Turbine engine temp vs. Torque
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Turbine engine temp vs. Torque
Specifically regarding the turboprop engines, would this be a true generalization? Torque is the limiting factor for the gearbox and engine temp is the limiting factor for the engine. Technical replies are more than welcome!
Interesting question and your presumed answer for discussion.
I'll start off with the premise that the engine and gearbox are rated for the design torque and temperature.
If you exceed the turbine inlet temperature (too much fuel or too little air) than the turbine blades will rapidly detriorate. A small accunulated amount over hours or within a minute or less.
Torque needs two parts, driving torque and load. I presume that the driving torque from the turbine (typically a free turbine) is already sized to the gearbox in normal operation. If for some reason you managed to greatly increase the load torque (typically pitch changes on props) beyond design limits the turbine would slow down way earlier than a gearbox failure. This may have the result of a back pressure on the engine compressors and a droop in operating RPM which in turn results in less driving torque and may result in the engine becomming unstable.
I suppose that barrit1 when may some better words on this
I'll start off with the premise that the engine and gearbox are rated for the design torque and temperature.
If you exceed the turbine inlet temperature (too much fuel or too little air) than the turbine blades will rapidly detriorate. A small accunulated amount over hours or within a minute or less.
Torque needs two parts, driving torque and load. I presume that the driving torque from the turbine (typically a free turbine) is already sized to the gearbox in normal operation. If for some reason you managed to greatly increase the load torque (typically pitch changes on props) beyond design limits the turbine would slow down way earlier than a gearbox failure. This may have the result of a back pressure on the engine compressors and a droop in operating RPM which in turn results in less driving torque and may result in the engine becomming unstable.
I suppose that barrit1 when may some better words on this
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The reason I came up with that generalization is because engine temp is definitely not going to affect the gearbox, just like a car it's going to be limited by the amount of torque. Re: the engine, if the compressors and turbines and spools can all support the torque then that wouldn't be a limiting factor. The amount of torsion that these parts could withstand before deforming is immense. The only real limitation I can think of would be the engine temp; if it gets too hot like you said it will just melt and destroy the engine. From what I understand, engine temp limits can be based on TIT, ITT or EGT. What's the reasoning when selecting a main temperature to monitor? I would think TIT would be the most important to monitor. I don't think all airplanes with turbines have all those engine temp sensors.
Re: the gearbox, engine temp really shouldn't be a factor when considering operational limits. Torque seems to be the only limiting factor.
So if you're operating at high altitude in cruise and you have really cold temperatures, your main concern would typically be the torque in my opinion; you'd hit your torque limit before you hit your engine temp limit. But if you're taking off on a hot day you'd most likely hit your engine temp limit first. Any turbine pilots that can comment on that?
Re: the gearbox, engine temp really shouldn't be a factor when considering operational limits. Torque seems to be the only limiting factor.
So if you're operating at high altitude in cruise and you have really cold temperatures, your main concern would typically be the torque in my opinion; you'd hit your torque limit before you hit your engine temp limit. But if you're taking off on a hot day you'd most likely hit your engine temp limit first. Any turbine pilots that can comment on that?
Not sure which engines you are involved in, but the one I used to operate was the Allison 501-D13 on the Lockheed L188 Electra.
The Limits were TIT 971 C for take off and max torque of 4000 Horse Power, (gearbox limit).
we used to set 971 for TO. (or 932 for reduced power TO.)
The only time we were Torque limited was if the temp was below Zero, and then we hit the 4000 HP limit with less than 971 TIT.
At altitude, say FL 220, you would never reach anything like the Torque limit, in fact I seem to recall that in cruise the set TIT of 847 would give about 1500 HP so if you pushed the power lever forward to 971 I doubt you would get 2500HP. The air is too thin for the engine to develop that much power.
This is for an engine with a single shaft and constant speed prop. I don't know if the same would apply for a free turbine engine like the PW 125 or PT-6 engines.
The Limits were TIT 971 C for take off and max torque of 4000 Horse Power, (gearbox limit).
we used to set 971 for TO. (or 932 for reduced power TO.)
