View Full Version : why itt limit higher with higher oat?
16th Jun 2012, 13:24
I have a question about the ITT limits of a turbine eng, specifically the pw125b.
For 'GA' power, there is a flat limit (800).
For 'TO' (normal) power, there is a variable limit, below 800. (It's flat up to a certain temp, goes up, and is flat again.)
That limit is lower with lower temps, and higher with higher temps.
I understand that is is lower for TO compared to GA, but why does it go UP with higher temps?
Does that have to do with air density? does the ITT not go up as much with higher temps as opposed to lower temps on higher temps?
17th Jun 2012, 14:34
I am merely speculating here, but I can see some logic in the ITT limits you describe. I suspect the cert limit is 800.
But at lower OAT, the published (reduced) operating limit varies as a means of identifying a deteriorated engine. If you can find the TC data sheet for the engine, that would help resolve the question.
18th Jun 2012, 04:06
Maybe ITT is"normal" at high OAT and decreases at lower OAT. It might be a way of squeezing a bit more life out of the engine by setting lower limit when typically less power would be needed.
18th Jun 2012, 10:43
I am led to believe (by an engineer who worked on our PW125B's) that it is due to the engine spools potentially seizing if an engine flamed out on takeoff. This would then prevent an in-flight restart for quite some time. This comes about due to the uneven cooling of various sections of the spools and the itt limit is there to stop that from happening. (Ie cooler OAT, then shock cooling would happen from a lower ITT, if higher OAT then less shock cooling so you can run it hotter)
Whether that is actually the case or not I would be very keen to find out too!! :ok:
21st Jun 2012, 20:26
Yes, the certified temp for continuous use is 800 degrees (for starting and transients there are other limits).
I've tried to find some more technical data on the engine, but since the engine is not in use that much at this time, it's not easy :( any help, from anyone, would be greatly appreciated :)
Regardless of the temperature, the SHP in TO mode should always be the same. TO mode is a specific power rating. We don't use flex/derated/etc. power settings.
I suspect that can't be the reason, since at any temperature the GA limit is 800 degrees. If it were to fail in GA power, the temp differences would be even greater as opposed to TO power.
22nd Jun 2012, 02:50
In the sloped portion of the ITT limit chart, what are the endpoints of the sloped line?
e.g. 800C at OAT=30C, down to 700C at OAT=0C (I'm making up these numbers... What are they really?)
22nd Jun 2012, 09:54
One of my favourite tech questions,
You will notice the reduced ITT limits are for "TakeOff" the reason is for certification requirements, the rules say if an engine flames out on takeoff it must be capable of being restarted. This does not mean you must restart the engine only that it must be capable of being restarted..
During a flame out in cruise flight with very low ambient temperatures the core of the engine will lock a condition known as thermal core lock. This is because the casing around the turbines cool faster than the blades themselves, On the 125B engine this will occur (core locked) in the period between 4 and 20 minutes after flame out, depending on the actual ITT and the ambient temperature.
Due to certification requirements the reduced ITT limits for takeoff means that the engine in this range will not core lock and therefore is capable of being restated, note that the takeoff ends at the end of the 3rd segment.
Hope that answers your question.
Further to that, how do you know and engine is thermal core locked. This will be displayed as an NH of 0 percent, generally an unlocked engine will spool at around 20 percent NH at around 160 knots, therefore ignition on, fuel in and light off. Any attempt to restart the engine with use of the stater motor whilst the engine is core locked (0 percent NH) the stater motor drive shaft will sheer off, this is a deliberate weak point to prevent engine core damage in an attempt to start the engine whilst it is locked.
Be aware once the engine is core locked it may remain that way for up to 20 minutes.
22nd Jun 2012, 13:38
Is this really an issue with modern turboprops? If so, the IFSD rate must really be rotten. :eek:
22nd Jun 2012, 13:53
the stater motor drive shaft will sheer off, this is a deliberate weak point to prevent engine core damage in an attempt to start the engine whilst it is locked.
I believe that it's the other way around. i.e. if the starter motor is seized you still want the core to windmill