School me on shock cooling.

cowman

cowman

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I had an instructor tell me we shouldn't be practicing any stalls or other maneuvers where there would be lots of big changes in power settings because of shock cooling.. this was on a day when surface temps were around 0F.

I also read something on the internet saying there's nothing supporting the idea that shock cooling happens/causes any harm.

I honestly don't know a lot about the subject but it seems suspect to me that such a thing would really be a problem. Can you guys educate me on the subject?
 
cowman said:
I had an instructor tell me we shouldn't be practicing any stalls or other maneuvers where there would be lots of big changes in power settings because of shock cooling.. this was on a day when surface temps were around 0F.

I also read something on the internet saying there's nothing supporting the idea that shock cooling happens/causes any harm.

I honestly don't know a lot about the subject but it seems suspect to me that such a thing would really be a problem. Can you guys educate me on the subject?
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If shock cooling were such a big problem why is it that most engines in trainer aircraft that are used daily with significant power changes going on all the time and students who could care less about shock cooling make it to TBO? It's because it is not a big problem. That being said, I would not dive to the ground from 10K at idle, but for the most part I would not worry about it. I have an engine monitor in my Mooney that will sound an alarm if the rate of change in Cylinder temperature is large enough that I need to be concerned about shock cooling. It has NEVER gone off and I have monitored that parameter closely during some extended steep descents at low power.


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Perezhr said:
If shock cooling were such a big problem why is it that most engines in trainer aircraft that are used daily with significant power changes going on all the time and students who could care less about shock cooling make it to TBO? It's because it is not a big problem. That being said, I would not dive to the ground from 10K at idle, but for the most part I would not worry about it. I have an engine monitor in my Mooney that will sound an alarm if the rate of change in Cylinder temperature is large enough that I need to be concerned about shock cooling. It has NEVER gone off and I have monitored that parameter closely during some extended steep descents at low power.


Sent from my iPad using Tapatalk HD
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Some of this just depends. For example, most trainer aircraft are using pretty bulletproof overbuilt underpowered engines. Just because you can get away with something on a trainer engine doesn't mean it's a good idea when you're pushing 300+ horsepower.
 
Shock cooling is what you get from a lot of POA members. :yikes:



Honestly, I don't know a lot about it either. I just keep the cowl flaps closed on the way down, and watch the temps, which they never seem to vary much. :dunno:

But I'm not in cold country.
 
I was instructed to not let the CHT decrease at greater than 1 degree per second between 350-400. Below a CHT of 350 it is not a concern. Also, avoid operations with CHT above 400 degrees as aluminum begins to soften at temperatures in the over 400 range.
 
We have a 182 jump plane operating out of our airport. Climbs to 10,000' full throttle, pulls the power, drops meat bombs, and dives to the runway with little to no throttle. Does this day in and day out year round.

Shock cooling is an issue only in the minds of CFIs trying to impress students. :mad2:
 
It is real in one particular operation, and that is glider-towing. Some big british training operation published a paper on this. When their SOP was to release the glider, pull power to idle and to immediately dive back to the field, they kept buying cylinders by the dozen. Once they changed their SOP to include 1min of level flight with a lower power setting before descending, their cylinder replacement frequency dropped back to baseline.

Skydiving operations dont seem to have a problem, they already include the jump-run with a reduced power setting. It seems to be the first 20degF that make the difference, once the cylinders have cooled down just a little bit from peak temp, it doesn't matter much how fast the further cooling takes place. If you never have hot cylinders to start with, it probably makes no difference either way.


(above is my personal opinion based on reading up on this a while ago, ymmv)
 
Computerjim said:
Shock cooling is a myth. Now that all cylinder engine monitors are becoming common, it is evident from observation that the feared fast cooling just does not happen.
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Oh, it happens, at least it does on my cylinders as observed by a UBG-16. I can easily producing cooling in the 35 to 45 degree/min range coming out of slow flight practice if I'm not careful depending on ambient temp and I could produce more if I was really reckless with my technique.

