Shock cooling ... is it really a thing?

Daleandee

Daleandee

Final Approach
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Dale Andee
I have read so many various opinions on this. I'm not really convinced that it is a real concern.

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
avweb.com avweb.com

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

Just wondering what others have experienced ...
 
I like the observation that if you believe in shock cooling, you have to believe in shock heating every time you start your engine. Based on what I've heard and read, I'm not sure how much empirical data there is to support the phenomenon.
 
I’ve never understood:

Why (after the oil is warmed up during pre-takeoff operations), following application of full power during the takeoff, creating half a thermal cycle (heating up / expanding the cylinders)…. how is that any different than the second half of the thermal cycle (cooling and contracting)?
 
“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.”
“Shock cooling of Continental engines is a risk caused by excessively rapid cooling, which can lead to cylinder cracks”.

It’s your plane, you can opt to ignore this advice, but if you are shock cooling an engine it indicates you have rather poor situational awareness and ADM skills and don’t operate your aircraft efficiently.
 
The thing about this topic is, even if shock cooling is not as big a deal as some make it out to be, why wouldn't you want to be gentle unless you have to make a sudden rapid power reduction. And, if you always seem to need to make a sudden rapid power reduction, see @Clip4'sinal comment in the post just above.
 
AV8R_87 said:
Flight school and aerobatic planes would replace cylinders every 100 hours if this was an issue.
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Which flight school did you attend where you transitioned from a high power / high cylinder temperature configurations to a power off / high airspeed configurations?

Most flight school operations are occurring in rather slow high drag aircraft, mixture full rich and at an altitude of 2000 feet AGL. Descents are power on and at standard descent rates. The cylinder head temperatures are not high and the descent rates and airspeeds are not high.
 
Clip4 said:
Which flight school did you attend where you transitioned from a high power / high cylinder temperature configurations to a power off / high airspeed configurations?
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All of them do simulated engine outs (from either cruise or take-off power), emergency descents, and power on/off stalls.
A 172 with a couple heavy guys in it will get decently high CHTs on departure, with not a lot of cooling on downwind, followed by pulling power pretty much to idle abeam the numbers. That's a lot of heat cycles doing pattern work.

Clip4 said:
Descents are power on and at standard descent rates.
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Maybe at the puppy mills on 10-mile finals.
Plenty of schools reduce power significantly and teach students to fly tight patterns with descent rates way greater than 500 fpm.

And what about the aerobatic guys? Our own Ed Haywood cracked his oil cooler mounts from ram air pressure during descents while having fun. I don't hear him (or other aerobatic pilots) replacing cylinders like underwear.
 
To truly shock cool an engine, you’d have to immerse a hot, running engine in ice-cold water. Simply chopping power and descending won’t do it - the cooling happens over minutes, far too slowly to qualify as a “shock.”

That said, large or abrupt power changes aren’t fine either. While jump pilots may routinely chop power and dive, for most pilots that kind of throttle pumping signals poor airmanship.

It's like driving with full acceleration, and then slamming on the brakes, and then back to full acceleration. You immediately cast an impression of a hot dog with a lot of muscle and no brains.
 
sarangan said:
It's like driving with full acceleration, and then slamming on the brakes, and then back to full acceleration. You immediately cast an impression of a hot dog with a lot of muscle and no brains.
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I resemble that statement. Sometimes when you're racing a paperclip you have no other options
 
I towed gliders with my 182A when I owned a soaring operation. That model did not have cowl flaps. I ran the oil temperature close to to the red line & the cylinder head temps were close to redline too. After the glider released it was a decent to the ground for the next tow. I ran the operation close to 4 years & did over 2,000 tows. I never replaced a cylinder during that time. The engine had about 1,000 hours on it. I'm not concerned about shook cooling unless I'm flying a tightly-cowled turbo.
 
The most thrilling example of shock cooling was at my mom’s. This basically happened:
 
sarangan said:
To truly shock cool an engine, you’d have to immerse a hot, running engine in ice-cold water. Simply chopping power and descending won’t do it - the cooling happens over minutes, far too slowly to qualify as a “shock.”

That said, large or abrupt power changes aren’t fine either. While jump pilots may routinely chop power and dive, for most pilots that kind of throttle pumping signals poor airmanship.

It's like driving with full acceleration, and then slamming on the brakes, and then back to full acceleration. You immediately cast an impression of a hot dog with a lot of muscle and no brains.
Click to expand...

Depends on the plane and or engine, you could definitely shock cool the engines on a dc3, lb-34 and dc4. On the Howard 350 you would have to close cowl flaps on the takeoff roll on a hot day in south Texas.
 
Daleandee said:
Like flying into a rain storm?
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In theory your engine is still generating heat when you fly into the rain storm. So you're not really shock cooling the cylinders.

