Shock cooling of engines

4

4RNB

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4RNB
I've seen this discussed as something to be avoided and have seen the impact of putting a very hot glass in the sink and having it come in touch with very cold water. Sometimes the glass breaks.

I've noticed my engine cooling rapidly at times and want to learn more. Is there more scientific data on shock cooling and its impact on engines? Is there a base rate of problems? Is there a certain time frame of concern, is it an instantaneous change as in the glass in the sink, or does over a minute or two really matter?

Yesterday I was 9500 VFR when my destination seemed to be going IFR. I turned around and the cloud picture ahead of me looked impressive. 20 miles out my options looked best if I could get beneath the clouds while I could, I did so, but it required some rapid descending. This cooled things down.

In addition to just wanting to learn, I am curious how others might manage their engines in such scenarios? I couldn't keep the temps up.
 
Mostly an owt imo.
www.avweb.com

Shock Cooling: Time To Kill The Myth

Some years ago, I had one of those "what in the world are they thinking?" conversations with a pilot who was towing gliders as a volunteer for the Civil Air
www.avweb.com www.avweb.com

Lycoming has some guidance

How to Avoid Sudden Cooling of Your Engine

Sudden cooling is detrimental to the good health of the piston aircraft engine. Lycoming Service Instruction 1094D recommends a maximum temperature change of 50˚ F per minute to avoid shock cooling of the cylinders. Operations that tend to induce rapid engine cooldown are often associated with a...
www.lycoming.com www.lycoming.com

To the extent it is a problem, lycoming says to limit cht drop to 50*/min. That's a lot. You basically have to go to idle and point the nose down. My engine monitor has a built in warning when you hit that 50*/min threshold. I've only ever seen it go off doing emergency descent practice on a cold day.
 
Jim K said:
Mostly an owt imo.
www.avweb.com

Shock Cooling: Time To Kill The Myth

Some years ago, I had one of those "what in the world are they thinking?" conversations with a pilot who was towing gliders as a volunteer for the Civil Air
www.avweb.com www.avweb.com

Lycoming has some guidance

How to Avoid Sudden Cooling of Your Engine

Sudden cooling is detrimental to the good health of the piston aircraft engine. Lycoming Service Instruction 1094D recommends a maximum temperature change of 50˚ F per minute to avoid shock cooling of the cylinders. Operations that tend to induce rapid engine cooldown are often associated with a...
www.lycoming.com www.lycoming.com

To the extent it is a problem, lycoming says to limit cht drop to 50*/min. That's a lot. You basically have to go to idle and point the nose down. My engine monitor has a built in warning when you hit that 50*/min threshold. I've only ever seen it go off doing emergency descent practice on a cold day.
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So what do you do for fast descents?
 
How fast are you needing to descend? You can’t compare an engine made of steel, iron and aluminum to a drinking glass. While I do believe you can add stress to the metals if the temperature change is drastic, I don’t think it’s as prevalent as it seems. If you absolutely need to descend at a high rate, which shouldn’t typically be required, keep the mixture leaned out and try to clear the engine if it’s prolonged. About all you can do.
 
RyanB said:
How fast are you needing to descend? You can’t compare an engine made of steel, iron and aluminum to a drinking glass. While I do believe you can add stress to the metals if the temperature change is drastic, I don’t think it’s as prevalent as it seems. If you absolutely need to descend at a high rate, which shouldn’t typically be required, keep the mixture leaned out and try to clear the engine if it’s prolonged. About all you can do.
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Yesterday I do not know how fast I needed. But once I got to the opening of choice, at the top of the clouds I wanted to get to the bottom with good cloud clearance. I agree on the comparison, just used glass as an example of fast thermal changes causing stress. I've read the AVWEB stuff on ignoring the overly hyped shock cooling myths/. I am just trying to learn science and facts.

I'd thought of the descent as full prop, full mixture but am reading about keeping a lean mixture.

I think I was 2500 fpm+ yesterday, saw 3000 with the factory rep on a test flight. He said to just push it to the ground and fly it there (RV10 different than a 172).
 
I like Mike Busch's comment that if you believe in shock-cooling, then you have to believe in shock-heating. Take a look at how quickly the CHTs rise from ambient to operating temperature after engine start. Then take a look at the sky-diving planes which chop power and dive all day, every day, without apparently harm. Same with aerobatics.