I don't think the question is "does it happen?", but rather does it hurt anything. I'm in the camp with those who don't worry about it much especially since I have to really be stupid to achieve the 50dF/min rate...but I've done it.
 
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The sort of shock cooling which can cause cylinders to crack immediately (the process actually called "quenching" by metallurgists, and the damage being "quench cracking") is not possible short of what Henning suggested (plunging the hot engine into a large bath of cold liquid). The problem which can occur due to a combination of large power reductions and high speeds is uneven contraction of the cylinders and the pistons, causing accelerated wear of the piston rings and cylinder walls. It's not something that is going to cause a catastrophic engine failure on the spot, just shorten cylinder/ring life, and thus something to be avoided, but not feared.
 
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A lot of damage attributed to 'shock cooling' is probably the result of cylinders getting too hot in the first place.
 
weilke said:
A lot of damage attributed to 'shock cooling' is probably the result of cylinders getting too hot in the first place.
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DING DING DING... WE HAVE A WINNER

Analysis by both the engine/cylinder manufacturers (a lot of which is coming to the head over the ECI cyl controversy) is that it is HEAT (usually during the takeoff/climb) that is far more damaging than the illusory shock cooling ever is.
 
flyingron said:
Analysis by both the engine/cylinder manufacturers (a lot of which is coming to the head over the ECI cyl controversy) is that it is HEAT (usually during the takeoff/climb) that is far more damaging than the illusory shock cooling ever is.
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Below is ECIs take on the issue. I am sure there are some who vehemently disagree with ECIs analysis of the barrel separations, absent of a better explanation, I find theirs quite reasonable:

http://www.eci.aero/pages/Cool-Is-The-Rule.aspx

Now, sometimes they do put out things that are rather bizarre:

http://www.eci.aero/pdf/93-6-7.pdf
 
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weilke said:
Now, sometimes they do put out things that are rather bizarre:

http://www.eci.aero/pdf/93-6-7.pdf
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That is pretty bizarre indeed. When I get to cruise altitude, level off, and if I don't wait for a minute or two before leaning, my shock cool alarm goes off immediately upon leaning. Admittedly, I have my alarm to set to go off at fairly low rate, @ 30°F. But ECI asserts that rapid leaning causes cylinder temperatures to rise not fall...I see CHTs fall.

So, I've learned to level off, set my engine power, let it run for a minute or two, and then lean.
 
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I have heard of issues where the IO-360 can shock cool and crack the cylinder tops between the plug hole and valve from the reduced power and cool fuel being injected onto the hot surface. The practice I have seen written out is to lean the engine in the pattern, which I am not comfortable with. I also have my JPI set to alarm at >30* of cooling and sometimes see it upon my power reduction from downwind to base. I have found that if I just ease in a couple more inches of MP it goes away.
 
Why is shock heating never an issue? I see temps rise just as fast as they fall off, but no one ever says to keep your heating delta at 30°(or whatever temp)/minute. Of course if we did, we would need 40,000' runways.
 
EdFred said:
Why is shock heating never an issue?
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Probably because unlike rapid cooling, rapid heating from the inside at the rates possible with normal engine operation cannot cause damage to engines such as ours, and the reasons will be clear to anyone who has read a book on metallurgy and materials engineering.
 
As in relationships it is always more fun to get heated than to cool down.

And as you know the sarcasm is never missed, only returned!!!!:yes:
 
Well I'm glad this has only gone to 22 replies including mine. I agree with Ron, and Weilke, and Jesse. It's theoretically possible, but doesn't appear likely in our airplanes, particularly if you make power changes gradually. So far, the only times I've heard of problems in my experience have been from towing operations, where the engine was at full power under towing loads and the power was immediately pulled to idle. That's the ONLY abuse (and I call it abuse) that seems to have resulted in damage.

So, to the OP, you shouldn't be afraid of pulling power when you need to, whether it's for practicing your emergency descent maneuver, or to practice a stall. Just do it smoothly and you'll be well within the normal operating envelope of your engine.
 
