cooldowns
 

grettings,
a quick question regarding turbine cooldowns; i used to work for an operator who insisted on a 3 minute cooldown for C20B's. is there any benefit to doing this? i have heard otherwise. on the other hand, what happens if the cooldown is shorter than what the flight manual states? i am flying astars (B2's) and 407's.
cheers and thanks
fp

In a Rolls Royce engine there is no benefit at all. In fact you do more harm than good. I have a graph from Rolls which shows that after 2 minutes things start to heat back up again. If more computer savvy I'd post it for you. So in your 407 2 minutes is it. They don't spend millions of dollars on research in these companies to have someone go "well 3 must be better than 2!" Can't answer for your Astar though.

My understanding is that 2 mins is the optimum period for reduction of temperature so that stabilisation (equilibrium) occurs in the hot section, and consequent carbon build-up is obviated in the oil system. I don'tknow if it is advised in teh RFM, but I DO recall a discussion with an Allison engineer years ago who said that much more than 2 mins will see an increase in internal temperature of the turbine. Don't know if one would see this on the tot gauge, though. Someone correct me if I am wrong? Thx - VFR

We just had the engine guy here from the USA before Christmas rebuilding one of ours, and i asked him that very question. The answer depends on the design of the engine. Some engines have a flight and idle setting, and the oil and bearings need the chance (and are designed to operate at an idle setting), to circulate the oil and to cool down. In the Astar, the engine has no idle setting and the bearings are designed to be lubricated when the engine is at flight. Turbomecca recomend 30 seconds at 70%NG to allow temps to stabalise and then shut down. Running the engine for longer than this means the bearings are no longer being lubricted properly and can cause coking and damage to the bearings. Thats what we were told anyway, open to other advice or thoughts though?

vaqueroaero is correct. Here is the chart of which he speaks:

http://i76.photobucket.com/albums/j3...r37Uih0Jf1.jpg

for Mi-17 manual states:
summer 2-3min, (we do 2min)
winter 1-2min (we do 1min)
no longer than 20min on ground idle...

In the C20 ( B ) i am not sure there is much to be gained by doing more than a minute or so . I was told that the MD 500,s which did many thousand engine off,s , when their engines went in for service they were absolutely perfect :ok: And they were often shutdown straightaway or after a few seconds of auto . Personally i would do 2 mins in morning to allow me to finish coffee and ciggy but 1 min on last shut down so could get a drink . So average of 1.30 ............engines fine .

interesting replies, thanks guys.
does any of this have to do with preventing turbine rubs or is it just a lubricating issue?
cheers
fp

It's to do with coking of the engine oil at the bearings listed in the diagram.

Always follow the manufacturer's recommendation.

The 407 has a RR250-C47 engine and if I am not mistaken, requires a 2 min cooldown as well as most of the RR250 applications that I know of.
I don't remember in the B2 but the B3 requires a 30 second cooldown. Different design, different animal. As a matter of fact as helirally pointed out, Turbomeca calls this the "thermal stabilization" period instead of cooldown.

One exception to the rule of following the manufacturer's recommendation is the Arrius 2F in the EC120B. The RFM calls for a 30 second stabilization but there is a service letter from Turbomeca from 1999 that recommends 1 minute. And that is what the factory instructors taught me.

I was told in the 250 better to do a 2min shutdown, and then after blades stopped it is recommended to do a full turn of the rotors by hand to help remove the coking from the oil. (I combine this in my preflight)

And post-flight....

Been a long time since I flew the 206B, but from memory I was told (by a Bell prod. test pilot) that by turning the blades backwards after landing to tie the blades down would help against the coking, and during pre-flight it would tell you if you would get the blades to turn during enginestart.... If coked, you would only get a bloody high TOT with no blade-turning
We always did 2 min at idle before shutdown.

The 350, I've been instructed to 30 sec, but some companies want 1 min as well.

And how would the FADEC differentiate between ground idle waiting for something not ready to shut down and ground idle shutting down?
135 P&W has no prescribed cooling period in the FLM but we tend to leave it running for 1' ground idle anyway and than we switch it off with the switch, not auto function.

The reasons behind cooldown runs are perfectly clear and imho & experience always have been.

What is not clear to me is why anyone would even dream of doing anything different from the manufacturer's instructions?
Varying those on a personal whim - no matter what they refer to seems a rather worrying habit to say the least.

What about the times when you need to switch off as soon as you land? Waiting for the optimum temperature for shutdown might be a hazard by itself as you don't have the normal control over shutoff. I would go with a software equipment measuring the parameters and advising when is the optimum time to do something, but I would leave the control over shutoff to the pic.


@Agaricus bisporus: I can't see what is being done wrong using common sense where the manufacturer had nothing to say?
Because of certain idiots out there EC's flight manual is turning into lawyers dream diverting from a good practical manual.

Anyway, it takes approx 1' to switch off all the extra equipment via checklist prior to eng shutoff, its not waiting 1'.

Even EC factory instructors say "treat her like a lady and she will be good to you".

Those who pay attention would note that the TOT at flat pitch, but flight RPM is probably not much different than at idle (and may be lower). Not sure if it's a good idea to shut down from flight if you've been waiting for the next passenger at flight RPM and then decide it's time to quit.

Regarding FADECs - I often wondered why they didn't do a controlled deceleration like they control the start acceleration. The start acceleration on a FADEC equipped engine is much cooler than the hydro-mechanical FCU start, so it would seem to make sense to this engineer that you should be able to schedule the shutdown fuel flow as well.
Just pay me a small royalty if anyone decides to take up this idea.

Makes sense. At flight RPM with flat pitch, the only work the engine is doing is overcoming the mechanical friction in the drive train and the flat pitch drag on the blades, but there is far more airflow through the engine than at idle.

krypton
 

er , and turning the compressor ... ?

Indeed!

The point is - in either case the work done is not much different but the air flow is.

related question...
 

does anyone recommend turning the blades backwards to turn the (turbine) engine on helicopters which don't run often? once per week say? (obviously flying is better)