Shock Cooling

[AlanA]

Glad to hear there are others of you with oil temps that that are slow to move into the green.  My J is the same.

[jetdriven]

As a data point I ran my engine for 5 minutes a couple days ago at 1200 RPM to warm the oil and mix it in order to drain an oil sample. The oil temp was approximately 110 degrees when drained from the sump. The white dot is 75 degrees. The POH warning about not doing a run up until the oil temp is off the white dot is deleted from later model POHs but I am not sure which year. Straight weight oils may have pumpability issues when cold but I don't see this with multigrade oil.

[Shadrach]

I instructed at BDU in the early 90s and taught mountain flying. We required a mandatory mountain checkout for all renters regardless of experience. Shock cooling was one of the reasons for this. I'm not going to bore anyone with the details of some of my experiences with some of these checkouts, especially going into ASE via DBL, but i will say this... The reason for implementing our required mountain checkout regardless of experience was the result of numerous cylinder replacements on our rental aircraft, among other things.

 

So, I'm gonna have to go with Lycoming on this one 100%. If you wanna chop and drop, I would suggest you switch to burning kerosene. 

 

On a side, I flew from LMO to C08 and on to ALS today. The colors are amazing right now and the weather today and tomorrow is ideal for enjoyable mountain flying. Cheers. Brian

Brian,

Please bore me with the details... Also, what does mountain flying have to do with "shock cooling"? I mean I often use 11.5 and 12.5 as cruising altitudes, does being at a lower AGL altitude somehow cause the cylinders to fail. Lots of 182s out there going to TBO and beyond dropping their human payload at 14,000 and doing the chop and drop approach back to base at SL +/- 1000. Without more info, who knows what caused all those cyl replacements...but I don't think the mountains should take the blame...

[Sabremech]

I instructed at BDU in the early 90s and taught mountain flying. We required a mandatory mountain checkout for all renters regardless of experience. Shock cooling was one of the reasons for this. I'm not going to bore anyone with the details of some of my experiences with some of these checkouts, especially going into ASE via DBL, but i will say this... The reason for implementing our required mountain checkout regardless of experience was the result of numerous cylinder replacements on our rental aircraft, among other things.

 

So, I'm gonna have to go with Lycoming on this one 100%. If you wanna chop and drop, I would suggest you switch to burning kerosene. 

 

On a side, I flew from LMO to C08 and on to ALS today. The colors are amazing right now and the weather today and tomorrow is ideal for enjoyable mountain flying. Cheers. Brian

 

 

Hi Brian,

 I too would like to hear more details, specifically details that point to failures as a result of shock cooling and how it was determined this was the cause. Reading that this is related to rental aircraft says a lot.

 

 David

[pinerunner]

One FBO I used fly at wouldn't let us fly at lower than 20 degrees as I recall and gave us some specific training about not decending too fast, chopping power with hot engine in very cold air. Apparently someone had once managed to cool the cylinders so fast that they locked onto the pistons, stopped the engine, and ended in some sort of exciting, expensive, event. Just hearsay. There must be some excellent, firsthand, knowledge on this coming out of deepfreeze places like Minnesota and I'd like to hear some if it.

 

I don't take everything the manufacturers say as necessarily the last word even on their own engines, ie the folks at GAMI are clearly very sharp, but if they bothered to publish that they likely knew  what they were talking about and had some data too. Deakins wrote the article describing using low RPM in descent to slow the cooling rate down and still get good descent. it was quoted in a thread here asking about WOT descent. Lycoming mentions leaning to keep the heat up when droping power and lower RPM so they seem to agree with Deakins.

 

I won't worry much about shock cooling but following both Deakins and Lycoming when they agree makes sense to me.

[Cruiser]

If I remember right from my mechanical engineering days, the ratio of elasticity to induced stress (Young's modulus) changes with temperature. At higher temperatures, the material is less likely to crack under stress as the Young's modulus is lower (the same force will induce less stress at higher temps than at lower temps). That explains why things crack when cooling not heating. Sharp edges, holes, inclusions and minor cracks greatly concentrate stress. Also if the surface cools faster than the rest of the material, it will induce internal stress (pouring cold water on hot glass for example). Uneven temp changes lead to internal stresses. The faster the change happens, the more likely it will be uneven.