The only time we were Torque limited was if the temp was below Zero, and then we hit the 4000 HP limit with less than 971 TIT.
At altitude, say FL 220, you would never reach anything like the Torque limit, in fact I seem to recall that in cruise the set TIT of 847 would give about 1500 HP so if you pushed the power lever forward to 971 I doubt you would get 2500HP. The air is too thin for the engine to develop that much power.
This is for an engine with a single shaft and constant speed prop. I don't know if the same would apply for a free turbine engine like the PW 125 or PT-6 engines.
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Ah ok, that makes sense. I don't fly turbine right now but I will be going onto the Metro 2 & 3 soon. They have the TPE-331. I'd assume the PT6s would be similar with regard to the characteristics of the limits you talked about.
Has anybody heard of a gerabox failure due to overtorque limit being exceeded?
Food for thought.
Turbine temperatures are measured at their easiest and most reliable points.
Torque is also measures at it easiest and most reliable point.
The actual failure condition for torque or temperature may have nothing to do with the actual location of the measurement (could be downstream or upstream from the measurement location)
Food for thought.
Turbine temperatures are measured at their easiest and most reliable points.
Torque is also measures at it easiest and most reliable point.
The actual failure condition for torque or temperature may have nothing to do with the actual location of the measurement (could be downstream or upstream from the measurement location)
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I have never flown the 2, but have much experience in the 23.
The 23 has a -12 engine, the 3 a -11.
They are both similar.
The TPE331 engine is a very good engine and quite robust. I much prefer them to the PT6.
You would be hard pressed to over torque the engine, requiring about -6 degrees to develop over 110% torque.
They are very sensitive to temperature however especially during start. You must have sufficient power to start them else you will certainly over temp the engine. This is especially so if the engine is "hung" and you try to add more fuel to build rpm.
Generally a symptom of this behaviour is bits of rather expensive metal alighting themselves via the tailpipe. This is never complimentary to your resume, or of your piloting prowess.
Luckily once you have an agreeable SRL computer online and the rpm exceeds 90% you have temp limiting to save your backside. This will limit the temp below 650 degrees and save your job. Of course if you give it a fistful of power at low level, you may over fuel the capacity of the temp limiter to bypass and have an expensive day. Still, on an engine like a Garrett, torque is rarely a consideration. It's always temp and the required torque at a particular temp given the ambient conditions.
The 23 has a -12 engine, the 3 a -11.
They are both similar.
The TPE331 engine is a very good engine and quite robust. I much prefer them to the PT6.
You would be hard pressed to over torque the engine, requiring about -6 degrees to develop over 110% torque.
They are very sensitive to temperature however especially during start. You must have sufficient power to start them else you will certainly over temp the engine. This is especially so if the engine is "hung" and you try to add more fuel to build rpm.
Generally a symptom of this behaviour is bits of rather expensive metal alighting themselves via the tailpipe. This is never complimentary to your resume, or of your piloting prowess.
Luckily once you have an agreeable SRL computer online and the rpm exceeds 90% you have temp limiting to save your backside. This will limit the temp below 650 degrees and save your job. Of course if you give it a fistful of power at low level, you may over fuel the capacity of the temp limiter to bypass and have an expensive day. Still, on an engine like a Garrett, torque is rarely a consideration. It's always temp and the required torque at a particular temp given the ambient conditions.
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"Has anybody heard of a gerabox failure due to overtorque limit being exceeded?"
On P&W 127 a torque above 120% for 5min will be followed by RGB replacement and inspection.Same for 2min with 140%...
On P&W 127 a torque above 120% for 5min will be followed by RGB replacement and inspection.Same for 2min with 140%...
"Gearbox failure due to overtorque"
Zantop had a gearbox failure on an Electra in the cruise due to fatigue. The Prop and most of the gearbox came off and ended up in a tree in upstate New York. The only airframe damage was a slight nick on the stab. Lucky guys.
This may be related to over torque events.
