ITT split

[Cecco]

What is the maximum permissible ITT split on your jet engine (whatever you fly)? A colleague told me that on their CJ1, they ususally had 40-50 degrees difference (climb and cruise), which seems a lot to me.

Cecco

[hawker750]

I would agree with you, needs investigating. Does not always mean a major engine problem, more likely a bleed air leak or a stuck ant-ice valve. First thing to find out is whether the split was gradual over time (normally bad news) or happened quickly.
On the Hawker an ITT rise of about 35 degrees when you put engine ant ice on but do not know the CJ but 40-50 is too high

[adverse-bump]

we have no limit

however i have seen splits in the temps, normally associated with a bleed stuck on/off or a problem in the engine.

[Cecco]

Ironically, the 40-50 degree difference in ITT turned up after the HSI, so yes from one day to the other. Now, we had the gearbox and oil cooler exchanged at our right jet engine (CJ1+) 3 days ago, which has nothing to do with the bleed air system/ducts. Nevertheless, now, at the first flight after the maintenance, the ITT in the right engine is about 15 degrees higher than in the left one so I´m wondering if taking apart half of the engine and re-assembling it with the two new parts did have some effect on that (never observed that phenomenon before).

[Cecco]

Stuck anti-ice valve? you mean like engine anti-ice is always one on the engine with the higher ITT?

Cheers
Cecco

[CJ Driver]

Ours has had an ITT split since the day it came from the factory. There is an "allowable" limit - after all, you would never expect them to be completely identical - and our was close to, but inside, the limit.

And indeed, 1800 hours later, the split is the same as always, including no change after the HSI.

The indication of a fault is when either of the ITT's change, so you should really be doing some basic monitoring, but the underlying split is nothing to be worried about.

[FE Hoppy]

Agree with CJ driver to a certain degree. No two engines are the same but a large difference would indicate either a big cycle difference between the engines or a fault, and bleed air loss would certainly do the trick. The other thing to think about is if the hot engine has any ITT margin at max to thrust. Do you de-rate or reduce thrust for take off? If so do you do periodic full thrust take offs?

[Cecco]

We always do the take-off with the throttles in TO detent so yes, max.take-off thrust. As opposed to the big jets, we have no performance table for reduced TO thrust. I was thinking, however, that given a long runway and considering the obstacles/required climb gradient, you could do a Take-Off with MCT to save the engines. Any of you CJ drivers are doing that?

[hawker750]

Cecco
Yes a stuck open engine anti-ice valve would give that sort of split. But when you said it happened after an HSI that is the major clue.
4 or 5 years ago we had an HSI done on one engine of our Citation 2. (different engine I know). After the HSI the ITT was 40 hotter than before. The overhaul shop said it was in limits and it was just bad luck. After about 30 hours we demanded an investigation and the LPT blades were clipping the shroud ring. I am sure if we were paying the bill the LPT blades would have been scrapped but as the overhaul agency were paying they managed to crop the blades! After the temp went down 40.
Another question: are the two fuel flows identical and what are the 2 fuel flows at identical temps? It could be that a temp probe has gone short circuit.
I would not be happy with this situation, there may be something wrong inside that the company that did the HSI company will refuse to rectify if it is left too long.

[FE Hoppy]

Cecco
Yes a stuck open engine anti-ice valve would give that sort of split. But when you said it happened after an HSI that is the major clue.
4 or 5 years ago we had an HSI done on one engine of our Citation 2. (different engine I know). After the HSI the ITT was 40 hotter than before. The overhaul shop said it was in limits and it was just bad luck. After about 30 hours we demanded an investigation and the LPT blades were clipping the shroud ring. I am sure if we were paying the bill the LPT blades would have been scrapped but as the overhaul agency were paying they managed to crop the blades! After the temp went down 40.
Another question: are the two fuel flows identical and what are the 2 fuel flows at identical temps? It could be that a temp probe has gone short circuit.
I would not be happy with this situation, there may be something wrong inside that the company that did the HSI company will refuse to rectify if it is left too long.

This is a very good shout!

[FJ44]

Had the same thing happen on a CJ after overhall.
Engines went away a perfect matched pair and came back with different N2 and ITT readings in the cruise.
Was told by the maint. organistation that it was standard and that nothing can be done about it.

Have just had a CJ2 back from a HSI and there is now a split in the N2's of about .5% and 10oC in the ITT's. Same answer.

Looks like once they change some of the parts there never the same again.

[barit1]

...are the two fuel flows identical and what are the 2 fuel flows at identical temps? It could be that a temp probe has gone short circuit.

Don't think so. The probes are wired in parallel, thus a short (actually a cold junction) affects the entire thermocouple circuit, not just one probe. The error would be huge - several hundred degrees.

An open thermocouple in a parallel array could result in either a + or - error, depending on whether it's at a hotter or cooler spot in the circumferential temperature profile. (The temperature of the gas is never perfectly uniform around the case.)

For that matter, fuel flowmeters have been known to disagree. For one thing, make sure both flowmeters are THE SAME p/n. I once diagnosed a flowmeter mismatch by remote data - 5000 miles away! :=

[Cecco]

New observation. The ITT split occured with the engine synchronizer on (the split wouldn't start until above FL250 and then in cruise). With the engine synchronizer turned off, no ITT split; the N2 is 0.2-0.3% higher in the right engine (with engine synchronizer on, the right engine is 10-15 degrees hotter, N2 is 0.7-0.8% higher).

[bcgallacher]

I am not familiar with the particular engine but with the N2 being higher and also ITT higher I would be looking at vigvs ,if fitted ,to be off schedule or bleed problems - it could also be that the hp compressor is getting a bit past it. As long as the temps are within limits just monitor - any sudden increase is a cause for concern .

[Cecco]

What is vigvs?

[spannersatcx]

Variable Inlet Guide Vanes, used for airflow control through the compressor.

[His dudeness]

Variable Inlet Guide VaneS I´d think. The Wiliams doesn´t have them IIRC.

Sometimes its just a bleed air valve setting prob. I think the CJ bleed valve flow rates cant be changed manually, but if they differ from engine to engine you´d see a split.
40-50° is alot and I would just not accept it from the mx shop.

We had a KingAir having HSI on both sides, which was a near perfect match afterwards. (before we had a 20-25° difference)
A few hours later it had a boroscope inspection that reavealed 2 missing stators on one side. One could positively see no real difference between the engines....

[orion1210]

You shouldn't have a temp split on a new a/c from the factory as one engine, at the least, is always trimmed to give matching ITTs, not sure if this is just a williams policy?

To increase your n2 you need to burn more fuel thus higher ITT. If you need a higher N2 to achieve the same N1 there are a number of possible causes, dirty damaged fan/IP rotor or fan stator, a compressor bleed leak, and or a gas loss after the HPTurbine such as excessive LPturbine tip clearence.

A stuck bleed valve, anti-ice valve or leak is easily found on the 525 series. Give your engineering dept a call. A bleed leak in the tailcone could cause significant damage so if your concerned give them a shout.