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High CHTs after TB21 turbo engine overhaul

Dear all,

I hope to find some more knowledgeable people with regards to Lycoming turbo engines than myself.

I have had my Lycoming TSIO 540 overhauled last year and since then I battle very high CHT in full power climb out. While I expected to have higher temps initially I did not expect CHTs to redline after 4-5K ft of climb.

First I thought it might have something to do with baffling but that part seems ok and it would be strange to see temps getting so high after just 5K ft of climb. I then checked if fuel flow was still the same as before and it also looks correct. I get about 115lt/h on full power.

I then realized that the MP pressure was getting to 35-36 psi on take-off where according to book it should be around 33psi. Could it be that 3 psi more boost could have such a noticeable impact on CHTs? I don’t gain much by throttling back to 33 psi as at the same time my fuel flow drops.

Could this be the reason for temps to rise to 470 in such a short period of time?

thanks for your help.

Placido

LSZH

Something looks very wrong indeed.

How are your EGTs ?

LFPT, LFEH

placido wrote:

I then realized that the MP pressure was getting to 35-36 psi on take-off where according to book it should be around 33psi. Could it be that 3 psi more boost could have such a noticeable impact on CHTs? I don’t gain much by throttling back to 33 psi as at the same time my fuel flow drops.

Hi Placido, I think I saw your plane flying recently… taking off right before mine.

As for the MP: what is the red line? a book value of 33 with full throttle suggests it would be about the max power this engine should produce? With 35/36 is that overboost? What kind of wastegate does your turbo have, automatic or fixed? If it’s automatic, I reckon this is where I’d start looking. If it’s fixed, you would have to set your TOP manually anyway, like e.g. in the Seneca II or Turbo Arrows.

If it overboosts then the CHT’s are very much explained.

Who did the overhaul? I’d involve them asap.

LSZH, Switzerland

Insufficient fuel flow at full power will definitely cause high CHTs. Have the shop set it at the higher limit of what is allowed. With a turbo, you’ll be able to use it.

Also, have the wastegate looked at, I don’t know how the turbo in your engine is managed – if it is fully automatic, or not. In mine I have to consciously restrict the overboost on take-off, but mine is designed like that, with a fixed wastegate tuned for a 18k critical altitude.

tmo
EPKP - Kraków, Poland

The TB21 has an automatic waste gate but it needs to be set to a certain psi level at sea level. That number would be 32.8 psi at SL and 15 degrees OAT.

Mine pushes between 35 and 36 on take-off from Zurich which is about 1700ft MSL. I wonder if 3 psi more boost without increasing fuel flow can have such a noticeable effect on CHT.

LSZH

I wonder if 3 psi more boost without increasing fuel flow can have such a noticeable effect on CHT.
eq.

I believe you have a combination of two problems, your wastegate is not operating properly and this is causing your engine to produce 10% more horsepower.
Having the same fuelflow but with 10% extra horsepower means that you are running too lean- this further aggravates the problem.

As Jujupilote said : EGTs? I would add : compare TIT too.

I believe it is an easy fix:
1. readjust the wastegate or replace if defective and
2. set the fuel flow to the upper limit of what is allowable . in my case I even had them exceed this limit and never looked back. It is very easy to lean from excessive fuel flow but impossible to do the opposite in flight

LSGG, LFEY, Switzerland

A lot of TB21 owners had lots of downtime due to turbo issues and a lack of expertise among European dealers.

However, others say the turbo installation is straightforward…

10% more air would certainly push the engine from the required ~ 150F ROP to somewhere leaner and would push the EGTs and thus CHTs well up. Same with the TIT which I believe is a limiting factor on turbo engines.

Maybe @paul knows more; he used to have a TB21 for some years, and got to know his turbo better than most

Administrator
Shoreham EGKA, United Kingdom

OK, here are my thoughts.

Check the easiest things first.

1. What airspeed do you use for the climb? The POH says optimum climb is 94 KIAS however I think this is too slow. I used to keep the speed up to about 120 KIAS, at least for the first 8,000’ or so. If you are at max weight and going high, you will need to bring the speed back. I found that CHTs are correlated to OAT, but only by about 10 degrees or so and even at 94 KIAS you should still be able to manage the temperatures within reasonable limits (i.e. closer to 400).

