My -28’s ITT limits in cruise are 750 deg. With nothing more scientific than the notion the harder you run something mechanical, the more you wear it out, I get uncomfortable operating them over 700. For example, during a short flight today at FL230 and ISA + 17, initial cruise power setting yielded 710 and 720 deg. Pulling back 100 lbs. of torque per side resulted in 695/695 degrees and a 6 kt loss true airspeed. Will it materially affect engine health/life in a positive way to limit them to 50 deg. below limits in cruise rather than 25 deg. below? What’s the whole story on ITT limits vs. engine longevity?

No. In fact if you run your PT6A too cool it can be detrimental to your blade life. They are designed to be run at close to the ITT limit and that is where they have the best combustion with the least residue on the blades.

Hmm. My plane came with an OAT temp based torque setting guide. That's what I use to set torque and ITT just becomes a byproduct of the setting. I think it's about making TBO.

Chip-

This is a good question... A MX TBM/PT6 guru, not working for Pratt, recommended when I got the TBM to run her 10-15 below max temp for available max TQ (the altitude and OAT) charts.

Assuming Pratt over specs the PT6 why shouldn't we run it max out. There's hound be a safety factor, spend less hours flying which makes it cheaper per NM.

Of course I've been following that advice since because he knew and still does much more than I. What do others with more experience think?

Post repeated

I know in the Meridian, one of the issues that was occurring was blade sulfidation. Causally, it was discussed that this was because you never ran the engine hot enough by setting with the power charts for OAT. Obviously, you have redlines that you cannot exceed (it was always torque, not ITT for me.)

Jason, that is interesting. My new M500 running per max torque levels is the coolest I have seen. I am running 675 ITT at max cruise around ISA. Would love to see the performance if I ran it by the ITT limits. probably outrun a TBM7

As you know, Pratt says if you run the torque settings you will make TBO and also have no performance degradation out to 3600 hrs. My assumption is that if you run it hotter, that at some point, you may have trouble making book torque within ITT limits. Just had my last 600 hour Meridian borescoped, and was told it looked new. So we are probably babying them a little.

Folks, something to keep in mind from the engineering and materials science point of view, the creep-temperature curve is usually pretty flat until just above max continuous ITT.

That is, you probably won't save significant life by routinely operating 10 degrees or 100 degrees below that number. It probably won't significantly hurt engine life several degrees hotter; if you're talking extended climbs or cruising 50-100 or even hotter than the turbine blades are going to grow and start rubbing in a matter of hours (maybe sooner). Another way of illustrating this is to look at the proportional difference between temperatures when an engine also has published limits for OEI, takeoff, and max continuous.

Local differences of 10-20 degrees aren't unusual downstream of an annual combustor (I actually saw that when I was involved in PW150 development and other engines in the 1990s). Greater than that means something is likely wrong with the engine, although you'd still see the average ITT on the gauge.

Clear as mud?

Jim when did you work at Pratt? Pre Navy I worked in the Applied Mechanics lab in East Hartford

Little Pratt for a couple years in the mid nineties. Just a small department using a few leased buildings and test cells at the NRC campus on Montreal Road in Ottawa, Canada. Technically my employment was with a mechanical contractor that provided a handful of skilled and semi-skilled bodies to them.

Paul jones tells me run the -28 between 705-708deg, if there ever was an authority on how to run the pt6 its him

Hmm, this is the first time I've heard of this. Especially considering that the B36T running the -21 engine will run way below max ITT most of it's life. You don't have to have a hot section inspection because of this so to say it's detrimental to run them too cool is something I haven't heard of.

You can't use the smallest, simplest, PT6A as the example of how you should run the entire fleet. The motors get more complex as they get larger.

See Jason Talleys comment above about sulfidation on the blades caused by running too cool. That can be the result.

As described in an earlier thread describing my double hot section experience, I’m a big admirer of Paul Jones. Philip Eisinger recommended him and Paul more than lived up to his reputation as a first class guy and resource on turbine engines. Today, I asked his opinion on the ITT limits issue. As usual, Paul provided superb information and guidance. I join in recommending Paul for anyone needing advice with these engines. With his permission, here’s what he had to say:


"There are have been a lot of changes over the years for the PT6-28 engine and some of those changes influence how we in the maintenance environment council our clients. When the PT6-28 engine was first introduced, they were built with a cobalt based turbine guide vane and compressor turbine blades which were the same as used in the PT6-20 engines The combination of those two components did not hold up well in engines operating in the higher temperature ranges. (740-750 ITT). Therefore we advised operators with these parts to operate at a conservative 700 to 710 degree limit to avoid premature damage to those parts.
When Pratt introduced service bulletins 1346 and 1348, this introduced a nickel based turbine guide vane which is still in use today (1348) and also a newly designed turbine blade 3102401-01 (SB 1346) which are triple tapered and also still in use today.
Experience with this combination of components has proven these parts to be more resilient to thermal distress and they can easily take the additional 25 degrees ITT over a period of time. Therefore operating at 725 degrees will provide good -performance with out sacrificing damage to the hot section parts. As you get closer to the 740-750 area, you still don;t have a high risk of thermal distress, but hot gas erosion starts to come into the picture. We find that the 725 is ideal and provides the best of both worlds. I hope this helps and finds you doing well.
Contact me any time with questions relating to your engines.

Have a great holiday."

Paul Jones
President
Specialty Turbine Service, Inc
111 Mirage Drive
Cranberry Twp, PA 16066

Phone: 724-776-3925
Cell: 724-272-3589
e-mail: sts7630@aol.com
www.specialtyturbine.com

I follow what Pratt recommends, they built the engine so knows how we should run it.,

In the G1000 aircraft its easy, you follow the bug as it has the chart built in and it follows torque with regards with OAT.

I agree with Jason and Charles, the meridian runs way too cool in my opinion.
You are limited by torque from what I see. In the TBM we are limited by ITT most of the time.
You can see here in the 850, im on the high side of the bug that gives me ITT limited to 787. Red line is 850.
311 kts TAS at FL280, -35 OAT burning 58 gph, not too shabby.