In the absence of solid data which shows shock-cooling to a) exist and b) cause perceptible engine harm, I don't give it a lot of thought. That said, I think avoiding abrupt engine power and altitude changes through good situational awareness and proper planning is generally good pilot technique.
 
It's going to be different with every plane. Do you have an engine monitor? Your best bet is to replicate your conditions and see how fast the chts drop. My Lance is the only plane I've flown with cht. It has a lot of cooling, so my cylinders never get that hot. Tightly coweled planes that run higher chts will have more concern as the temperature delta will be greater. I don't know enough about the -10 to comment.

I dont think I've ever descended that fast outside of emergency descent practice. That should be such a rare occurrence as to not worry about the few times it'll happen over the life of the engine. I think 1000fpm is about a fast as I've ever descended on a normal flight, and that's because I screwed up and stayed high too late. That's easily achievable with some power still in.

There's no reason to go full rich in a descent. You do have to enrichen on the way down, although the worst case scenario if you don't is the engine stops producing power until you give it some gas. If you go too rich, that's just more cooling, wasted gas, and washing the oil of the cylinder walls.

If I owned a skydiving plane that regularly did fast descents, I'd expect that the period during which they dropped the meat bombs would cool the cylinders off from the climb enough to allow them to survive a chop & drop afterwards.
 
4RNB said:
I've noticed my engine cooling rapidly at times and want to learn more. Is there more scientific data on shock cooling and its impact on engines? Is there a base rate of problems? Is there a certain time frame of concern, is it an instantaneous change as in the glass in the sink, or does over a minute or two really matter?
Click to expand...
When it comes to aviation topics context is everything. Unfortunately, over the years and especially with advent of the internet age that context has become altered or even cherry-picked over time. In the case of cylinder shock cooling, yes, there can be concerns so long as the original context of those recommendations is understood. Which for cylinder shock cooling is simply "when pilots consistently, make fast letdowns with little or no power" with the key word being consistently, i.e., in every case or on every occasion. So, in general, shock cooling is not an issue for your average recreational pilot. But to add, one of the most detrimental conditions one can induce on an air-cooled aircraft engine is uneven operating temps across the engine. So whenever possible its better to manage off-power letdowns, limit ground runs without cowling, and maintain engine baffles to spec.
 
4RNB said:
I'd thought of the descent as full prop, full mixture but am reading about keeping a lean mixture.
Click to expand...
You wouldn't run full rich at at idle on the ground, why would you do it in the air, much less at altitude?

I would be thinking more about the sequence of decisions that led to me being at 9500 and needing to get down in that much of a hurry.
 
I remember when I was in a glider club they were always replacing cylinders in the tow planes. Would those cylinders have cracked if the tow planes were run at almost constant MP and RPM? Who knows. But the cylinders did seem to need replacement more often than in other aircraft. The tow pilots were instructed to avoid shock cooling.

I don't know if there is anything to it or not. I don't know if it's the same thing as shock heating, but in reverse. With engine metals, heat makes components expand, and that increases clearances.
 
I don't have the experience to offer my own opinion. But El Jefe, who is ultimately on the hook for us as a tow pilots, quotes folks like Bob Hoover in telling us there isn't any reputable evidence that making big power reductions, or even just shutting one down mid flight, will cause cylinder walls to crack, or reduce the longevity of an O-360. Once the glider releases, our procedure is to smoothly pull power to the desired setting in order to get back to pattern ASAP while aggressively reducing mixture.
 
LWGII said:
I remember when I was in a glider club they were always replacing cylinders in the tow planes. Would those cylinders have cracked if the tow planes were run at almost constant MP and RPM? Who knows. But the cylinders did seem to need replacement more often than in other aircraft. The tow pilots were instructed to avoid shock cooling.

I don't know if there is anything to it or not. I don't know if it's the same thing as shock heating, but in reverse. With engine metals, heat makes components expand, and that increases clearances.
Click to expand...
The stuff on heard from Busch made it seem like clearances decrease as they heat.

podcasts.apple.com

Shockingly Hot - Aviation Podcast

Podcast Episode · The Finer Points - Aviation Podcast · 02/25/2025 · 13m
podcasts.apple.com podcasts.apple.com
 
GeorgeC said:
You wouldn't run full rich at at idle on the ground, why would you do it in the air, much less at altitude?

I would be thinking more about the sequence of decisions that led to me being at 9500 and needing to get down in that much of a hurry.
Click to expand...