But your question does help disprove the shock cooling myth. If it were a thing, then people flying into rain, all of us that fly at higher altitudes or those that fly in the winter would all be loosing cooling fins off our cylinders if they were constantly being being shock cooled and breaking down the metal.
 
Rick A said:
I’ve never understood:

Why (after the oil is warmed up during pre-takeoff operations), following application of full power during the takeoff, creating half a thermal cycle (heating up / expanding the cylinders)…. how is that any different than the second half of the thermal cycle (cooling and contracting)?
Click to expand...

I think SC only applies to turbos. Think of a turbo running at 75% power, pulling back and then descending into very cold rain. The cold rain can quickly cool the engine, especially the heads with its cooling fins, far faster than air.

Now cylinders are made from steel and aluminum heads are screwed on with steel cooled and aluminum hot, making a tight fit when they return to equal temps (the cooled steel expands as it warms and hot aluminum contracts as it cools). Now if you super cool the heads they contract even more and the steel is still very hot which means they expand much more than when assembled, this stress could crack the cylinder heads.

As least that’s my theory.
 
If shock cooling is not a real thing, that's good for the operators of several backcountry flying outfits in Idaho. I watch them coming in to land, sounds like about 2500 rpm dragging it in, then once over the runway straight to idle, then after a minute taxi, shut down.

The guy I flew for in Alaska was super anal about cooling. His rule was no more than 2" manifold pressure reduction per minute, no pitch down and drop 2". Pitch down, then minimum one minute later you could drop 2", then again minimum one minute later.
 
I’ve often wondered if this was a carry over of operating practice warnings from the ultralight world back in the day that has grown over the years into being a thing from the “telephone game” of information being passed along. With the ultralight two strokes cutting the power also effectively cut the oil…so cutting power and descend while the prop windmills the hot engine (now starved of oil).. they were prone to lock up.
 
AV8R_87 said:
And what about the aerobatic guys? Our own Ed Haywood cracked his oil cooler mounts from ram air pressure during descents while having fun. I don't hear him (or other aerobatic pilots) replacing cylinders like underwear.
Click to expand...

Most of us just accept we're going to get 500 to 1,000 hours on our engines, due to the wide variety of stresses we inflict on them. But since most acro planes see little XC, that works out to 10-20 years like many regular planes.

That said, there is not much opportunity for shock cooling. Most maneuvers are flown at WOT, even vertical dives.
 
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You can't shock cool an engine if it doesn't get too hot. Keep your cht's at normal temps and do whatever you want.
 
Mikey52 said:
Depends on the plane and or engine, you could definitely shock cool the engines on a dc3, lb-34 and dc4. On the Howard 350 you would have to close cowl flaps on the takeoff roll on a hot day in south Texas.
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On a large radial engines as found on DC-3s, DC-4s, Howard 350....etc you are going to destroy your master rod bearings well before you have to worry about shock cooling cylinders...
 
This might be an OWT, but a mechanic I used to use told me that shock cooling OWT/fear dates back to WW2 and radial engines; and was an actual issue on those planes. It has nothing to do with the horizontally opposed engines we fly today and were designed decades ago.

So this is what I was told: on the old radial engines from WW2 if the engine changed temperature unevenly which often happened on final approach and landing when running hot. The issue was with the engine hot and near max temp, on final and approach the bottom bottom of the engine in comparison would cool much faster. The difference in thermal cooling rates caused slight changes to the master rod bearing between the top and bottom of the engine. Effectively causing a slight "binding" of the master rod bearing, which then caused the master rod bearing to fail much earlier.

It has never had anything to do with the cylinders themselves.

BWTHIDK?

Tim
 
tspear said:
This might be an OWT, but a mechanic I used to use told me that shock cooling OWT/fear dates back to WW2 and radial engines; and was an actual issue on those planes. It has nothing to do with the horizontally opposed engines we fly today and were designed decades ago.

So this is what I was told: on the old radial engines from WW2 if the engine changed temperature unevenly which often happened on final approach and landing when running hot. The issue was with the engine hot and near max temp, on final and approach the bottom bottom of the engine in comparison would cool much faster. The difference in thermal cooling rates caused slight changes to the master rod bearing between the top and bottom of the engine. Effectively causing a slight "binding" of the master rod bearing, which then caused the master rod bearing to fail much earlier.

It has never had anything to do with the cylinders themselves.

BWTHIDK?

Tim
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What it sounds like you are describing WRT the 'binding' and failure of the MRB is not related to shock cooling but reverse loading on the engine from pulling the power back too far for a given RPM. Has nothing to do with temperatures.

In large radials if you are running at say 1800 RPM and you pull the power back to something like 15" or less, the windmilling prop starts to drive the engine. Due to the design of the lubrication ports in the engine, that reverse loading effect can result in insufficient lubrication of the MRB and can cause damage.

