Danger of shock cooling

Danger of shock cooling

I always hear that you should not shock cool the engine by abrupt power changes. I was wondering why, what does this do to the engine?

Is it from the expansion and contraction of different metals at different rates?

Also wondering if there is a certain temp that would be too cold to perform spins for this reason? C-172

Thanks for any feedback

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Re: Danger of shock cooling

Kas Thomas wrote a good article on AvWeb, "Shock Cooling: Myth or Reality?" a few years ago. You can access it online at <http://www.avweb.com/news/maint/182883-1.html>.

I've experienced stuck exhaust valves from throttling back from cruise to idle too quickly, so I've learned to gradually reduce power over a few minutes when beginning my descents. I'm pretty sure the stuck valves were caused by the difference in thermal expansion coefficients between the steel exhaust valve stems and the valve guide material.

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Re: Danger of shock cooling

It is our contention that shock cooling is a myth.  I routinely reduce power significantly and never worry about shock cooling the engine.  How can you shock cool something that's not hot already?  If my CHTs ar eunder 380 or so, I can't shock cool them.

This is an interesting Old Wive's Tale that will not die.  Prior to the introduction of the turbo Twin Cessnas, there is no reference to the phenomenon of Shock cooling anywhere in any of the aviation literature.  So how did it all get started?  In the 421, pilots were descending from the flight levels to land. The pressurized aircraft could descend quickly and not be an ear-popper.   The fuel in the tanks was very cold from being at altitude.  They'd reduce the power and when they got to pattern altitude, the common practice was to go full rich.  This blast of very cold fuel on the inside wall of the warm intake chamber caused cracking.  That's where all of this shock cooling stuff came from.  The cylinders were not having trouble at all.  The problem was the super-cooled fuel from flying high hitting the warm metal.

Shock cooling is a myth for operational purposes unless the CHT is near redline and you chop the power and go full rich.  That stupid pilot trick can cause problems. <g>

Walter Atkinson
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Re: Danger of shock cooling

Thank you Walter.  I have also wondered about this so called problem.
I will sleep better tonight.  Hopefully Santa will leave me something aviation related this year. 
I just wish I had been a better boy.  lol
Merry Christmas
Clay Story

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Re: Danger of shock cooling

I agree with Walter, I think it's an old wives tale. In my game of AG flying there is a lot of throttling off to land, with aerial fertilizing there is a landing every 6 or 7 minutes. You go from full power to criuse to throttle off in that time with landing throttling right off until touchdown. I have flown behind Pratt&Whitney, Continental and Lycoming, none seem prone to "shock cooling". In all the old flight manuals and pilot notes on aircraft before WW2 the instructions are to just trottle off and land, even up to the early 60's that was the way to land. It's only been with the introduction of shallow approaches that power has been carried to landing.

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Re: Danger of shock cooling

Thanks for the replies, I am still not totally convinced. I will still pull power out and increase power gradually. I know that this wont hurt the engine.

Check out this article:
http://www.liquidcooledairpower.com/lc- … ling.shtml

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Re: Danger of shock cooling

I forgot to add there are always exceptions, glider towing and parachuting are very hard on engines. Low speed high power climbs then low power high speed decsents, you can run into trouble with those but "normal" operations OK.

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Re: Danger of shock cooling

Jim:

**I will still pull power out and increase power gradually. I know that this wont hurt the engine.**

That is true, but please do not continue to spread what is clearly, proven to be an Old Wive's Tale about shock cooling.  Tain't so.

Walter Atkinson
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Re: Danger of shock cooling

**I forgot to add there are always exceptions, glider towing and parachuting are very hard on engines. Low speed high power climbs then low power high speed decsents, you can run into trouble with those but "normal" operations OK**

OK, let's take this at face value.

Why then do those engines generally make TBO? 


That reality is at odds with the assumption that those operations are hard on the engine.  They are not.  So, the question is, "Why aren't those operations as hard on the engine as it apprears they should be?"

Walter Atkinson
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Re: Danger of shock cooling

Walter said:

That is true, but please do not continue to spread what is clearly, proven to be an Old Wive's Tale about shock cooling. Tain't so.

