Turbo-charger cool down procedures

[A37575]

Often observe Chieftains taxiing in at my local airport using what looks like very high idle rpm (1200?). THe power is kept high until the aircraft is parked in the lines. The pilots must be deliberately dragging the brakes to keep taxiing speed down at such high revs. I was always taught it is a bad thing to taxi brakes against power. After aircraft has stopped the engines are left at relatively high rpm for 30 seconds before they are suddenly cut. Presumably the prolonged running is to stabilise EGT and/or cool down the turbo-chargers before the engine is stopped?

I recall the POH recommends running down at idle rpm (which is 850 rpm according to POH I think) for two minutes to stabilise temps (which temps?)

So what is the point in running the engines at higher than idle (850) if the reason for the run down procedure is to drop the turbo-charger temperature as low as practicable before shutting down the engines. Surely 1200 rpm will give slower rate of temp reduction than 850rpm? Prefer facts as well as personal opinions on the subject.

[Jet_A_Knight]

From memory...
Turbos will not be 'on boost' at 1200rpm and < 25"MAP.
Turbocharger Run-down period usually about 2mins in order 'to stop spinning' before the oil supply stops when engine is shut down.
The cooldown period is usually for CHT.
Normal taxi speed for 2-3 mins with cowl flaps open is usually sufficient to achieve all of the above, (unless you have been making an appch & ldg using 30"/2500rpm and dragging the aeroplane in on it's @rse on the back end of the drag curve := )

[OpsNormal]

Often observe Chieftains taxiing in at my local airport using what looks like very high idle rpm (1200?). THe power is kept high until the aircraft is parked in the lines. The pilots must be deliberately dragging the brakes to keep taxiing speed down at such high revs. I was always taught it is a bad thing to taxi brakes against power. After aircraft has stopped the engines are left at relatively high rpm for 30 seconds before they are suddenly cut. Presumably the prolonged running is to stabilise EGT and/or cool down the turbo-chargers before the engine is stopped?
I recall the POH recommends running down at idle rpm (which is 850 rpm according to POH I think) for two minutes to stabilise temps (which temps?)
So what is the point in running the engines at higher than idle (850) if the reason for the run down procedure is to drop the turbo-charger temperature as low as practicable before shutting down the engines. Surely 1200 rpm will give slower rate of temp reduction than 850rpm? Prefer facts as well as personal opinions on the subject.

Seems odd. You don't understand the reasoning behind the cool down, yet you can quote "figures" from the POH? If you've seen someone running these things on the ground actually at 1200 rpm idle you can be sure if his/her boss knows, then s/he probably has another a#$@hole by now courtesy of the boss. Dragging the brakes? A Pa31 that has any sort of a load in it will need most if not all of that 1000rpm to taxi with, especially turning small radius corners. If you had or have flown one you'd know that....

I do not know why this is, but I am yet to see a Pa31 that will idle smoothly and without suddenly stumbling and loping under 950-1000rpm straight after landing (especially with the pumps off after landing) no matter how gentle you try and be on the machine and coax it to idle lower. With the pumps on the gear just gets blacker and blacker.... Whereas the 402 will quite happily idle down around 5-600rpm and if not for the gearboxes, so would a Titan.

That run-down time is to allow the turbo to equalise temps through the turbine wheel and its associated ceramic bearings, thus removing any left over hot spot or hot(er) area. Don't get me wrong, that turbo is still bloody hot, it has just been allowed to settle temps to a dull roar after the 3minute run-down after landing. There is also a requirement for the EGT to stabilise at or below 800F or you wait until it has. The run-down does cool the turbo, however it is only to equalise and stabilise temps in the turbo, not primarily to lower it to ambient temperature.

Why don't you go and ask these questions of the pilot concerned? Most people will give you the time of day or help you to understand something that you are not sure of if you ask them.

Jet_A. Atmospheric @ SL is 29.92 inches, so wouldn't "on boost" be anything above that (minus about an inch or so of MAP to allow for induction system losses)? ;)

[The Messiah]

Sorry OpsNormal but I don't agree with you at all. For a start EGT is instantaneous and has no relevance whatsoever when taxying after landing when you will have very low EGT unless it is a gas turbine obviously. Not arguing with you just giving my little bit of knowledge.

