Engine Shock Cooling: Myth or Reality??

Is shock cooling a myth?

  • yes

    Votes: 38 58.5%

  • no

    Votes: 15 23.1%

  • other

    Votes: 12 18.5%
  • Total voters
    65
Capt.Crash'n'Burn

Capt.Crash'n'Burn

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Capt.Crash'n'Burn
Was farting aroung on wikipedia when I came across their article on shock cooling. Whoever wrote the article seems to consider the phenomenon a myth, and the damages atributed to it are the result of other causes.

What's your $.02??

Wiki article
 
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Isn't it a given that turbocharged engines are subject to shock cooling?

Perhaps a double-blind, placebo-controlled trial is needed to determine the answer once and for all.

BTW that Wiki article isn't anywhere near Wiki quality standards.
 
N801BH said:
I can't shock cool the engine in my plane no matter how hard I try. :cheerswine:

Ben
www.haaspowerair.com
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Exactly.

Of course shock cooling is not a myth. What is a myth, however, is that it's something that is a problem for most aviation engines. Also, since many people do not run their engines correctly (eg. they run at CHTs that are way too hot), shock cooling can become a problem if, for example, you run at 460 CHT and go full rich and bring the throttle to idle. If you run your engine sensibly, though, it's never an issue.
 
IMHO shock cooling is 100% real but is a problem only if you do it when the CHT is above a certain value.

What that value is, nobody really knows, but it is probably above 350F.

There are many training ops which fly full-rich (at least here in Europe) and thus their CHTs never go above 350F or so. They do constant simulated engine failures, etc. No problems.

I have an article (on my site) from a glider tug operation which found a strong correlation between cracked cylinders, and the CHT at which they cut power.

At least this makes engineering sense: the metal is more likely to crack if its strength is reduced, and that happens substantially above 400F.

All the other explanations fall down when you ask why cylinders don't crack due to shock heating, on takeoff.
 
I want to hear from the aerobatic guys - if anyone knows, they will.
Full power low airspeed to altitude then, yank the throttle & stuff the nose down to 120mph.
Unless you have a definition of SC that is confined to routine GA x-c machines.
 
Let'sgoflying! said:
I want to hear from the aerobatic guys - if anyone knows, they will.
Full power low airspeed to altitude then, yank the throttle & stuff the nose down to 120mph.
Unless you have a definition of SC that is confined to routine GA x-c machines.
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I've got nearly 400 hours on my AEIO-540 in my Pitts and the compressions haven't changed in 320 hours and the screen is clean at every oil change. The vast majority of those hours are at wide open throttle and low airspeeds punctuated with moments of slamming the throttle closed. There's lots of max power/low airspeed interspersed with power idle/high airspeed and any combination in between (and in my bird, it's more like dive at 180 - 200 mph :)) For those that this will mean anything to, I compete pretty regularly at the Intermediate level in IAC.

I change the oil every 25 hours and it looks barely dirty when I do.
 
You can't get the cooling rates necessary to cause quench cracking of steel cylinders by chopping the throttle. As any materials science text will tell you, that takes cooling rates in the 1000's of degrees per second, and the most I've ever seen on an engine analyzer is on the order of 100 degrees per minute -- two orders of magnitude off. OTOH, the cylinder heads will cool faster than the pistons/rings, so even at the sorts of rates you can see, they will tighten up on the pistons/rings, and accelerate the ring/cylinder wall wear. That's not the sort of thing that kills engines rapidly, but it may make some difference in the long term wear on the engine. For that reason, I try to do what Lycoming recommends, and limit my cooling rates to below 50F/minute. See Lycoming's Key Reprints Operations section for more on this issue (check page 17).
 
Here's what I tell my students: I don't care if it's real or not.

The basic operating techniques that are encouraged to prevent shock cooling from occurring are gradual power reductions. Whether you do the 1" per minute, 3" every 3 minutes, whatever, it means that you don't just chop the power and point the nose down. What does this result in? A smoother flight that makes your passengers happier. As such, I encourage my students to do these gradual power reductions. It makes for smoother flight, which makes me happier. Also, it certainly won't hurt the engine to treat it that way, and could only help.

As to what temperature to run your cylinders at, limits are not goals. Lycoming publishes recommended CHT guidelines for maximum service life, I suspect Continental does as well. The limits are meant to be that - limits.
 
Ron Levy said:
You can't get the cooling rates necessary to cause quench cracking of steel cylinders by chopping the throttle. As any materials science text will tell you, that takes cooling rates in the 1000's of degrees per second, and the most I've ever seen on an engine analyzer is on the order of 100 degrees per minute -- two orders of magnitude off. OTOH, the cylinder heads will cool faster than the pistons/rings, so even at the sorts of rates you can see, they will tighten up on the pistons/rings, and accelerate the ring/cylinder wall wear. That's not the sort of thing that kills engines rapidly, but it may make some difference in the long term wear on the engine. For that reason, I try to do what Lycoming recommends, and limit my cooling rates to below 50F/minute. See Lycoming's Key Reprints Operations section for more on this issue (check page 17).
Click to expand...

