EC 135 - TOT discrepancy

[UnDies]

Have noted a difference in the TOT reading whilst in flight - sometimes a failry large difference. Any ideas?

Thanks

[HeliComparator]

Could be a number of things but we had a problem with a Makila engine - it has multiple thermocouples to measure TOT and one was not fitted securely - it moved out just enough so that the tip was not in the gas flow, giving an under-read of about 80 degrees.

HC

[Thomas coupling]

What engines are they first?

Are you reading analogue or digital TOT? If the former there is an allowable 5% discrepancy. If they are digital the resisters that are in the cct could be duff?

Failing that, when was the last time you did a power assurance check and compared the graph?
That will tell you if one is running hotter than the other.

How big is your discrepancy?

[Shawn Coyle]

Is it different in all flight regimes?
In many other helicopters with intakes that are exposed to the airstream, there might be the possibility that the airflow entering one intake is not as good as the airflow entering the other intake. The difference would obviously be at a minimum in a hover and maximum in cruise.
With the EC-135, the intakes are buried in the transmission area. Unless you have something not being right in that area, you might look at the thermocouples as previously mentioned.

[wrench1]

UnDies,

Have all the electrical connectors been "treated" with Stabilant 22? Check PW&C MM for details. Don't have a lot of experience with the 135, but most old hands reference this first with any electrical issues first.

Beyond that, would need more info.

W1

[UnDies]

Gents, thanks for the posts - will get some more info and post it. standby.

[NickLappos]

There is no reason why the Engine Temps have to match. Only with a factory fresh pair of engines will they match at all. Typical installations can have one engine at +1% (borderline on power assurance) and the other ay +5% (typical, perhaps, depending on engine type.)

Each 1% torque is worth about 4 degrees TGT (or T5 or T4.5) so that a 5% difference between engines can yield a 20 degree "split". 10% would of course be 40 degrees.

If the split develops suddenly, there is probably something wrong, and you should pull power assurance and compare to the recent past. Look to bleed air leaks as the best single cause of power loss, but there could be internal problems that are potentially catastrophic, such as a cracked turbine section that allows massive quantities of gas to leak away from the power section. There could also be a problem in calibration of the power gages, not just engine temp, because the torque could be wrong and when you match torque, the temps look bad.

[maxtork]

To continue on what Nick has posted above there is always a trade off between a strong engine that runs warm and a not as strong engine that runs cooler. Even right off the test cell there is a window which allows two engines not to run identical but still pass power checks just fine. This is why power assurance checks are such a great tool for troubleshooting if we can look at the trend. If the torque margins stays the same and the temp suddenly changes we can start to look more at an indication problem as the two elements are closely related and affect each other. If the torque margins are shifting as well then we may very well have a core engine issue.

I'm not sure if you are running a PW powered or a TM powered 135 but if it is TM I would first look at the T4.5 conformation box readings and then pull the T4 harnesses out and look at the probes. Those probes are actually two thermocouples inside one metal probe. If the outter covering gets cracked or comprimised in any way the hot gases that the thermocouples are normally sheilded from are now getting right onto the thermocouples and the engine will read very high, even though the engine internal temp has not changed at all..

If nothing is found there I would look at the fuel manifolds. They have a history of coking inside the lower nozzels which is why they are replaced at a relatively short interval (200-800 hours depending on variant). One of the symptoms we have seen is high t4 on an engine which is starting to get clogged manifolds.

Again these are tips that apply to the TM powered 135. If thats what you fly then let me know and I can surely help get you squared away.


Good Luck!

Max

[NickLappos]

Good comments, Max, with one exception:

Strong engines run cool, weak ones need more temp for the same power (thus weak ones reach temp limits earlier, at lower power.)

Also, before I would have a mechanic do anything, I would perform power assurance. If both engines pass, stop all worry and just fly.

[Aser]

On this thread...
How is possible to have 5ºC difference in flight (or ground at 100%NF) and , 10ºC or more when you are at idle in NO WIND condition?

[NickLappos]

The answer is that no engine set is matched at all, and the change in operating conditions between a powered condition and an idle one is enormous. Slight differences in blade tip clearances, bleed valve port sizings and valve openings, etc. all contribute.
A digital gage is the real culpret, where one can read to the degree, one expects similar consistency in every way. Actually, most T5 gage systems are not good for more than about 5 to 10 degree accuracy.

[Brian Abraham]

The answer is that no engine set is matched at all

We had a 76C at one time where one maxed out on N1 and the other TQ. That was enough to uncage the eyeballs coming off a rig, and kept the left thumb busy.

[maxtork]

Good comments, Max, with one exception:

Strong engines run cool, weak ones need more temp for the same power (thus weak ones reach temp limits earlier, at lower power.)

Also, before I would have a mechanic do anything, I would perform power assurance. If both engines pass, stop all worry and just fly.


Nick,

This is exactly the difference I was trying to illustrate. There is a difference between engine calibration and engine health. I can take a perfectly healthy engine and calibrate it to make more power by closing down the NGV and it will run hotter. Then I can take the same engine and open the NGV and make less power but with less temp as well.

Example:

1) 98% N1 = 80% Tq at 800 C
now open the NGV on the same engine
2) 98% N1 = 75% Tq at 750 C

However as you stated engine health is different. If an engine has turbine rub, erosion, etc it is now a health issue and will in fact require more fuel and therefore more temp to reach the same power.

Example:

1) 98% N1 = 80% Tq at 800 C when engine is healthy
2) 98% N1 = 75% Tq at 850 C !when the engine has erosion, rub etc.

I run into this often where someone claims an engine is unhealthy when in fact it just isn't calibrated to their liking.

I agree though that first step would be a power check however I would look at the engine trend also and not just the latest numbers. An engine can still pass a power check but be on it's way out and letting you know it the whole time but if you don't look at the trend you won't identify a problem until you fail the power check and probably ground the machine!

Max