 

As for heating, a material's strength is not necessarily linear. As it reaches a certain temps (Seems like +400 in engines if I believe everyone) it tensile strength weakens sharply. Starting an engine cold and heating up quickly won't lead to cracking in the material. But uneven heating could lead to problems including cracking. I used to have an old Pontiac that had aluminum block and a cast iron head. This caused uneven heating and cooling and lead to gasket failures. I hope TCM did a better job than GM!

 

I can say that shock cooling or shock heating can't be good for any machined part or parts. Especially considering gasket fitment, critical clearances and all. That said, I don't worry too much about shock cooling or heating but do try to make power changes as smooth as possible. I assume most of this has been engineered into the system. I don't freak out when I fly into heavy rains or if I have to cut power and drop a few 000 feet. 

 

I agree, I don't know the history of the term "shock cooling" or its meaning but I would suspect "shock cooling" is more dangerous as it is repeated over time and not a single event. Large rapid changes in temperature cannot be good for your engine under any circumstance. 

[Shadrach]

One FBO I used fly at wouldn't let us fly at lower than 20 degrees as I recall and gave us some specific training about not decending too fast, chopping power with hot engine in very cold air. Apparently someone had once managed to cool the cylinders so fast that they locked onto the pistons, stopped the engine, and ended in some sort of exciting, expensive, event. Just hearsay. There must be some excellent, firsthand, knowledge on this coming out of deepfreeze places like Minnesota and I'd like to hear some if it.

 

I don't take everything the manufacturers say as necessarily the last word even on their own engines, ie the folks at GAMI are clearly very sharp, but if they bothered to publish that they likely knew  what they were talking about and had some data too. Deakins wrote the article describing using low RPM in descent to slow the cooling rate down and still get good descent. it was quoted in a thread here asking about WOT descent. Lycoming mentions leaning to keep the heat up when droping power and lower RPM so they seem to agree with Deakins.

 

I won't worry much about shock cooling but following both Deakins and Lycoming when they agree makes sense to me.

I seriously doubt that is a true story as told...there must be more to it.  The way I see it, engine manufacturers are incentivized to insist shock cooling is real. It is yet another way to redirect liability from metallurgy/assembly QC to the pilot.  What are the repeatable, definitive signs of shock cooling induced failure in a cylinder?  I've never seen it.  If you're a cylinder manufacturer, why take the blame for premature cylinder failure when you can blame the pilot and the bogyman...

[jlunseth]

Well, I am from Minnesota and fly in our weather as much as I can.  I am not a believer in shock cooling for most of the year, but am certainly careful when cold temps arrive.  The air can not only be very cold, it can be very dense and therefore cools things very fast.  The truly cold days - below zero F - usually come in Jan. and as a kid growing up around my relatives farms, we were always aware that metals behave differently in that kind of weather.  At a minimum you would hear the frame of the car creak and groan just from driving down the road.  Metal parts simply were not very resilient and super low temps, and would sometimes become brittle and break from a sudden stress. 

 

There are other things that start happening inside the engine compartment at low temps.  I once had an air/oil separator freeze up, with oil spewing out of every orifice in the engine.  The oil temp will seem cool but normal enough, and then suddenly will drop, then maybe come up again, then drop again, a sign that oil is not correctly circulating and the flow is stopping.  I have a high tech oil cooler block I use now (two pieces of foam of the right size taped together and stuffed in the cooler).  But the biggest help is to sit on the ground and let the engine and the whole engine compartment warm up for a good long time.  I do not use the cowl flaps for most operations when it is that cold, I just leave them shut. 

 

Bear in mind that during most of my longer trips I am in the flight levels where it is probably -25 C or more even during the summer, but -25 C at high altitude is a lot different from the same temp at ground level during the winter, because of the air density difference.  Nevertheless, I keep the power up during a descent from altitude, just to keep things warm in the engine compartment.