I agree with Ron up the point of locations of cracks.

I have NEVER seen the steel portion of a aircraft cylinder crack. It is the aluminum cyl head that suffers the cracking, ie, between the seats, from the seat to the spark plug hole, in the exhaust port by the guide boss are the most frequent places. Thats not to say they will not crack elsewhere.

IMHO

Ben.
 
most glider tow operators have learned that a gradual power reduction is key to cylinder life. in my non motor head mind i think of keeping the heat inside the cylinder. so in our 182 towplane after release instead of throttling to idle to dive down i bring the manifold and prop back squarely in the center of their green arcs and descend at about 130 mph. find some sinking air to get down faster.

in the super cub we would do a gradual power reduction coupled with a full slip to descend. we had a digital CHT and i always would adjsut the throttle to keep the cooling rate at about 1 deg/second until it was either below 300 or time to land.

in the 421 we didn't get too wound up about 1 inch per 1000 feet or whatever but we did make all power changes smoothly. of course we still would occasionally have a bad cylinder on a 100 hr but thats a 421...
 
I agree with teaching gradual power reductions, and do so with the simple explanation that we cruise at high power settings and flare with very low (little or none) power settings, so it's clear that the big knob(s) must be pulled at some point(s) along the way.

In order to determine how much pull and when, I ask the student to establish various targets at which power should be reduced, the reasons for doing it and the arithmetic required (power, descent rate, airspeed) to make it work. If the pilot wants to be stabilized at pattern speed and altitude 7 miles from the airport without exceeding the green arc, his planning will be different than if he chooses (or ATC requires) that he stay high and descend steeply. In the latter case, I want the student to recognize that a power reduction (slow-down) in cruise may be necessary in order to prepare for the arrival slam-dunk that he may expect from ATC, and plan accordingly.

Whether they learn anything or not, it keeps them busy and they don't disturb my sleep.


Ted DuPuis said:
Here's what I tell my students: I don't care if it's real or not.

The basic operating techniques that are encouraged to prevent shock cooling from occurring are gradual power reductions. Whether you do the 1" per minute, 3" every 3 minutes, whatever, it means that you don't just chop the power and point the nose down. What does this result in? A smoother flight that makes your passengers happier. As such, I encourage my students to do these gradual power reductions. It makes for smoother flight, which makes me happier. Also, it certainly won't hurt the engine to treat it that way, and could only help.

As to what temperature to run your cylinders at, limits are not goals. Lycoming publishes recommended CHT guidelines for maximum service life, I suspect Continental does as well. The limits are meant to be that - limits.
Click to expand...
 
Lots of flight school 150s and Skyhawks that spend all day getting the throttle slammed on and off by eager CFIs who know little about aircraft ownership or maintenance. They seem to run forever.
 
peter-h said:
I have an article (on my site) from a glider tug operation which found a strong correlation between cracked cylinders, and the CHT at which they cut power.
Click to expand...

I found the article, and that is GREAT info. Thanks!
 
N801BH said:
I have NEVER seen the steel portion of a aircraft cylinder crack. It is the aluminum cyl head that suffers the cracking, ie, between the seats, from the seat to the spark plug hole, in the exhaust port by the guide boss are the most frequent places. Thats not to say they will not crack elsewhere.
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Look very, very carefully about 3" above the orange band, just below the large cooling fins.

This engine was running in cruise at about 50% power for 2 hours at constant power over the little town of Hanna WY 30 miles east of Rawlins when this jug let go. The only thing holding it to the rest of the engine was the exhaust system. Thankful to merciful heavens the folks in Hanna let the kids build a dragstrip just south of town.

Jim
 

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weirdjim said:
Look very, very carefully about 3" above the orange band, just below the large cooling fins.

This engine was running in cruise at about 50% power for 2 hours at constant power over the little town of Hanna WY 30 miles east of Rawlins when this jug let go. The only thing holding it to the rest of the engine was the exhaust system. Thankful to merciful heavens the folks in Hanna let the kids build a dragstrip just south of town.

Jim
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WOW! that must've left a brown racing stripe in your underwear!

Do you have any idea what caused it to crack??
 
Capt.Crash'n'Burn said:
WOW! that must've left a brown racing stripe in your underwear!

Do you have any idea what caused it to crack??
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Nope, no brown racing stripe. After you teach this stuff for a few thousand hours, you are so used to going through the engine failure checklist that there really isn't time to be scared. Besides, I had the luxury of 5000' of air between me and the rocks to plan the landing on that little dragstrip fairly carefully. My favorite technique is to come in clean about 500' higher than normal, do a standard pattern, and then when you KNOW without a shadow of a doubt you've got the field made, dump full flaps and you can put it on "the numbers" (except that there weren't any numbers) every time. Flaps fail? That's why we teach slips to a landing.