TheTraveler said:
It is, just not in planes, I guess.

You never fired up a cold 2-stroke that was liquid cooled, and ran it wide open without warming it up?
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I don't think I've ever operated a liquid cooled 2 stroke.
 
TMetzinger said:
So, to the OP, you shouldn't be afraid of pulling power when you need to, whether it's for practicing your emergency descent maneuver, or to practice a stall. Just do it smoothly and you'll be well within the normal operating envelope of your engine.
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Rather than smoothly, I would say that if you have to do something with a power-cut, dont do it from a situation where your cylinders are hot, e.g. a climb at full power or some sort of high cruise setting. If the training scenario is set up right, there should not be a need to do an abrupt power cut from a high power setting.

I have flown with people who adhere to the 'no more than 2in mp reduction per two minutes' school of thought. Not sure where that originated, but it makes it unduly complicated to plan a descent. Particularly with an engine monitor in place, I see no rationale to use MP as a proxy for the engines temperature state.
 
EdFred said:
I don't think I've ever operated a liquid cooled 2 stroke.
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Cold cease! First time I did it I was about 14, on a Suzuki RM 125.

Gorgeous Saturday morning, pulled it out of the barn, kicked it, took off through the woods wide open, and never looked back.

It only took about a minute or two until she died. Locked right up. Took me over a month of mowing grass to get that rebuilt.
 
weilke said:
I have flown with people who adhere to the 'no more than 2in mp reduction per two minutes' school of thought. Not sure where that originated, but it makes it unduly complicated to plan a descent. Particularly with an engine monitor in place, I see no rationale to use MP as a proxy for the engines temperature state.
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I get the concept, I think it's just being explained wrong. It's not so much that you are reducing it, rather than setting it and keeping it there. For me, I usually am WOT and 20-21" at cruise. I pull it back to 18" and since I gain an inch of MP on the descent for every 1000' feet, I just keep it at 18" (or by their explanation keep reducing it 1"/1000') until I get to within 1000' of TPA, then my MP has to be what it has to be to do what I've got to get done.
 
TheTraveler said:
Cold cease! First time I did it I was about 14, on a Suzuki RM 125.

Gorgeous Saturday morning, pulled it out of the barn, kicked it, took off through the woods wide open, and never looked back.

It only took about a minute or two until she died. Locked right up. Took me over a month of mowing grass to get that rebuilt.
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Those were liquid cooled? I thought those were all air cooled. I was lucky growing up, I had a Yamaha MX80 4 stroke. No mixing oil for me. I still have the bike.
 
EdFred said:
Those were liquid cooled? I thought those were all air cooled. I was lucky growing up, I had a Yamaha MX80 4 stroke. No mixing oil for me. I still have the bike.
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Late 80's liquid cooling started coming into powersports. Now the largest collection of them are liquid cooled, although since the early '00's 2-strokes have been going bye-bye.
 
TheTraveler said:
It is, just not in planes, I guess.

You never fired up a cold 2-stroke that was liquid cooled, and ran it wide open without warming it up?
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My ex-brother-in-law was a jetski racer out in Commiefornia, he built his race engines right out there on the ragged edge and he would have to be very careful about warming them up before a race to avoid cold-seizing them. Even then, he would still wipe out about every tenth engine during the warmup, we got very good at changing engines quickly in the pits, could do it in about 30 minutes.

It certainly is possible to damage an engine by heating it too quickly - but the scenario does not apply to air-cooled engines.
 
Geico266 said:
We have a 182 jump plane operating out of our airport. Climbs to 10,000' full throttle, pulls the power, drops meat bombs, and dives to the runway with little to no throttle. Does this day in and day out year round.

Shock cooling is an issue only in the minds of CFIs trying to impress students. :mad2:
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FWIW on the actual jump run there is a few minutes of straight and level at a reduced power setting to maintain about 80mph. Once the jumpers are away the cowl flaps are closed and manifold pressure is reduced to about 15" followed by the RPM to the bottom of the green arc. The power really never reaches idle until just before touchdown.