 

A bigger concern than shock cooling is ring slap.  A prolonged high speed descent where the prop is driving the engine can cause it.  The compression rings normally are sealed against the piston walls by pressure from inside the engine, but when the engine is not producing power they lose that pressure, and bounce around inside the ring groves.  The rings, ring grooves and cylinder walls can all suffer.

[Z W]

Mountain flying sometimes produces the worst potential for shock cooling in the winter. You may have to be at 16k to clear the terrain, and then descend to an airport at 7k in a very short distance, meaning a very steep descent angle. The easiest way to accomplish that is to clear the terrain, pull the throttle to idle, drop the gear and flaps, and point the nose down. If there is such a thing as shock cooling, that's how you do it.

In that circumstance it's more of an immediate safety concern than an engine wear concern. After 5 minutes with an idle engine and a high airspeed (nose pointed down) your engine will be very cold and may not produce full horsepower if you need to do a go-around when you get to the airport. Since density altitude is still high, with the airport at 7k or higher, you really need all the horsepower you can get. Failure to keep a warm engine has gotten more than one pilot in the mountains.

You can do the same thing coming down from the high teens or flight levels on any given flight with a turbocharged aircraft in the winter. It's harder to do in a naturally aspirated plane cruising at 10-12k, but possible.

The way to avoid it is to watch your CHTs and keep them in the green at all times. It's just good practice, and easy to do if you can plan your descent from far enough out. If you can't, due to ATC or terrain, use the tools your aircraft has - close the cowl flaps, keep some power in, pop the speed brakes, put the gear down, put the flaps down, etc. Some aircraft lack some of those tools and so have more trouble with this than others.

Whether any of that relates to increased engine life, I have no idea, and I don't think anybody else does either. But you should keep your engine warm anyways.

[DaV8or]

And on the other side of the argument, you have training airplanes that are notorious to reach their TBO on a very regular basis. They suffer from extreme power changes all day, every day...

 

When I belonged to a flying club, the planes there were used primarily for training. I can tell you that they didn't all make it to TBO without a new cylinder here and there. The break in rules for the new cylinders, or engine was always the same, cross country flights only. They were always trying to encourage guys like me who flew for fun to fly these planes and break them in so they could get back to training. I flew a number of these break in missions. I think the training planes regularly make it to TBO with out any major work is a myth.

[jetdriven]

It depends on what the green is, depending on Lycoming or Continental, your POH, and which year, the minimum CHT is 300, 250, or 200. According to APS there is no minimum CHT.   I like to see 250 as a minimum but if flying around at minimum power on a cold day, you might not be able to maintain that.  Regarding the bringing the power to idle for several minutes with the gear and flaps out to descend, that sounds like terrible technique in any airplane regardless of temperature, and I have never had to do that in a piston airplane. Set minimum power and if needed, do a couple 360s to get down.  I havent seen an MEA or a step-down on an approach which requires such a descent.

[DaV8or]

That said, I still have yet to see immediate evidence of harm done by supposed "shock cooling" an engine.

 

Question is, how would you ever see this evidence? Cylinders and Cases keep cracking and when they do, those in one camp will say, "Looks like you probably decended too fast one time." and those in the other will look at the same parts and say either, "Looks like you run 50 degrees ROP.", or "The manufacturer made a crappy part." Unless somebody spends a bunch of money to develop a test to test the theory of shock cooling, or running 50 ROP, we'll never know. To my knowledge, this test has never been done.

 

In the mean time, the conservative thing to do, is avoid shock cooling just in case Lycoming might on the off chance be right about something.

[DaV8or]

Martin,

If shock cooling existed, then we couldn't fly in the rain. Or cruise into a cold front.

 

There is a big difference between these scenarios and shock cooling. In the case of flying into a rain storm, or a cold front, the cooling temperature change is only on one side of the engine. The fires that keep the engine warm continue, so the cooling change is only on the surface. In the case of shock cooling, the fires are greatly reduced and cold air rushes over the engine simultaneously, so it is rapidly cooled from both sides. This cooling is through out the engine, rather than just the surface.