2. How are you measuring the CHTs – is this with the factory gauge or do you have an engine monitor. If you have an engine monitor:
- Are the CHTs the same as the factory gauge?
- Are you using bayonet probes or gasket probes? Bayonet probes give more accurate readings but usually cannot be fitted on the cylinder with the factory probe (usually #6)
- Did you had any of the CHT probes changed with the overhaul? Bear in mind that there are two types of CHT probe, the J type used in GEM monitors and the K type used in JPI monitors. Note you can use J type probes in a JPI monitor but you have to adjust the settings otherwise the monitor will over-read by about 100 degrees. I’ve know engineers to fit the wrong probes before now!
- Have you had the engineers fire a temperate gun at the engine on a ground run? It’s not particularly accurate but might give you an indication whether the readings you are seeing are in the right ball park.

3. Baffling could be an issue, but in my experience, unlike some aircraft, the turbo engine in the TB21 is not particularly sensitive to poor baffling (bear in mid there are no cowl flaps on the TB21). Just make sure there is a good seal at the back where it meets the top cowling.

4. Dumb question, but are you sure the engine overhaul fitted the correct cylinders? I can’t imagine that this is the issue but, unlike the TB20, the cylinders on the TB21 have extra long cooling fins. I’m not even sure if it’s possible to fit the wrong cylinders but maybe worth checking. They normally have a blue paint mark to indicate nitrided cylinders.

5. How many hours do you have on the new cylinders and were they properly broken in? New cylinders can give slightly higher readings for the first few hours until everything has bedded in. Also, what oil are you using now? a synthetic multi-grade is by far the best for a turbo engine rather than a straight w100.

6. Are there any signs of exhaust leaks under the cowling? Look carefully around the exhaust manifold as any escaping gasses that fine their way to the cylinders will cause an erroneous reading.

7. Fuel flow doesn’t seem to be an issue here. I agree with the comments that fuel flow needs to be set on the ground to give the maximum possible flow (about 28 USG/Hr). 115 Litres/Hr seems fine to me – I used to get about 105 at full power and that was fine.

8. Waste gate issues are unlikely. This is just a dumb lever that is controlled by oil pressure which is regulated by the density controller (more on this below). By all means check on the ground that the waste gate is moving freely. You will probably need a spanner to do this and you can use plenty of mouse milk to lubricate it.

9. The MP is a bit on the high side and the book says that the density controller should be set to 32.5" at sea level in ISA standard conditions. However, the red line on the MP is 38" to allow for high OAT conditions at high altitudes where you will naturally see a higher MP. 35-36 doesn’t sound to me to be a big issue, especially wit the higher fuel flow that you are getting, and this could be quite a normal reading in some circumstances. The only way to adjust the maximum MP is to adjust the density controller on the ground. To do this properly requires quite an expensive instrument that not many shops will have. However, small adjustments can be made by breaking the locking wire and turning the adjusting bolt a small amount (only about 1/16th" at a time), and then re-locking. This is quite difficult to do accurately as re-locking the adjusting nut can cause it to move!

In summary, first make absolutely sure that the temperature readings you are seeing are genuine and are not artificially over-reading simply because you have the wrong probes. If you are convinced the temperatures are genuine, then I think you will need to speak to the overhaul shop in the first instance.

Good luck!

Paul wrote:

However, the red line on the MP is 38"

Ok, that means no overboost. That is something already.

Paul wrote:

I think you will need to speak to the overhaul shop in the first instance.

This I think so too. They might have a pretty good idea what the reason could be from the reports.

Last Edited by Mooney_Driver at 18 Nov 19:49
LSZH, Switzerland

Mooney_Driver wrote:

38"
Ok, that means no overboost. That is something already.

Even if the extra 3" will not cause damage,they will indeed cause higher temps. If the reading of other causes in @Paul’s excellent list does n ot drive you in other direction, then I would lower the max MP setting. And yes, closing the throttle slightly to adjust MP will not have the same effect as having the correct mas MP setting since since throttle positioning will affect FF negatively.

On my aircraft, however, closing the throttle after t/o is standard procedure (T/O is 36.5" but normal climb is 31", max climb is 34"). Is that not the case on the TB21 or are you supposed to climb full throttle (WOT) like the TB20?

If the former, I would definitely adjust the max MP. On our engine, the extra 3" will definitely cause a substantial overheating.

Has that max MP number changed after the OH?

BTW: MP is measured in equivalent inches of a vertical column of liquid mercury (inHg or ") . Psi is a different unit, about 2 inHg for one psi.

Antonio
LESB, Spain
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