Yeah, I know. The book says to make a forced landing just like any other landing and you get two engine "clears" to make up lousy altitude judgments on the practical exam. That engine wouldn't have cleared for love nor money. And "the book" wasn't looking at large rocks in every direction except for that little strip of dirt.

The only thing I could find is that this particular jug was manufactured about thirty years before the other 5 (by serial number), so it probably had several thousand hours more time in service than the others. Magnaflux will only find cracks that are present, not those that are incipient. The overhaul done some years before I became the owner was done by a company that evidently does not match cylinders. It was the only chrome jug on the engine with the rest being standard steel.

Jim
 
Our club had an 0-200 lose a cyl recently. It looked much like yours. Happened in the pattern, On the first T&G on his First, post solo , solo flight to a nearby airport. Matt , the lucky student, did a superb job of flying the plane and making a safe landing. . and yes He kept training.
 
I cracked one in just about the same place many years ago, on a rental 172 as a 100-hour pilot. Happened on the downwind leg shooting touch-and-goes, I just pulled the power all the way off and tightened up my turn to final, non-event. Had I been in a less hospitable location it could have been ugly.
 
The four cylinder Lycomings rarely have shock-cooling related problems; this is also true for the O-200 in 150's. There have been exceptions, though, and good "throttle discipline" is a good thing to teach.

The six cylinder Continentals, especially the geared variety, do seem to be more susceptible to cracks due to rapid cooling. They'd invariably show up when training a new pilot on a 421 or a 404. It appears to be more of a temperature gradient problem as opposed to total cooling of the cylinder, but I don't have data to back up that hypothesis - and Continental probably ain't talking. The correlation between training (sudden simulated engine failures, approaches to stall, etc.) was unmistakable, though. :(
 
rotorhead1026 said:
The four cylinder Lycomings rarely have shock-cooling related problems; this is also true for the O-200 in 150's. There have been exceptions, though, and good "throttle discipline" is a good thing to teach.

The six cylinder Continentals, especially the geared variety, do seem to be more susceptible to cracks due to rapid cooling. They'd invariably show up when training a new pilot on a 421 or a 404. It appears to be more of a temperature gradient problem as opposed to total cooling of the cylinder, but I don't have data to back up that hypothesis - and Continental probably ain't talking. The correlation between training (sudden simulated engine failures, approaches to stall, etc.) was unmistakable, though. :(
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No. There's no proven correlation between engine failures and so-called "shock-cooling". The AvWeb article linked above makes for an excellent read in case you're interested.

Good throttle discipline is not important - the cooling rates one can achieve when manipulating the throttle is insignificant as long as the engine is run sensibly. Of course, training aircraft engines are rarely run sensibly - ie. 450+ CHTs in power-on stalls. Even in that case, though, the temperature is only one factor. By itself, it probably wouldn't be that terrible. But since those power on stalls are also often done at very high power settings (which increase internal cyl pressures), temp. + pressure combine to make sure your engine doesn't last long.

I wish this issue would stop being discussed. It's sort of like the "LOP ruins your engine" argument that's still somewhat prevalent. Both of these arguments were proven to be baseless years (decades in the automotive industry) ago.
 
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I'm over TBO on my second TN/TC plane (three engines). With a JPI, it's very easy to see how the cylinders are cooling. Once reasonably cool (low 300 CHTs), one can't shock what's already cool.

I'm sure there is a basis for it. We don't have to worry much about it if using reasonable engine management principals.

Best,

Dave
 
OFF topic

WHY does PayPal try to put a cookie on my computer when I view this topic??
I've been following this topic and get a cookie notice from paypal.com everytime I view the thread

Paul
N1431A
2AZ1
 
sba55 said:
No. There's no proven correlation between engine failures and so-called "shock-cooling".
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Agreed. My post was about cylinder head cracks. They're usually detected well before failure. <sigh>

Good throttle discipline is not important
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Wonderful ... :cryin:
 
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pmanton said:
OFF topic

WHY does PayPal try to put a cookie on my computer when I view this topic??
I've been following this topic and get a cookie notice from paypal.com everytime I view the thread

Paul
N1431A
2AZ1
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Ted's signature has a link to a Paypal donation button.
 
Let'sgoflying! said:
Further Off:
I never get a cookie notice from any site even when I am visiting it. How come you do?
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Different privacy settings. Check yours out. Or, better yet, check out all the cookies stored on your computer. :eek:
 
I am not really an engineer (software engineer) and know nothing about shock cooling, but I know if you pull the throttle back sharply two things can happen:
1) engine backfire
2) passenger lunch backfire

Neither of these make you look like a professional.
 
AuntPeggy said:
I am not really an engineer (software engineer) and know nothing about shock cooling, but I know if you pull the throttle back sharply two things can happen:
1) engine backfire
2) passenger lunch backfire

Neither of these make you look like a professional.
Click to expand...

And I know which I'd rather clean up...
 
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