[M20F]

There are all kinds of stories about all kinds of things. From my perspective there is no need for me to shock cool the engine, not preheat it, not let the oil get into the green prior to taxing, not taking off without the cylinders above 250, etc. so the whole thing becomes a moot point. It is like pulling an airplane around by the tips of the propeller, I am sure half the people here will say its ok; half will disagree. My point is why the hell would you want to do it in the first place when there are so many easier ways of pulling the airplane.

[Rhumbline]

This one's about beat up but I'll still give my licks and take my lumps!  In a distant past, I too conducted mountain checkouts regularly but I do not recall much focus on shock cooling unless the aircraft in use happened to be turbo-charged.  And then procedures in this respect did not vary from anywhere else except that one may be circling down over a field due to prickly terrain intervening where a more direct descent might otherwise have been accomplished.  Because of the unyieldingly confining nature cumulogranite, this was often the case while operating normally aspirated variants as well anyway.   It was not until I was working for an outfit operating a number of models with GTSIO's that a steadfast procedure of reducing an inch or two of manifold pressure every two or three minutes (or some such... I forget the precise proportion) during the descent was followed and that was ostensibly as much to avoid slapping gears around in those particular engines as for moderated cooling.

 

On the other hand, I do have an anecdotal tale suggesting the reality of "shock cooling".  It was at DEN (In the olden-times 'cause it was still Stapleton) in mid winter and during one of those cold snaps where the highs never climb out of the teens or twenties.  The C-402 I was operating had flaps which had been deferred for a day or two and were thus inoperative.  Sequenced among the typical assortment of liners we were fast on final and the FO (PF) was forced to chop the throttles a couple-three miles from the runway.  Upon doing so there was a loud pop from the right wing and several, nearly simultaneous, cracks against the fuselage.  I immediatley advanced the mixtures to full rich and was going for the props when it occurred to me that we were going nowhere but to the runway ahead unless the guy ahead balled it up and forced a go-around.  We landed without incident and, but for the fact that the 402 does not taxi worth a $hit on one engine, taxiied to the gate.  We were then able to examine the oil soaked nacelle and wing as well as all the holes in the cowling created by migrating parts; a catastrophic failure for which the passengers had front row seats.  With the cowl removed, it was obvious by looking into the large hole in the crankcase that at least the #1 connecting rod had failed.

 

It was at about the same time that another pilot approached and stated that he was wondering how long it would be before that engine canned.  He unabashedly explained that, about a week prior and in the same plane, he had flown a charter to MSP.  The left engine lost oil pressure and he shut it down as a precaution and over-boosted the right engine for 15 or 20 minutes as he had a full boat and the unprotected portions of the airframe had so iced up.  I'll forego any discussion pertaining to the idiocy of this creature.  Is it coincidence, however, that that connecting rod just happened to fail upon that drastic power reduction in frigid temperatures or, and I can't help but wonder, did a contracing cylinder grab that piston and cause the failure of an already compromised component or components?  And if the latter is true, what's it doing to an otherwise healthy motor?                  

[DS1980]

There is a big difference between these scenarios and shock cooling. In the case of flying into a rain storm, or a cold front, the cooling temperature change is only on one side of the engine. The fires that keep the engine warm continue, so the cooling change is only on the surface. In the case of shock cooling, the fires are greatly reduced and cold air rushes over the engine simultaneously, so it is rapidly cooled from both sides. This cooling is through out the engine, rather than just the surface

Yea, I was trying to have a hint of sarcasm. Should have just stayed out of it. I'll drop the cold front argument, I was being silly. But the rain argument is valid. Inducing a liquid into an accelerated stream of air is exactly how A/C units work (and how carb ice is formed). This is exactly what comes into the air inlet when flying in the rain. If shock cooling existed, there would be a lot of airplanes falling out of the sky in Washington. (The state of Washington, not the one that is taking a vacation at the moment because they dont know how to balance a check book. 800,000 non essential employees............)