Just after this event I was training on the Electra with Zantop and we were doing circuits at Lincoln Nebraska. The temp was about 10 degrees F (-12C). Doing a 3 engine GA the PF fire walled the power levers and and kept his hand on them preventing me from pulling them back to limit the HP to 4000. I saw about 4500HP. I don't think this was written up in the log.
There were several other gearbox events on this type of engine over the years.
When the Electra was put on the UK register the CAA required new gearbox casings to be fitted as there was no record of the number of cycles they had done as they were zeroed lifed at each overhaul.
Zantop had a gearbox failure on an Electra in the cruise due to fatigue. The Prop and most of the gearbox came off and ended up in a tree in upstate New York. The only airframe damage was a slight nick on the stab. Lucky guys.
This may be related to over torque events.
Just after this event I was training on the Electra with Zantop and we were doing circuits at Lincoln Nebraska. The temp was about 10 degrees F (-12C). Doing a 3 engine GA the PF fire walled the power levers and and kept his hand on them preventing me from pulling them back to limit the HP to 4000. I saw about 4500HP. I don't think this was written up in the log.
There were several other gearbox events on this type of engine over the years.
When the Electra was put on the UK register the CAA required new gearbox casings to be fitted as there was no record of the number of cycles they had done as they were zeroed lifed at each overhaul.
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There is one method to which an over torque on a 331 is quite certain however. That is to activate the water methanol once you are producing over 40% torque.
I have seen the engines produce over 120% for a transient period after such an error, however it was not necessarily sustained for a prolonged duration.
The RPM of the shaft is more concerning measured as a % of RPM. From memory without checking my old flight safety manuals if you produced over 106% rpm you were required to remove the engine. They were quite prone with the speeds at high rpm in a cruise climb to exceed 101.5% quite rapidly if the error was not caught.
A real mans aeroplane the metro and after flying such, your scan and manual flying skills will never be greater.
I have seen the engines produce over 120% for a transient period after such an error, however it was not necessarily sustained for a prolonged duration.
The RPM of the shaft is more concerning measured as a % of RPM. From memory without checking my old flight safety manuals if you produced over 106% rpm you were required to remove the engine. They were quite prone with the speeds at high rpm in a cruise climb to exceed 101.5% quite rapidly if the error was not caught.
A real mans aeroplane the metro and after flying such, your scan and manual flying skills will never be greater.
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Re: Hitting the limits
On the PT6A-27 (Twin Otter) torque is our reference to set power at any stage. In day to day operations, it is uncommon to hit the temp limit before the torque limit, even at our hub (south american airport at 5000ft AMSL, usually between ISA+15 and ISA+20). Since torque is our main reference value, it is adjusted by temperature.
Regarding destroying the engine if there is an over-torque condition, the emergency section of our AFM states that we can redline the engine to the first limit we hit (Torque, temp or compressor rotational speed) if an emergency condition is present. That difference is in the order of 10PSI (80% of the power available) if taking off from a high elevation airport at high temperature. The engine is flat-rated from 715EHP to 625SHP, which surely must play a big part in this discussion (but since my knowledge doesn't go that deep, that's as far as I can get
)
Regarding destroying the engine if there is an over-torque condition, the emergency section of our AFM states that we can redline the engine to the first limit we hit (Torque, temp or compressor rotational speed) if an emergency condition is present. That difference is in the order of 10PSI (80% of the power available) if taking off from a high elevation airport at high temperature. The engine is flat-rated from 715EHP to 625SHP, which surely must play a big part in this discussion (but since my knowledge doesn't go that deep, that's as far as I can get
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The above responses cover the subject pretty well I think.
Yes, torque redline is normally the gearbox limit, and is more likely to be reached at low altitude/low OAT because of the increased air density and engine mass airflow, thus more physical HP.
And turbine temp (TGT, ITT, ...) is more like to be reached at higher OAT and/or altitude. It's an engine limitation, unrelated to the gearbox.
Yes, torque redline is normally the gearbox limit, and is more likely to be reached at low altitude/low OAT because of the increased air density and engine mass airflow, thus more physical HP.
And turbine temp (TGT, ITT, ...) is more like to be reached at higher OAT and/or altitude. It's an engine limitation, unrelated to the gearbox.