Thanks, I am. I am happy to share my decision process yesterday if you want to suggest where I was off.
My handling of the mixture seems to be a point to improve upon.
 
Jim K said:
It's going to be different with every plane. Do you have an engine monitor? Your best bet is to replicate your conditions and see how fast the chts drop. My Lance is the only plane I've flown with cht. It has a lot of cooling, so my cylinders never get that hot. Tightly coweled planes that run higher chts will have more concern as the temperature delta will be greater. I don't know enough about the -10 to comment.

I dont think I've ever descended that fast outside of emergency descent practice. That should be such a rare occurrence as to not worry about the few times it'll happen over the life of the engine. I think 1000fpm is about a fast as I've ever descended on a normal flight, and that's because I screwed up and stayed high too late. That's easily achievable with some power still in.

There's no reason to go full rich in a descent. You do have to enrichen on the way down, although the worst case scenario if you don't is the engine stops producing power until you give it some gas. If you go too rich, that's just more cooling, wasted gas, and washing the oil of the cylinder walls.

If I owned a skydiving plane that regularly did fast descents, I'd expect that the period during which they dropped the meat bombs would cool the cylinders off from the climb enough to allow them to survive a chop & drop afterwards.
Click to expand...
1000 fpm was the most I recall in the C172. But both the factory rep and my RV10 checkout CFI pushed it over for 3000. I was surprised it felt ok, not disturbing.
 
GeorgeC said:
You wouldn't run full rich at at idle on the ground, why would you do it in the air, much less at altitude?

I would be thinking more about the sequence of decisions that led to me being at 9500 and needing to get down in that much of a hurry.
Click to expand...
If you're descending that fast, shock cooling is the last thing to worry about. Ahead on the list would be "Did my wing fall off" or "Am I going to tear my wing off pulling out of this",
 
Ah yes, the dry-rotted 50hr a year brigade fretting (pun very much inteded) about heat high-cycle fatigue failure LOL.

This is of course the same people who overpay for the price risk of 2000 hours when definitionally they were lucky to get 600 hours by their own use-case's market resale TBO period. And they think I'm the ham-handed one bwahaha. Pennywise pounds fools.
 
Not a big deal for acro. The only time we descend without power is a spin, and that only lasts maybe 10 seconds.

 
hindsight2020 said:
Ah yes, the dry-rotted 50hr a year brigade fretting (pun very much inteded) about heat high-cycle fatigue failure LOL.

This is of course the same people who overpay for the price risk of 2000 hours when definitionally they were lucky to get 600 hours by their own use-case's market resale TBO period. And they think I'm the ham-handed one bwahaha. Pennywise pounds fools.
Click to expand...
I don’t understand this response and can’t tell it’s usefulness or helpfulness to what I’m asking
 
The drop zone near me usually has to do engine work on their 182s around mid-time. Usually by 1300-1400 hours it is more cost effective to just do a major overhaul. They're also often flown by wet ink CPLs. That said, the dropzone can afford it. The jump plane probably brings in more money per hour than a flight school plane does per day.
 
4RNB said:
I don’t understand this response and can’t tell it’s usefulness or helpfulness to what I’m asking
Click to expand...
Don't worry about it; he's harmless. The overall arc of his posts is one of anger and frustration at various aspects of the GA community, manifesting as delightful, sometimes entertaining, and occasionally borderline comprehensible abuses of his thesaurus, in lieu of the professional counseling and possibly medication that would probably be more appropriate. In this case, the thrust of his post was that you are foolishly concerned about an edge-case occurrence expense when he rejects the entire concept of your owning a certified aircraft as a reasonable financial risk given that you're unlikely to make TBO anyway.

I think. :D
 
AvNavCom said:
Don't worry about it; he's harmless. The overall arc of his posts is one of anger and frustration at various aspects of the GA community, manifesting as delightful, sometimes entertaining, and occasionally borderline comprehensible abuses of his thesaurus, in lieu of the professional counseling and possibly medication that would probably be more appropriate. In this case, the thrust of his post was that you are foolishly concerned about an edge-case occurrence expense when he rejects the entire concept of your owning a certified aircraft as a reasonable financial risk given that you're unlikely to make TBO anyway.

I think. :D
Click to expand...

Kinda like Fred needing an interpreter ...
 