[DS1980]

There are all kinds of stories about all kinds of things. From my perspective there is no need for me to shock cool the engine, not preheat it, not let the oil get into the green prior to taxing, not taking off without the cylinders above 250, etc. so the whole thing becomes a moot point. It is like pulling an airplane around by the tips of the propeller, I am sure half the people here will say its ok; half will disagree. My point is why the hell would you want to do it in the first place when there are so many easier ways of pulling the airplane.

You my friend, need to be the first to comment on every post.

Whats your thought on LOP operations. I'm kidding! Please don't respond to that.

[PMcClure]

Considering an engine running continuously at 65-75% power is moving considerable volume of air (we could do the math!) and producing a lot of heat, the cooling effect of rain is probably minimal. It sounds bad, but the temperature change is going to be minimal. A fast decent at idle power will cool the engine much faster. I am still in the "not much concern" camp about shock cooling. 

[201er]

Considering an engine running continuously at 65-75% power is moving considerable volume of air (we could do the math!) and producing a lot of heat, the cooling effect of rain is probably minimal. It sounds bad, but the temperature change is going to be minimal. A fast decent at idle power will cool the engine much faster. I am still in the "not much concern" camp about shock cooling. 

Ever toss water drops on a hot pan?

[mike_elliott]

Considering an engine running continuously at 65-75% power is moving considerable volume of air (we could do the math!) and producing a lot of heat, the cooling effect of rain is probably minimal. It sounds bad, but the temperature change is going to be minimal. A fast decent at idle power will cool the engine much faster. I am still in the "not much concern" camp about shock cooling. 

The thermal transfer properties of water are much greater than air. Bathing a hot engine with water will cool it much faster than air will. Shock cooling if it is real will be very real flying thru a cold rain.

Geothermal heat pumps are much more efficient that air xfer heat pumps, for example.

[PMcClure]

The thermal transfer properties of water are much greater than air. Bathing a hot engine with water will cool it much faster than air will. Shock cooling if it is real will be very real flying thru a cold rain.

Geothermal heat pumps are much more efficient that air xfer heat pumps, for example.

I agree with your first sentence and the last, but not the second sentence. Add water and flow, a wet surface can have as much as 2500 times the  thermal transfer rate of a dry surface with no air over it. But I just don't buy that there is enough mass of water coming onto the cooling fins to matter in a rain storm. I found this article, http://www.avweb.com/news/maint/182883-1.html?redirected=1

 

 

they actually did the math which would suggest rain does matter, but I don't agree with their assumption that 100% of the rain water contacts the fins and evaporates. But I do agree with their conclusion that it doesn't matter.

[PMcClure]

Ever toss water drops on a hot pan?

 

yes, but the pan stays hot, especially if the burner is still under it. 

[PMcClure]

A visual demonstration!

[orionflt]

The thermal transfer properties of water are much greater than air. Bathing a hot engine with water will cool it much faster than air will. Shock cooling if it is real will be very real flying thru a cold rain.

Geothermal heat pumps are much more efficient that air xfer heat pumps, for example.

 we spray approximately 8 gallons of water in about 7 minutes on the cylinders of the plane we race at reno, when the water cooling stops the chts among other temps climb very quickly. rain, even very heavy rain will get no where near that rate of flow and most of the droplets will be evaporated before that can cause much if any noticeable thermal cooling. I know the conditions we put the race engine thru are extreme but it not totally different from the way we operate our engine, in fact most of the engines are stock just run at extremely high manifold pressures(50-70"). also after the race all the planes do cooling laps to prevent shock cooling on the engine. I think that most normal flying techniques naturally negate shock cooling, and there are so many different factors that play a part in whether or not shock cooling will damage your engine. one thing i do know that it is possible and if i can avoid it by planning my descent i do, I see no reason to tease the tiger because i know when he bits it is going to hurt!

[DS1980]

We are trying to validate something that doesn't exist. IF shock cooling existed, it would exist in a cold rain and when we pull power and greatly increase airflow. It doesn't, which is why nobody can agree under what circumstances it will happen.

There's just too much heat and pressure in the engine, even under idle, to cool an engine quickly enough to cause concern. A more revevant topic is the sloppy airmanship that this accompanies.