What would you do in this flight scenario? Potential for shock cooling

[hazek]

1 hour ago, kortopates said:

Maybe this article will help some:

It didn't, at all. The only perhaps valuable evidence based nugget of knowledge this article has is what supposedly Kas Thomas has written some long time ago about some test stand data. My personal experience teaches me however that cooling rates of -50F/min are reached quite easily. At least on my engine measured by my EDM.

The article is riddled with contrarian folklore and anecdotes. What I'm looking for is hard data and facts. Precise and hard answers to:

• procedures that were followed, exactly
• EDM data
• engine longevity data

Anything short of analyzing that are conclusions which are not rooted in facts. That's another good nugget from the article: “Everyone is entitled to his own opinion, but not his own facts.” - Yes indeed, so what are the facts is my question! And I don't hear any.

[Paul Thomas]

I'd just pull power and slow down.

The concept of "shock cooling" in engines is primarily a metallurgy-based issue. The potential for damage typically doesn't manifest until the cylinder heads have already been deformed. This deformation is unlikely to occur at normal cruising temperatures; I can't remember the numbers because it's been almost 20 years ago since I looked at the topic in depth, but something like 425 or 450. You run the risk of "shock cooling" when CHT are so hot that your cylinder heads are oblong.

The irony of shock cooling is that if your engine was at redline on CHT (500), you'd want to get that down to 400 ASAP but we worry a lot more about temps at which you can't shock cool the engine.

[flyboy0681]

On 5/22/2025 at 1:23 PM, N201MKTurbo said:

Shock cooling is a myth.

 

Someone once commented here that the greatest shock cooling occurs when then engine is shutdown.

[PeteMc]

1 hour ago, hazek said:

The article is riddled with contrarian folklore and anecdotes. What I'm looking for is hard data and facts. Precise and hard answers to:

Okay, I'll bite...  So there are multiple articles and VERY SMART people that say shock cooling is a myth.  Many also say that if shock cooling was an issue, then shock heating would be an issue since you're also putting expansion stress on the metal.  But please show me the proof that it is NOT a myth as you seem to be indicating.  Your credentials, documentation, longevity data, etc., etc.  

As for longevity on why it's a myth.  Look at the flight hours of jump planes, tow planes and all the training flights over the last 50 years or so and note that there is no overwhelming data to show that chopping the power completely will cause any problems.  But again, I'm open to looking at your data and how you present that it shows the other flights that do go from Takeoff or Cruise power to idle or near idle multiple times a day (actually within an hour in many cases). 

 

 

[varlajo]

5 minutes ago, PeteMc said:

27 minutes ago, flyboy0681 said:

Someone once commented here that the greatest shock cooling occurs when then engine is shutdown.

Not sure this is true. Cooling from say 375F cruise CHT at altitude means equalizing to a potentially sub-freezing ambient temperature and a lot of air moving through the engine. Shutdown takes the engine from a much lower descent/landing CHT to a (generally) much higher ambient temperature, and no oncoming air. I'd say the delta is 350-400F vs. 150-200. 

Shock cooling is still a myth though 

[hazek]

https://en.wikipedia.org/wiki/List_of_fallacies

Truth emerges under rigorous reasoning. Reliable conclusions rest on valid logic. Below, each quoted statement is paired with its logical fallacy.

1 hour ago, PeteMc said:

So there are multiple articles and VERY SMART people that say shock cooling is a myth.

• Appeal to authority (argument from authority, argumentum ad verecundiam) – an assertion is deemed true because of the position or authority of the person asserting it.
• Appeal to accomplishment – an assertion is deemed true or false based on the accomplishments of the proposer. This may often also have elements of appeal to emotion see below.

1 hour ago, PeteMc said:

Many also say that if shock cooling was an issue, then shock heating would be an issue since you're also putting expansion stress on the metal.

• Argumentum ad populum (appeal to widespread belief, bandwagon argument, appeal to the majority, appeal to the people) – a proposition is claimed to be true or good solely because a majority or many people believe it to be so.

1 hour ago, PeteMc said:

But please show me the proof that it is NOT a myth as you seem to be indicating. Your credentials, documentation, longevity data, etc., etc.  

• Argument from ignorance (appeal to ignorance, argumentum ad ignorantiam) – assuming that a claim is true because it has not been or cannot be proven false, or vice versa.

1 hour ago, PeteMc said:

As for longevity on why it's a myth.  Look at the flight hours of jump planes, tow planes and all the training flights over the last 50 years or so and note that there is no overwhelming data to show that chopping the power completely will cause any problems. 

• Argument from anecdote – a fallacy where anecdotal evidence is presented as an argument; without any other contributory evidence or reasoning.
   

1 hour ago, PeteMc said:

But again, I'm open to looking at your data and how you present that it shows the other flights that do go from Takeoff or Cruise power to idle or near idle multiple times a day (actually within an hour in many cases).