IO-470s are also about the strongest most rock solid engine there is.

Not saying shock cooling is really a big problem by any means but efforts are made to make the reduction from thirty minutes of full power to idle as smooth as possible while not impacting the efficiency of the operation.
 
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EdFred said:
Those were liquid cooled? I thought those were all air cooled. I was lucky growing up, I had a Yamaha MX80 4 stroke. No mixing oil for me. I still have the bike.
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I had a Yamaha RD350. Adding 2 cycle oil was a pleasure. That thing was a rocket.

Wish I still had the bike.
 
Ron Levy said:
The sort of shock cooling which can cause cylinders to crack immediately (the process actually called "quenching" by metallurgists, and the damage being "quench cracking") is not possible short of what Henning suggested (plunging the hot engine into a large bath of cold liquid). The problem which can occur due to a combination of large power reductions and high speeds is uneven contraction of the cylinders and the pistons, causing accelerated wear of the piston rings and cylinder walls. It's not something that is going to cause a catastrophic engine failure on the spot, just shorten cylinder/ring life, and thus something to be avoided, but not feared.
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This is particularly troubling when the cylinders are new, the piston to ring gaps and piston to cylinder wall clearances are at the new spec. (minimum) This is the only time I like to see those new cylinders stay nice and warm.

This is one of the reasons ECI went to the new fine hone process they break in quicker and get us out of the time period quicker.
 
Yep. Shock cooling is a total myth. Doesn't ever happen and there are even internet articles by columnists to prove it. And yet...

There are whole industries dedicated to the welding of cracked airplane engines.

The shock cooling critics will tell you that the cracks we see with way to much frequency, are caused by bad manufacturing and by prolonged operation in the "red box" and have nothing to do with cooling. They could be right, or not. Nobody really knows because there hasn't been any real study done on the subject. All we have is anecdotal evidence, suppositions and unproven theories on either side of the argument.

I choose to error on the conservative side and treat my engine with respect. The same way that I don't slam my throttle forward on take off, I also don't yank it back to slow down. Chopping and dropping is rarely required and a planned approach including a more gradual reduction in engine power is the better plan IMO.

Having said that, if the safe conduct of flight requires a chop and drop for some reason, I would not fear it. Once in a while, in moderation won't destroy the engine IMO. Everything in moderation.
 
weilke said:
I have flown with people who adhere to the 'no more than 2in mp reduction per two minutes' school of thought. Not sure where that originated, but it makes it unduly complicated to plan a descent. Particularly with an engine monitor in place, I see no rationale to use MP as a proxy for the engines temperature state.
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It's a pretty good guideline for some of the big turbocharged engines where the temps can vary faster and more widely when you make large power reductions.
 
Dav8or said:
Yep. Shock cooling is a total myth. Doesn't ever happen and there are even internet articles by columnists to prove it. And yet...

There are whole industries dedicated to the welding of cracked airplane engines.

The shock cooling critics will tell you that the cracks we see with way to much frequency, are caused by bad manufacturing and by prolonged operation in the "red box" and have nothing to do with cooling. They could be right, or not. Nobody really knows because there hasn't been any real study done on the subject. All we have is anecdotal evidence, suppositions and unproven theories on either side of the argument.

I choose to error on the conservative side and treat my engine with respect. The same way that I don't slam my throttle forward on take off, I also don't yank it back to slow down. Chopping and dropping is rarely required and a planned approach including a more gradual reduction in engine power is the better plan IMO.

Having said that, if the safe conduct of flight requires a chop and drop for some reason, I would not fear it. Once in a while, in moderation won't destroy the engine IMO. Everything in moderation.
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Dave, when ATC tells you to drop 2000 ft, do you just nose over or pull power (or a combination). My CFII taught me to pull power to lose altitude, but this almost always triggers my shock cooling alarm on my UBG17 monitor.
 
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