As far as rapid temp changes in radials goes, fighter combat maneuvering would be far harder/more of a shock on cylinder temps than anything else. The most rapid temp changes Ive ever seen in a T6, for example, occur during maneuvers like loops/ split-S. Basically maneuvers involving high speed/rapid descent. Not normal descent, approach or landing.
 
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tspear said:
This might be an OWT, but a mechanic I used to use told me that shock cooling OWT/fear dates back to WW2 and radial engines; and was an actual issue on those planes. It has nothing to do with the horizontally opposed engines we fly today and were designed decades ago.

So this is what I was told: on the old radial engines from WW2 if the engine changed temperature unevenly which often happened on final approach and landing when running hot. The issue was with the engine hot and near max temp, on final and approach the bottom bottom of the engine in comparison would cool much faster. The difference in thermal cooling rates caused slight changes to the master rod bearing between the top and bottom of the engine. Effectively causing a slight "binding" of the master rod bearing, which then caused the master rod bearing to fail much earlier.

It has never had anything to do with the cylinders themselves.

BWTHIDK?

Tim
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Deakin has written that radials is where the shock cooling OWT came from.
 
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So how many documented, verifiable issues attributable to shock cooling on Lycoming and/or Continental horizontally opposed engines are there? I'm going to go out on a limb and guess that it's substantially less than the number of posts on this topic over the years.
 
tsts4 said:
So how many documented, verifiable issues attributable to shock cooling on Lycoming and/or Continental horizontally opposed engines are there? I'm going to go out on a limb and guess that it's substantially less than the number of posts on this topic over the years.
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Documented? Verifiable? Not sure you are going to find anything along those lines.

I think the limb you are on is sound...

The closest I've ever seen to documented maintenance issues related to shock cooling came from jump operators using 182s and even that was inconclusive. Some operators reported higher number of engine changes related to rapid descents and others insisted they had no problems.

I think it really comes down to individual technique. If you take a 'fly it like you stole it' approach, you are probably going to experience more problems than pilots who fly more conservatively. Don't abuse your engine and it will likely last longer, but I wouldn't sweat worrying about shock cooling in GA ops.

Now, if I'm flying jumpers and the company wants me to chop and drop, fine with me - it's their airplane and they're paying the bills....
 
Fearless Tower said:
What it sounds like you are describing WRT the 'binding' and failure of the MRB is not related to shock cooling but reverse loading on the engine from pulling the power back too far for a given RPM. Has nothing to do with temperatures.

In large radials if you are running at say 1800 RPM and you pull the power back to something like 15" or less, the windmilling prop starts to drive the engine. Due to the design of the lubrication ports in the engine, that reverse loading effect can result in insufficient lubrication of the MRB and can cause damage.

As far as rapid temp changes in radials goes, fighter combat maneuvering would be far harder/more of a shock on cylinder temps than anything else. The most rapid temp changes Ive ever seen in a T6, for example, occur during maneuvers like loops/ split-S. Basically maneuvers involving high speed/rapid descent. Not normal descent, approach or landing.
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Nah, the mechanic was very specific that the issue about shock cooling came from radial engines. And it was due to uneven cooling, not rapid cooling. Loops or split S, does not matter the airflow over the is not sustained long enough for the engine to cool unevenly, there is a lot of thermal mass in those engines so it takes a bit for the uneven airflow to have an effect. It takes sustained higher angles of attack at slower speeds after running the engine hard to cause the issue.
Overspeed of the engine is something completely separate.

Tim
 
tspear said:
Nah, the mechanic was very specific that the issue about shock cooling came from radial engines. And it was due to uneven cooling, not rapid cooling. Loops or split S, does not matter the airflow over the is not sustained long enough for the engine to cool unevenly, there is a lot of thermal mass in those engines so it takes a bit for the uneven airflow to have an effect. It takes sustained higher angles of attack at slower speeds after running the engine hard to cause the issue.
Overspeed of the engine is something completely separate.

Tim
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I think you are missing what I'm saying...or your mechanic was wrong. The master rod bearing problems that you describe are from heat CAUSED by lack of lubrication that is created by reverse loading the engine (letting the prop drive the engine). Both P&W and Curtiss Wright put out SBs in the 50s warning about this.

If the problem was from loaded engines operating at higher angles of attack at slower speeds then ONLY Navy carrier pilots would ever have that problem because the Army sure didn't fly them that way.
 
@Fearless Tower

No, I understand exactly what you are saying. I am only passing on what one mechanic told me was the source for the OWT shock cooling.
I do not believe in shock cooling for our horizontally opposed engines. There is too much counter evidence, and the math does not support it when you dig into the numbers.

Having found over the years a number of OWT that date back to previous engine designs or airplanes (such as running over square); I have a rather jaundiced viewpoint to a lot of the aviation adages.

Tim
 
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