Furthermore I don't agree with your theory for turbocharger cooldown regarding stabilizing of temps. The only reason to run at idle or 1200 RPM or whatever for 3 or so minutes is to keep an oil supply for the turbo while it spins down, which is why it should be referred to as a turbo rundown period not a cooldown period because it implies the wrong principle.

Any questions on this matter should be directed at the LAME signing it out not the driver who may very well be damaging it.

[OpsNormal]

Messiah, would you care to have another guess? I don't mind you disagreeing with me at all if you can supply evidence of your theory. This isn't a "theory" of mine, it is in the many books AFMs/POHs relating to the aircraft and their engines. Apart from a previous life as a mechanic and having spent more than just a significant part of my previous life income on developing, building and racing turbo'd cars.

Also, why is it any different in a turbine? Even with a PT6-41/2 you still must achieve less than 585 dgrees for at least a minute, yet the only difference being that a turbo charger is a purely centrifugal design, and not a hybrid of axial and centrifugal designs like a PT6.

EGT is not completely instantaneous at all. As the engine has been producing power for the duration of the flight (bearing in mind that even in the circuit the engine/propellors are still driving the airframe), then after landing what air there is passing through the cowls is still cooling the heads/cylinders/crank case/oil cooler etc. In practice the EGT is a function of combustion temperature, which is obviously lower at a given idle speed than in flight. The EGT does not all of a sudden drop to below 800F (for example on the Pa31) straight away after landing, but drops markedly towards this, then slowly for the rest of the way as the thermal loads of the engine are dispersed into both the exhaust stream and the air passing through the cowls.

Likewise with a turbo charger. The turbo charger shaft RPM is a function of power output (exhaust flow) of the engine (bearing in mind that there is no physical connection to the engine crankshaft except for an exhaust pipe and an air inlet manifolding), otherwise it would tend to stop the engine from being able to decellerate to an idle speed (much like a rootes type supercharger without a throttle actuated blow-off valve). To suggest that a turbo is still "spinning down" during the run-down period is misguided and completely wrong. Because it has no real inertia (OK, things that spin at 60-90K rpm will have a certain mass inertia, but you would be wrong thinking that on base/final/roll-out a turbo is still spinning at anything more than 15-20% of max rpm) the turbo has already done its winding down, and now only needs to get rid of its heat.

The purpose of stabilising the temperatures in a turbo system is to allow the turbine wheel, the ceramic bearings and the shaft (the three hotest running parts of the device) to cool while the engine is still running (yes, as well as supplying oil) so that they are not subjected to thermal stresses or shocks which will either cause them to a: sieze the bearings in either the case or the shaft, or b: crack or distort the turbine wheel if it cools too quickly without a thermal path, and c: allow the oil to help cool those bearings otherwise if the turbo was shut down red hot, the surface of those bearings would be burned into a scaly varnish, forever damaging the bearings.

With all due respect Messiah, your almost but not quite right. I'd be keen for you to back-up your claims with verifiable data and links.

OpsN.;)

[tlf]

Also, why is it any different in a turbine? Even with a PT6-41/2 you still must achieve less than 585 dgrees for at least a minute, yet the only difference being that a turbo charger is a purely centrifugal design, and not a hybrid of axial and centrifugal designs like a PT6.




It's different because the turbine engine is it's own oil supply, the trubocharger relies on oil from the engine it's attatched to. The turbine engine as it winds down will continue to pump oil. Once the piston engine stops there is no oil to the turbocharger.

EGT, that's GAS temperature does change instantaneously with any change in parameters of the components leading up to combustion, that does NOT mean that the temperature of the exhaust components changes instantaneously. The EGT on a chieftain will be very low when the power is pulled off for tounchdown, but the turbo will still be quite hot. At idle the exhaust gas is still passing through the turbo so to suggest that it ran down during the low power approach is nonsense.

From the Teledyne Continental Operators Manual for the GTSIO-520-H

"Stopping engine

2. Run the engine at idle for approximately 5 minutes to allow the turbochargers to cool off and slow down.

NOTE...TAxi time after landing may be considered part of the 5 minutes.

The turbochargerswill generally spin from 1 to 2 minutes after the engines have stopped. If the engines are stopped too soon, and the turbochargers are still turning at high speed, the shaft bearings will become starved for lubrication."