• Proof by assertion – a proposition is repeatedly restated regardless of contradiction; sometimes confused with argument from repetition (argumentum ad infinitum, argumentum ad nauseam).

I’m glad you’re open to data. I’m equally ready to revise my view if solid evidence contradicts mine. Until then, I hope these clarified fallacies help us keep the discussion on sound footing.

[N201MKTurbo]

Wouldn’t it be glorious to have some data. 
 

Don't  the Savvy guys have a mountain of engine monitor data? I wonder if they have analyzed it for shock cooling? How would you determine that there was a problem? Maybe if the owner reported a bad cylinder. I guess you could correlate engine life to the number of shock cooling events.

[varlajo]

35 minutes ago, hazek said:

Below, each quoted statement is paired with its logical fallacy.

Brutal!! 

[PeteMc]

1 hour ago, hazek said:

Truth emerges under rigorous reasoning. Reliable conclusions rest on valid logic. Below, each quoted statement is paired with its logical fallacy.

Love it.  So the fact that shock cooling is a myth is not really the issue. 

[MikeOH]

@hazek

And, I would point out that every one of your "List of fallacies" is equally applicable to YOUR view that shock cooling is real!  As you point out, we have not met your standard of an irrefutable mountain of data to prove beyond a doubt that shock cooling a myth, but at least those of us that believe it is a myth have quite a bit of anecedotal evidence, while you have NONE.

Anecdotal evidence is certainly not rigid measured data, but it is hardly the worthless pile of nothing you seem to believe, and the stories are repeatable; we're not talking about a single he said/she said story.  People in a given field are not stupid and ignorant about the goings on around them. You complain that hundreds of jump zones and planes around the country do NOT have all the data and procedures documented so you are unconvinced. Do you honestly believe that these operators have rigorous procedures in place that prevent it, or is it far more likely they do chop power and dive back to the ground? Consequently, if they, and flight schools with twins shutting down engines, were cracking cylinders in any statistically significant fashion that we would NOT be hearing those ANECDOTAL stories??

And, as far as actual data, take a look at the temp rise on start up and take off...why aren't your cylinders a shattered wreck of cracked pieces by now?  EVERY flight, EVERY take off....OH, THE HORROR, OH THE HUMANITY!  My expectation is that you will respond with an unsupported (i.e. won't have the mountain of hard data to prove) contention THAT is somehow different and "doesn't count'!

[Hank]

7 minutes ago, PeteMc said:

Love it.  So the fact that shock cooling is a myth is not really the issue. 

There seems to be an appalling lack of data for shock cooling to harm our engines to be true or false . . .

[MikeOH]

10 minutes ago, Hank said:

There seems to be an appalling lack of data for shock cooling to harm our engines to be true or false . . .

While not a rigorous level of proof, I find the LACK of any reporting of mass quantities of cracked cylinders to be relevant data supporting that shock cooling is a myth.  I think the theory that@PT20J related  that cylinder cracks near the spark plug hole are caused by over torqued plugs to be far more plausible.

As also pointed out elsewhere, cracks seem to appear more frequently in cylinders that have been overhauled multiple times; since aluminum has NO fatigue limit, this is not only not surprising, but expected.  If shock cooling was the cause I would think first run cylinders would have a higher incidence (more thermal stress in a short period of time; more fatigue, early failure).

[Hank]

2 hours ago, MikeOH said:

While not a rigorous level of proof, I find the LACK of any reporting of mass quantities of cracked cylinders to be relevant data ...

Lack of evidence for damage is not evidence of lack of damage. Similarly, lack of evidence for no damage is not evidence for lack of no damage.

So, in the lack of evidence either way, each of us has to:

• think for ourselves;
• read the authoritative positions of reputed experts based on their personal experiences and and knowledge;
• and see what believable anecdotes we collect for each position.

Then make up our own minds, and operate our own aircraft accordingly. 

[PT20J]

There was never to my knowledge any hard evidence that shock cooling caused cylinder heads to crack. It was conjecture. So, it seems to me kind of silly to require evidence that it’s not true to stop believing it. 

BTW, I recall another theory to explain head cracks was shoving the mixture control full rich for landing. The idea was that the sudden increase in fuel flow would shock cool the head around the intake valve by evaporative cooling. This theory never seemed to get the traction that the shock cooling by rapid power reduction gained.

[hazek]

12 hours ago, MikeOH said:

YOUR view that shock cooling is real

Where did I say that??

[MikeOH]

4 hours ago, hazek said:

Where did I say that??

Well, this is pretty simple: you have been arguing rather forcefully that shock cooling isn't a myth absent near absolute proof otherwise (or, are you going to claim you haven't ) AND this isn't a multiple choice question; it's either true or false.

The real question is what do YOU do?  If you don't chop power because you are afraid of shock cooling, then why do I need to prove what you did or didn't say?

So, do YOU chop power, or not?

