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.

Without a specific model/engine I can only guess. But the max ITT imit should only be for the "low idle" setting on those engines with a high and low idle settings like on a King Air. Not all PW engines have a high/low setting which if I recall correctly the low idle max is due to reduced air flow through the engine at that RPM.

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

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.

I'm going to use that line, Hilife - many times :ok:. 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.

So shouldn’t that situation be controlled/avoided by having a minimum idle N1/Ng limit and not a min ITT limit?

FYI: I believe the OP is referring to a maximum ITT limit at idle. See "min. idle" below.
https://cimg4.ibsrv.net/gimg/pprune....91aa191a69.png

I would hazard a guess (without any engine specs) the the maximum idle temperature is set lower than the normal max continuous temp simple due to a lack of cooling air to the air cooled stages of the turbine (discs and blades).

51 Ng. I have never seen that low an idle used in a helicopter installation. Must be a fixed wing thing…Twin Otter or King Air perhaps.

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. 🤔

If you want the full answer to your question perhaps PM John Dixson. He was one of the Sikorsky test pilots on the CH-54 Tarhe (S64 Skycrane). I'm sure he can give you the inside scoop on the idle limits.

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.