Obviously the worst thing you can do to a turbo'ed aircraft engine is to give it a kick in the guts (spinning up the turbo again) turning into the parking spot and then killing the engines straight away depriving the unit of oil.

[The Messiah]

I'm not backing anything up as this is not a court of law but a rumour network. Ultimately so long as any pilot realises they need to run the turbo'd engine at low RPM for 3 or so minutes that is all that matters, whether they understand why I guess is not so important. As for us we just have to agree to disagree I guess.

I will reiterate however that any EGT reading is instantaneous as the probe is fitted directly in the exhaust so it cannot be anything else. I agree it is a function of many things but ultimately it is simply a measure of the temperature in the exhaust and nothing more than that, therefore after landing a turbo piston the EGT will read very low very quickly.

As for 3 or so minutes at idle, it is my belief (and every LAME I have ever met) that the most important reason is for lubrication while the turbo spinsdown.

No need to give me your resume but as you did I will say that I have also been around a bit of motor racing in my life and I know that the turbo charger on any racing car has a much shorter life expectancy and far shorter service intervals than a turbo off any TIO-540 for instance.

By the way none of this is a guess as you say.

[Ozgrade3]

I think its safe to say the only people who REALLY know what happens are the people at GAMI in the US who have a Chieftain engine permanently mounted in a specially instrumented test cell. They are the only ones who can scientifically verifiable data, the rest is all hearsay and supposition.

[PLovett]

Often observe Chieftains taxiing in at my local airport using what looks like very high idle rpm (1200?). THe power is kept high until the aircraft is parked in the lines.

I'll let the others debate the rest of your query but I suggest that if pilots are keeping the RPM around 1200 then the aircraft may be equipped with an airconditioner which I believe needs that sort of RPM to keep operating.

[sundown]

2minutes....3 minutes.....4 minutes....egt's.....cht's-who cares.
What's that saying: As long as you can use the ole girl again-everything must be A.O.K:ok:

[Fred Gassit]

One possible reason for the high idle could be to avoid fouling plugs. The lead scavenging agents in AVGAS dont work efficiently until a certain combustion temperature is reached and this is achieved at around 1200rpm. Prolonged idling can foul plugs to the point where no amount of running up can clear them. This applies to boosted and naturally aspirated engines.

[Air Ace]

A37575. Have you been here, to the manufacturer's web site (www.lycoming.textron.com/) and researched the facts, or do you intend to rely on the arm chair "experts" in this thread?

You are certainly getting the usual crop of "personal theories"!

I suggest you don't take some of them too seriously!!

Read the manufacturer's data - available on the internet - read the aircraft POH and have a chat with a good LAME! :ok: :ok:

:}

[Jet_A_Knight]

Jet_A. Atmospheric @ SL is 29.92 inches, so wouldn't "on boost" be anything above that (minus about an inch or so of MAP to allow for induction system losses)?

Ahhh, no. Where have you ever seen an engine that is running at sea level having a minimum MAP of 29.92"???

Once the engine is started, like any normally aspirated engine, MAP at idle can be down to about 16"MAP. It's got nothing to do with the ambient air px, until we talk about max throttle height etc etc etc etc.

On a Chieftain, the turbos start to provide boost from about 25"MAP.

You can over-intellectualise it as much as you like, but power in the circuit would be 20-24" MAP (depending on weight) in the circuit, so the turbos were therefore 'off boost' in the circuit til landing, and after that, a further 3 minutes for CHT to stabilise below 300degrees (I can't remember if that's the exact temp) and the turbochargers to RUN-DOWN before stopping the oil supply to them after engine shutdown.

PS I never worked on race cars, I only have about 800ish trouble free hrs on PA31's.

Someone might know or remember better.

[pa31hero]

Oh please. Turn it on, turn it off. How the hell do you know what the RPM is as he taxies in?

Turn the engines on. Lets go. Turn em off when your done. Who give's a fark the boss pays for new turbos anyway.

As if ANY of you give a hoot?

Fark off losers.

Stop standing at the fence and learn some real flying skills you pr*cks.

[Woomera]

Degenerated to the usual, predictable parlous standards!! :ugh:

http://img.photobucket.com/albums/v315/Woomera/Closed.gif

Sunny Woomera