I dont believe i have spread any information about this not being an old wives tale. I simply asked a question and wanted feedback from both sides.

Jim
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Re: Danger of shock cooling

The comments I made about Parachuting and Glider Tugging come from seeing the results. Seeing those results are not the full story though, are we to assume all those cracked cylinders were new before fitting? Both mobs are known for their penny pinching so fitting of dubious cylinders would be standard for some. Would those cylinders have cracked with use other than Parachuting or Tugging?

Another good story is about geared engines, your not supposed to throttle off and let the prop push the engine. There is no mention in operating handbooks with P&W engines, or mention about this with other older types. There is mention about it with the GO-300 in the 175 Cessna, the mention there is only because of the noise it makes not the damage it does. Has the same thing happened to  the geared engine? In some Aircraft configurations throttling right off can blanket the control surfaces as the prop disk becomes a big brake, good to use for a steep approach but a bit unnerving if your not used to it, is this part of the old wives tale with geared engines? Was it Continental that made the Tiara? good engine bad reputation, was it an old wives tale and gossip that cause people to avoid this engine?

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Re: Danger of shock cooling

Student Pilot:

**The comments I made about Parachuting and Glider Tugging come from seeing the results. Seeing those results are not the full story though, are we to assume all those cracked cylinders were new before fitting? Both mobs are known for their penny pinching so fitting of dubious cylinders would be standard for some. Would those cylinders have cracked with use other than Parachuting or Tugging?**

My experience with these operations is that they last quite well unless they are using junk parts to begine with.  Why?  They operate outside of the Red Box at all times.  Either full rich while climbing or at very low power during descent.  Neither are hard on the engine.

**Another good story is about geared engines, your not supposed to throttle off and let the prop push the engine.**

That is true.  Allowing the prop to drive those engines is very hard on the reduction gears in the nose case.  That is not an OWT.

**There is no mention in operating handbooks with P&W engines, or mention about this with other older types.**

Whoa, there.  There sure is.  The radials are a little different in this regard.  It's not the gearing that's the issue, it's the oil ports on the main bearing that make this an issue.  When the prop drives the enigne, the oil port is covered and the main bearing will be starved for oil.  Allowing a radial to descend with high rpms and low MP puts this at risk.  Ten demerits.  This is not a problem in flat engines becasue there are oil ports on both sides of the journal.

**There is mention about it with the GO-300 in the 175 Cessna, the mention there is only because of the noise it makes not the damage it does. **

That is not correct.  Gear-slashing in the nose case can result and prop failure is possible.  We take great care in the C-421 to not reduce power to allow the props to drive the engine until the wheels are on the runway. In doing so, the nose case gears are making TBO quite reguarly.

None of this applies to direct drive, flat engines.

Walter

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Re: Danger of shock cooling

Where in the 175 and the 421 POH's or owners manual's does it state the engine management for geared engines you suggest.

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Re: Danger of shock cooling

Student Pilot:

**Where in the 175 and the 421 POH's or owners manual's does it state the engine management for geared engines you suggest.**

The following information is from the 1960 Cessna 175/Skylark Owners Manual, page 3-2.

LET DOWN.
     The crusing glide should begin far enough away from destination so that a gradual descent can be made with power on, with mixture full rich.  On approaching the landing field, the engine should be throttled down gradually and the glide, with closed throttle, should not be longer than neccessary.

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Re: Danger of shock cooling

From a 1962 Owners manual.........Approach glides are normally made at 70 to 80 mph with flaps up, or 65 to 75 with flaps down........that's it, There is mention of cowl flap use to keep temps up, no mention of special throttling off proceedure or propblems with gear lash.

In the 421 owners manual there is no mention of special proceedures either, you would think if this is a problem the manufacturer's would have stated the problem and the cure.

With regard to Pratt's to which "oil port" are you refering to is uncovered by letting the prop drive the engine?

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Re: Danger of shock cooling

Student Pilot:

**In the 421 owners manual there is no mention of special proceedures either, you would think if this is a problem the manufacturer's would have stated the problem and the cure.**

Thinking that is a fairy tale.  The manual was written before the first airplane was sold and seldom, if ever, are they revised.

