are closed. I can also state from experience, because I wrote the checks, that P&W Canada will not warranty factory performed hot sections without new probes.

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18 hours ago, ilovecornfields said:

I guess my next questions should be if anyone knows the part number for the factory EGT probe in an Ovation 2. Seems easy enough to replace it and see what the readings are.

Generally, the lead to the probe will have an Alcor part# on a tag that you can match up. I looked it up and it shows an 86255 Alcor for S/Ns 29-001 thru 29-332 ( I am assuming you are NOT G1000). I would call them 1-800-354-7233 and they can verify the correct part.

https://alcoraero.com/product/egt-tit-clamp-style-probe/

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I understand turbine prices, and turbine ITT harnesses aren’t single probes either, I’m trying to remember a Garrett may be as it’s an EGT measurement, not an ITT on a Garrett.

While PWC may not warranty their hot sections without changing the harness it’s not a life limited item, it’s not required to be changed. No one that I knew sent an airplane to PWC for a hot section inspection, usually done by an IA or Covington Aircraft. I guess Corporate is a different world.

None of this has anything to do with the fact that a K type EGT probe, is a K type EGT probe It’s simply an Alumel and Chromel wire connected together that makes a tiny current when heated, that’s it. Bigger wires for higher temp.

I’ve not seen one wear out 

On edit, aging and drift exist, but the changes are minor a few degrees over 1,000’s of hours.

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13 hours ago, PT20J said:

I haven't studied thermocouples since an undergraduate instrumentation class many years ago. But, I don't remember drift over time in service being a really big deal. I did a little prowling around the Internet. Type K thermocouples have a sensitivity of about 41 microvolts per deg C. I found many descriptions of aging and drift in Type-K thermocouples, but it seems that it's only an issue where precision measurements are required and a change of a couple of degrees makes a difference. An aging curve I found shows that the effect of running at 500 deg C for 1000 hours causes an increase of about 65 microvolts or 1.6 deg C (3.6 deg F). From: https://www.msm.cam.ac.uk/utc/thermocouple/pages/Drift.html

DRIFTbis.jpg.a10370569518351d7ca4de08fc150602.jpg

I suspect the drift that's happening on my airplane may be a ground or something changing, but it's strange behavior.   The CHTs have been fine, the reported live system voltage has been consistent, etc.   If and/or when it gets bad enough that it needs attention I'll try to figure it out. 

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35 minutes ago, EricJ said:

I suspect the drift that's happening on my airplane may be a ground or something changing, but it's strange behavior.   The CHTs have been fine, the reported live system voltage has been consistent, etc.   If and/or when it gets bad enough that it needs attention I'll try to figure it out. 

I think the primary failure mode of thermocouples is open precipitated by erosion of the enclosure. Other issues are probably wiring or the electronics.

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Thermocouples by their very nature cannot be inaccurate. I believe that the primary failure mode is insulation failure of the wires which allows wire to wire shorts downstream from the tip of the probe. this will form multiple junctions and what you see is average temperature of multiple parallel junctions. Considering that the false junctions can be a ways from the hot spot where you are trying to measure, it will indicate a lower temperature then the process you are trying to measure. There is really no failure mode that will make them read higher then they should, with the exception of a short to ground. This is highly dependant on the amplifier circuit reading the thermocouple voltage. Some amplifiers are designed to read grounded thermocouples and won't be affected, some are cheaper and could be affected. The other failure mode is burning the junction open, in which case you will either get no reading or some full scale reading depending on the amplifier.

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