ITT (T5) idle limit less than continuous operating limit
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From: USA
ITT (T5) idle limit less than continuous operating limit
I’ve ran across a P&W engine that has an idle T5 temp limit of which is considerably less than the normal continuous operating T5 limit. Does anyone have thoughts on the logic behind such a limitation? I’ve flown other P&W equipped aircraft without any idle limit. Hoping it’s not a blazingly obvious reason that would make a fool of me 🙄Thanks in advance.
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From: USA
I’ve ran across a P&W engine that has an idle T5 temp limit of which is considerably less than the normal continuous operating T5 limit. Does anyone have thoughts on the logic behind such a limitation? I’ve flown other P&W equipped aircraft without any idle limit. Hoping it’s not a blazingly obvious reason that would make a fool of me 🙄Thanks in advance.
Last edited by wrench1; 1st May 2022 at 12:14.
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From: Canada
Well if ground idle is set at 61 N1 or Ng then in very cold temps the T5 is going to be low. Perhaps P+W are worried that the temp could be low enough to cause a flameout followed. by a big “WHOMP ” and T5 spike if it relights. Just a thought.
What engine and what is the min T5 idle limit?
Joke)As a bit of thread drift I talked to a Puma pilot who when slinging a load in very heavy rain had the temp of both engines just hovering at the very bottom of the green arc It got their attention as it occasionally dipped below the green. They were slinging drill pipe so were pulling a lot of power. Nothing happened, no surging or Torque fluctuations. Strange way to do a compressor wash. ) Joke Over
What engine and what is the min T5 idle limit?
Joke)As a bit of thread drift I talked to a Puma pilot who when slinging a load in very heavy rain had the temp of both engines just hovering at the very bottom of the green arc It got their attention as it occasionally dipped below the green. They were slinging drill pipe so were pulling a lot of power. Nothing happened, no surging or Torque fluctuations. Strange way to do a compressor wash. ) Joke Over
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From: Land of the Angles
My legs are grey. My ears are gnarled. My eyes are old and bent, so I'm getting rusty on my PT6 theory in my old age, but......
Were your platform a twin power plant, then my educated guess would be to prevent the potential for an overtemp at low idle speeds on the first engine now running at idle, during the run-up of the second engine start sequence.
Were the NG on the initial start engine running at idle to drop noticeably as a result of a high load created during the 2nd engine start, then the governing protection offered by P3 > PY bellows would be reduced and the FCU's start acceleration fuel scheduling may kick in additional fuel, but with a declining mass air flow past the fuel nozzles in the burner can, may result in the risk of a surge in ITT, as such the OEM sets a much lower idle ITT limit with plenty of margin.
Were your platform a twin power plant, then my educated guess would be to prevent the potential for an overtemp at low idle speeds on the first engine now running at idle, during the run-up of the second engine start sequence.
Were the NG on the initial start engine running at idle to drop noticeably as a result of a high load created during the 2nd engine start, then the governing protection offered by P3 > PY bellows would be reduced and the FCU's start acceleration fuel scheduling may kick in additional fuel, but with a declining mass air flow past the fuel nozzles in the burner can, may result in the risk of a surge in ITT, as such the OEM sets a much lower idle ITT limit with plenty of margin.
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From: SW England
I'm going to use that line, Hilife - many times 
. Best intro to a post I've seen in ages. My only PT6 time was in very warm places a fairly long time ago so have no recall of any such limit.
. Best intro to a post I've seen in ages. My only PT6 time was in very warm places a fairly long time ago so have no recall of any such limit.
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From: Canada
My legs are grey. My ears are gnarled. My eyes are old and bent, so I'm getting rusty on my PT6 theory in my old age, but......
Were your platform a twin power plant, then my educated guess would be to prevent the potential for an overtemp at low idle speeds on the first engine now running at idle, during the run-up of the second engine start sequence.
Were the NG on the initial start engine running at idle to drop noticeably as a result of a high load created during the 2nd engine start, then the governing protection offered by P3 > PY bellows would be reduced and the FCU's start acceleration fuel scheduling may kick in additional fuel, but with a declining mass air flow past the fuel nozzles in the burner can, may result in the risk of a surge in ITT, as such the OEM sets a much lower idle ITT limit with plenty of margin.
Were your platform a twin power plant, then my educated guess would be to prevent the potential for an overtemp at low idle speeds on the first engine now running at idle, during the run-up of the second engine start sequence.
Were the NG on the initial start engine running at idle to drop noticeably as a result of a high load created during the 2nd engine start, then the governing protection offered by P3 > PY bellows would be reduced and the FCU's start acceleration fuel scheduling may kick in additional fuel, but with a declining mass air flow past the fuel nozzles in the burner can, may result in the risk of a surge in ITT, as such the OEM sets a much lower idle ITT limit with plenty of margin.
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From: USA
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From: UK
I’ve ran across a P&W engine that has an idle T5 temp limit of which is considerably less than the normal continuous operating T5 limit. Does anyone have thoughts on the logic behind such a limitation? I’ve flown other P&W equipped aircraft without any idle limit. Hoping it’s not a blazingly obvious reason that would make a fool of me 🙄Thanks in advance.
Joined: Sep 2004
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From: Canada
Thread Starter
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From: USA
Great feedback guys - thank you. The idle temp limit is 515C and max continuous limit is 675C. The engine in question is actually a T73/JFTD12 as used in S64 cranes, an engine derived from the JT12 used in the sabreliners. I’m think the idea of increased engine load and subsequent T5 spike during a second engine start sequence would not be applicable in this scenario because it utilizes hydraulic starters driven from an accessory gearbox furnished from APP, but maybe it’s an old carryover from the Sabreliner limit. 🤔
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From: USA
Great feedback guys - thank you. The idle temp limit is 515C and max continuous limit is 675C. The engine in question is actually a T73/JFTD12 as used in S64 cranes, an engine derived from the JT12 used in the sabreliners. I’m think the idea of increased engine load and subsequent T5 spike during a second engine start sequence would not be applicable in this scenario because it utilizes hydraulic starters driven from an accessory gearbox furnished from APP, but maybe it’s an old carryover from the Sabreliner limit. 🤔
Last edited by wrench1; 1st May 2022 at 20:37.
Joined: Oct 2006
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From: USA
True. With the newer turbine designs it's mostly a fixed wing application where you see the lower idle settings as noted in the chart above. However, in some older helicopters especially those with direct-drive turbines you'll see engine idle settings in the mid-50% N1 range. For example, the SA315B Lama and S61 had engine idle settings in the 53-56% range.
Last edited by wrench1; 1st May 2022 at 20:36.