**With regard to Pratt's to which "oil port" are you refering to is uncovered by letting the prop drive the engine?**

The main bearing.  It has only one oil port, on one side of the journal.  Flat engines have two oil ports, one on each side of the journal.  This allows for oil entrance regardless to which side of the journal pressure is applied.

Walter Atkinson
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Re: Danger of shock cooling

The 421 owners manual might have been written before the Aircraft was out and about but General Aviation had a bit to do with geared engines before then. The Queenair had geared Lycomings, Pigaggio had Geared pusher Lycomings and Cessna had the 175 with the Continental. There were also other not so well known Aircraft with like early PC-6 Pilatus, Dornier Do-27 and DO-28. All of these Aircraft had geared engines well before the 421 was released, if there is no mention of special operating proceedures in the manual it smacks of an old wives tale.


Are the geared Pratt and Whitney's the only ones with the "single port" crank? are you talking of oil pressure when you talk of "pressure on one side"?

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Re: Danger of shock cooling

Student Pilot:

**Are the geared Pratt and Whitney's the only ones with the "single port" crank? are you talking of oil pressure when you talk of "pressure on one side"?**

Yes, oil port.  Yes, oil pressure on one side of the bearing.  I think they are all the same.  I don't think this had anything to do with the nose case gearing at all.  I know the 985s are like that.  There might be an exception, but I am not aware of it.

Walter

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Re: Danger of shock cooling

I've got a couple of years behind 985's, I used to throttle right off to get braking from the prop to make steep or quick approachs. On the Op's we used them for there was a landing every 6 or 7 minutes. I've never heard of any problem with oil feed, if the prop is driving the engine then you would think the oil pump would be doing the same revs and the oil pressure would be OK.

The work was aerial fertilising, taking of with up to a ton of fertiliser then spreading it at a rate of 1 cwt/acre then back to land. The strips were no more than 2 mile from the job.

The engines would go full time and some of the operators used to only put half the time down, with some of the engines doing up to 2,500 oil switch time (at the time a complete overhauled engine was cheaper to buy than doing up your own). The main thing with the 985 seemed to be warm them up and they would keep on going. The blokes that didn't warm them up properly seem to do the supercharger bearing after a couple of hundred hours. They rekon the oil feed was a small diameter and the thick oil would'nt flow, from memory the drill was no more than 800 revs until 20 degrees then no more than 1200 revs until 40 degrees, then run up's. Gen online at 1200, prop at 1400 (full movement with counterweight prop) then mags 1750 and full power and pressure check at 2000 with MP at 30 inches. Ready set go.

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Re: Danger of shock cooling

My experiences exactly. 

That situation was special in that the time that you had reduced throttle and were letting the prop drive the engine was very short.   Quick turns and hot turns, too.  Lotta dusters around here, too.

It's very different in a long decent.  Do that in a DC-3 or my Twin Beech and the engines won't last too long.  You'll risk spinning a main bearing.  (ask me how I know?  Damned wise-ass pilot who insisted on doing things his way!   Arrrrgh)

<ng>

Walter

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Re: Danger of shock cooling

To Walter A. You have enlightened me through the years on the shock cooling myth as well as running the engine lean of peak.
I routinely run my engine LOP in my Bonanza. The ABS service clinic commented on the cleanliness of my plugs, cylinder walls and exhaust. The oil stays cleaner longer too. Good, smooth pilot technique keeps me from "chopping" power. Not the fear of "shock cooling. It is funny to watch peoples' reactions when I teach them how to operate LOP and when I tell them shock cooling myths.

Dan R.

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Re: Danger of shock cooling

Dan:

Thank you for your kind words.  It's guys like you who spread the good techniques who make aviation better.

Thank you.

Walter

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Re: Danger of shock cooling

What is LOP?  Low Operating Power?

Dan

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Re: Danger of shock cooling

LOP =>  Lean of Peak EGT.

Operating the engine on the lean side of peak EGT.

ROP =>  Rich of Peak EGT.

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Re: Danger of shock cooling

Thanks, I am a new pilot and one of the hardest things is the abreviation of almost everything, I went so far as to make a list of them and keep it with me just in case.

Dan

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