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Thanks for all the replies! I have another question... why would the engine also be limited to a certain RPM? Obviously it makes sense it can't just increase to infinity, but what would too much RPM do to the engine if it was staying within it's temp and torque limits?
EDIT: would it have to do with the expansion of the metals, due to the extreme G force, resulting in reduced clearances?
EDIT: would it have to do with the expansion of the metals, due to the extreme G force, resulting in reduced clearances?
Many things can happen due to too high RPM. High centrifugal forces, high loads on the bearings together with insufficient cooling via the oil flow, maybe as well aerodynamic problems on the blades.
I have heard of two incidents, one to a DC-9 of OS and another one to a Tu-134 of Interflug, where one turbine shaft broke and the turbine was briefly running without any load. They rapidly spun up to above 100.000rpm; the bearings did not like this and the wheels soon departed the engine casings. In the case of the OS plane the parts seemingly crossed the rear fuselage and narrowly missed the other engine. Unfortunately, I cannot find details on those occurences on the web.
I have heard of two incidents, one to a DC-9 of OS and another one to a Tu-134 of Interflug, where one turbine shaft broke and the turbine was briefly running without any load. They rapidly spun up to above 100.000rpm; the bearings did not like this and the wheels soon departed the engine casings. In the case of the OS plane the parts seemingly crossed the rear fuselage and narrowly missed the other engine. Unfortunately, I cannot find details on those occurences on the web.
Adding to what's stated above, excessive RPM will cause structural overload of the compressor and turbine wheels and probable uncontained failure if allowed to progress very far beyond design limits. Typically, aviation turbine engines have 3 levels of overspeed protection to meet certification requirements and prevent catastrophic failures related to engine runaways.
several things happen when you run (overspeed) an engine above its design operating speed >110% redline RPM
The weakest disk will probably grow in size and rub the blade tips and loosen itself on its shaft
The compressors will likely run into their surge line and barf enough to prevent additional speed increases
Both the compressor and Turbine rotors will likely run into an unintended blade-disk coupled frequency and break either the disk or the blades
An unloaded turbine will likely runaway to 150-175% speed and start shedding blades or worse, the disk itself.
The weakest disk will probably grow in size and rub the blade tips and loosen itself on its shaft
The compressors will likely run into their surge line and barf enough to prevent additional speed increases
Both the compressor and Turbine rotors will likely run into an unintended blade-disk coupled frequency and break either the disk or the blades
An unloaded turbine will likely runaway to 150-175% speed and start shedding blades or worse, the disk itself.
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Generally speaking torque is the gearbox limit and temp is the engine internals. In some cases (like the -34 installation on the Twin Otter), the original -27 torque limit of 50psi is still the limit for airframe and Vmc reasons. If you exceed the design airframe, Vmc, torque limit, actual Vmc WILL increase.
Sick engines will reach temp first before reaching the torque limit...but that's another discussion.
Sick engines will reach temp first before reaching the torque limit...but that's another discussion.
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If you exceed the design airframe, Vmc, torque limit, actual Vmc WILL increase.
What do you mean by sick engines? Is that similar to a hot start when the engine air flow isn't high enough?
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Sick turbines are just like sick piston engines (old car with plenty of wear does not accelerate as fast as a brand new one).
As turbines wear, they don't produce as much power for a given turbine temperature.
This is most noticeable in a 331 for instance when one engine is much newer than the other.
To match the torque, one engine might be at 640 EGT, while the other at 605. .
Once the turbine cannot produce the rated torque for a given temperature on takeoff it will be pulled off the wing. I never had much trouble with Garretts and TBOs however. They are generally pretty good once they are started.
As turbines wear, they don't produce as much power for a given turbine temperature.
This is most noticeable in a 331 for instance when one engine is much newer than the other.
To match the torque, one engine might be at 640 EGT, while the other at 605. .
Once the turbine cannot produce the rated torque for a given temperature on takeoff it will be pulled off the wing. I never had much trouble with Garretts and TBOs however. They are generally pretty good once they are started.