Forty years ago, I was taught about shock cooling and treated all my planes like it was bad for the engine so in winter when it was below 40* I would only take off 200 RPM a minute down to 1500 RPM and then do what ever was needed. Now reading some of these posts I am going to have to go watch so of Mikes and Avweb stuff. Always learning. Thanks
 
I believe the theory is that cylinders are made by heating the head and cooling the barrel, then the head is threaded into the barrel, the head cools and shrinks while the barrel warms up and expands thus becoming permanent. If the head is shock cooled and shrinks while the barrel is still hot, this can cause the head to crack.
 
As a resident ignoramus, I thought the risk would be very different based on what's flying and where. A typical GA 'spam can' (N/A)? Flying below 10,000? Probably a myth. A high-performance, heavily turbo'ed twin? Flying in the 20s? Might be a concern.
 
AvNavCom said:
Don't worry about it; he's harmless. The overall arc of his posts is one of anger and frustration at various aspects of the GA community, manifesting as delightful, sometimes entertaining, and occasionally borderline comprehensible abuses of his thesaurus, in lieu of the professional counseling and possibly medication that would probably be more appropriate. In this case, the thrust of his post was that you are foolishly concerned about an edge-case occurrence expense when he rejects the entire concept of your owning a certified aircraft as a reasonable financial risk given that you're unlikely to make TBO anyway.

I think. :D
Click to expand...
*chefs kiss* Well done, masterfully translated. I can tell you've been reading my ish for a second. :thumbsup:
 
hindsight2020 said:
*chefs kiss* Well done, masterfully translated. I can tell you've been reading my ish for a second. :thumbsup:
Click to expand...
Thank goodness! For the proper translation mindset, I knew it was three shots of whiskey, but I couldn't remember if it was one mushroom or two. I'm glad I guessed correctly!
 
LWGII said:
But the cylinders did seem to need replacement more often than in other aircraft.
Click to expand...

since you're not really sure exactly how many hours it took to crack a cylinder I think your point is not indicative of anything. it could be poor maintenance it could be abuse of the aircraft it could just be simply that they were flown a lot, most people get a couple hundred hours before they replace their first cylinder even on freshly overhauled engines.
 
More engines have been killed with HEAT than COLD.
 
4RNB said:
I've seen this discussed as something to be avoided and have seen the impact of putting a very hot glass in the sink and having it come in touch with very cold water. Sometimes the glass breaks.

I've noticed my engine cooling rapidly at times and want to learn more. Is there more scientific data on shock cooling and its impact on engines? Is there a base rate of problems? Is there a certain time frame of concern, is it an instantaneous change as in the glass in the sink, or does over a minute or two really matter?

Yesterday I was 9500 VFR when my destination seemed to be going IFR. I turned around and the cloud picture ahead of me looked impressive. 20 miles out my options looked best if I could get beneath the clouds while I could, I did so, but it required some rapid descending. This cooled things down.

In addition to just wanting to learn, I am curious how others might manage their engines in such scenarios? I couldn't keep the temps up.
Click to expand...

Glass is a poor conductor of heat. When you dunk hot glass in cold water, the outside of the glass cools immediatly, but the interior take a few extra seconds. The difference in expansion causes stress and makes it crack.

It is very different when you cool a hot piece of metal by blowing cold air. The thermal capacity of air is much smaller than water. It takes minutes, not seconds. Combine that with the high thermal conductivity of metals, you won't have a large temperature differential to cause sufficient stress. If you take a red hot engine and dunk it in cold water, then maybe.
 
Daleandee said:
Fly through the rain ...
Click to expand...
Exactly my thoughts, I cannot see how rapidly descending under low power could come close to the thermal stress of cold water droplets hitting the engine and sucking up the entropy of phase change as they flash off as steam from the cylinder. I do understand that the piston would be slower to cool, and that a rapid cooling of the cylinder may reduce the clearances between the bore and piston, but I wonder if this idea didn’t originate in two cycle engines where reducing the throttle also effectively reduced the lubricant supply to the engine that was still “windmilling” at high rpm’s during rapid descents?
 
WDD said:
So - shock cooling can occur if you dive your plane under water.
Click to expand...
Nah...I can offer two counterexamples.

In the perfectly forgettable movie "Sky Captain and the World of Tomorrow", they had a whole squadron of fighter planes that could dive underwater and operate as fighter subs, just by slowing the prop way down.

Also, "Voyage to the Bottom of the Sea" had the flying sub. Though I don't know for certain if it was piston powered.

:)
 
Don't forget SkyDiver in the old UFO series.
 
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