[MikeOH]

14 hours ago, Hank said:

Lack of evidence for damage is not evidence of lack of damage. Similarly, lack of evidence for no damage is not evidence for lack of no damage.

Sure it is!  Anecdotal information is NOT proof, but is most certainly evidence.  The strength of which is highly dependent on quantity and source. But to act like "it doesn't count" and should be universally ignored simply because it doesn't fit your world view is a poor argument.

14 hours ago, Hank said:

in the lack of evidence either way, each of us has to:

• think for ourselves;

LOL! Again, there is evidence, and more of it, that indicates shock cooling is a myth.  Examples have been given of jump planes and twins used in multi-engine training. Where are the counter examples of piles of cracked cylinders?  And, yes one must think for themself; but do so with an open mind.

15 hours ago, Hank said:

• read the authoritative positions of reputed experts based on their personal experiences and and knowledge;
• and see what believable anecdotes we collect for each position.

In this very thread three pretty credible sources have been referenced/cited: John Deakin, Rick Darden, and John Schwaner.

Where are the cites of 'believable' credible sources of counter anecdotes?

15 hours ago, Hank said:

Then make up our own minds, and operate our own aircraft accordingly. 

As with pretty much everything in life, right?

[hazek]

2 hours ago, MikeOH said:

Well, this is pretty simple: you have been arguing rather forcefully that shock cooling isn't a myth absent near absolute proof otherwise (or, are you going to claim you haven't ) AND this isn't a multiple choice question; it's either true or false.

There is indeed a third option: I'm agnostic either way.

 

2 hours ago, MikeOH said:

The real question is what do YOU do?

I adhere to the POH and then the Lycoming best practices manual for my engine + an additional margin for a personal conservative approach on top. I do this because absent of hard evidence to the contrary, i.e. more than some anecdotes, I find that as the most credible source of information. I'm well aware though that the POH is marketing material, especially for the Bravo, so nearly worthless. And I'm also well aware that Lycoming's manuals in general are full of lawyer speak. On the other hand I know that metal expanding and contracting causes fatigue, this is established science and not up for debate. Also cylinders do crack and fail, this also cannot be up for debate, hell we have one such case in our closet at the club.

So that's why my position is in fact agnostic. I take a conservative cautious approach to give myself the best possible shot at preserving my engine, which I value above convenience or comfort At the same time I follow what ever new evidence might come out to disprove manuals and show a better way and am open to adjusting.

My only point of contention, which seems to bother people, is that I'm unconvinced that there is sufficient hard evidence to say that chopping power and allowing the engine to cool rapidly does not in fact severely impact the longevity of the cylinders. You say there is evidence and what you point to are nothing but anecdotes and fallacies. I just wish you'd actually supply evidence.

[IvanP]

Great discussion - kind of similar to the discussion about existence or non-existence of deity. 

On the one hand believers assert that the deity exists becaue the non-believers did not prove its non-existence, whereas, on the other hand, the non-believers assert that deity does not exist becauce noone is able to prove its existence. Neither side has definitive proof supported by empirical evidence, but both sides present vigorous arguments supported by sometimes questionable logic.  

Maybe we need to engage Myth-Busters to get definitive answer.

[PT20J]

And, as we all know, engines most often fail at the first power reduction. 

[PeteMc]

10 minutes ago, PT20J said:

And, as we all know, engines most often fail at the first power reduction. 

Well OF COURSE that is true!!!  My one partial engine failure (I learned later it was soon to be total failure) happened when I reduced power to duck down under the NY Class B.  So you're not going to convince me it's not true!!!!  

And it must have been shock cooling because I didn't take 20 minutes to reduce 10" of MP. 

Edited May 30 by PeteMc

[PT20J]

22 minutes ago, PeteMc said:

Well OF COURSE that is true!!!  My one partial engine failure (I learned later it was soon to be total failure) happened when I reduced power to duck down under the NY Class B.  So you're not going to convince me it's not true!!!!  

And it must have been shock cooling because I didn't take 20 minutes to reduce 10" of MP. 

So, ducking under Bravo airspace causes engine failure. Good to know. I’ll circumnavigate or get a clearance in the future. Thanks for the data. 

[N201MKTurbo]

1 hour ago, PT20J said:

So, ducking under Bravo airspace causes engine failure. Good to know. I’ll circumnavigate or get a clearance in the future. Thanks for the data. 

It is a wonder there are any airplanes left here in PHX!

[Pinecone]

23 hours ago, hazek said:

I adhere to the POH and then the Lycoming best practices manual for my engine + an additional margin for a personal conservative approach on top.

So, you don't operate LOP?

[Pinecone]

19 hours ago, PT20J said:

And, as we all know, engines most often fail at the first power reduction. 

I had heard that in the past, mainly when the idea of full power climbs to cruise altitude came about.  Prior to that, it was common wisdom to reduce power to 25/25 at about 1000 AGL.

Has anyone done a rigorous look at that OWT?