Debunking the Shock Cooling Myth

from SSA Clubs and Chapters:

Over the years I've come in contact with various club towplane operating procedures that made some attempt to address "shock cooling". These techniques often have the effect of substantially reducing the tow capability of a towplane during a busy day by increasing cycle time. Being the kind of person who always wants to know "why" when these kinds of facts are presented, I learned that there was no sound thermodynamic or metallurgical reasons for the practice. When I became our clubs Chief Tow Pilot and maintenance go-to guy I changed our procedures and worked with our towpilots to understand how our operational procedures might make our engines last longest yet yield the best tow service. Our procedure once the glider releases is to smoothly pull power to whatever setting is desired to get back to the pattern soonest and reduce mixture aggressively. If you belong to a club that has a prescribed "cool-down" procedure for towplane descents, you may be spending more on tow services than need be. The following from a more expert authority might be useful.
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Shock Cooling: Time To Kill The Myth

RICK DURDEN

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 Patrol. While he thought it was important to volunteer for a good group, he was ready to quit because of a screwy power reduction procedure imposed on the pilots by someone high up in the organization. The procedure was ostensibly to prevent cylinder cracking due to shock cooling during descent after the glider released. However, the procedure he described took so long that, even if the glider did several minutes of soaring during its flight, it was on the ground well before the tow plane. As a longtime tow pilot, this struck me as ludicrous.

The anti-shock-cooling exercise required a series of small reductions in manifold pressure, each followed by flying around for a period of time before making the next, while the airplane descended slowly, burning lots of fuel.. If shock cooling actually existed and caused cylinder cracking, it would probably be cheaper for the operation to have bought a bevy of cylinders and kept them on hand for replacement than pay for the fuel they were going through to avoid a phantasm.

I used to be astonished at how aviation myths, particularly when it came to engine operation, have such incredible staying power. Now, when I hear one spouted, I just shake my head in admiration of the influence of ignorance and belief over data. With some folks, the laws of physics, aerodynamics, metallurgy and thermodynamics are trumped by unwavering faith in their particular superstitions.

Nevertheless, when aviation superstitions get in the way of safe, efficient engine operation and addressing real risks of damage to engines, they need to be exposed for the nonsense they are, particularly when they are adversely affecting others—such as the glider operation that could only get off a few flights an hour. Such practices, especially when they are taught as fact to new pilots, only perpetuate the foolishness.

The widely respected Daniel Patrick Moynihan put it eloquently: “Everyone is entitled to his own opinion, but not his own facts.�

There is absolutely no hard evidence that making a large power reduction will cause cracking of the cylinders of a horizontally opposed piston aircraft engine. Because people like examples, we’ll start with a few: Bob Hoover regularly shut down and feathered the engines on his Aero Commander Shrike during airshows—going from max power to none—and never cracked a cylinder. That’s consistent with what skydiving and glider tow operators have known for decades—their engines hit TBO without much in the way of cylinder problems, even though they descend rapidly at low power settings. Flight schools, with their repeated touch and goes, don’t go through cylinders at a disproportionate rate.

Let’s look at the numbers involved in engine cooling, starting with the small role that the cylinder fins play. Only about 12 percent of the heat generated by combustion departs from the engine via the cooling fins. The biggest proportion, 44 percent, goes out the tailpipe. Eight percent, almost as much as is handled by the cooling fins, is dissipated through the oil. Most of the rest is dissipated via the big, metal prop bolted to the crankshaft.

The engine manufacturer that has published data on the potential for shock-cooling damage—Lycoming—said to avoid the risk of damage, pilots should limit CHT reduction in flight to 50 degrees F per minute. The good news is that, even assuming such a rate of cooling will damage an engine—Lycoming said that damage potential existed only if done "consistently"—it’s nearly impossible to cool an engine that fast in flight even by shutting it down. In an article written by Kas Thomas more than 20 years ago and reprinted in AVweb, he went through the published test data—which showed that cutting engine power by half only reduces CHT by 10 percent or so. That kind of CHT drop isn’t capable of trashing cylinders—and isn’t anywhere close to the CHT change that occurs in the opposite direction on takeoff—shock heating, so to speak. And there’s never been any data to indicate that the massive shock heating during takeoff harms the cylinders.

Thomas also pointed out that flying through rain reduces CHTs by nearly as much as a 50 percent power reduction. There’s no history of airplanes regularly flown through rain having to constantly replace cylinders.

In fact, the real shock cooling comes at the end of the flight when you pull the mixture to idle cutoff and the CHTs drop at more than 100 degrees per minute right away—yet every engine goes through that sort of shock cooling and manages to survive it.

In the last 20 years, graphic engine monitors have become common in general aviation—and the data they provide further support conclusions reached before they were around regarding the minor effect of big power changes. Many monitors are set to alarm if the CHTs show a drop at a rate of more than 60 degrees per minute. Pilots are discovering that it’s nearly impossible to hit that rate without slamming the throttle shut and diving—which isn’t comfortable for anyone in the airplane. Mike Busch, A&P and principal of Savvy Aircraft Maintenance Management, told me during a conversation at an AOPA Fly-In that he’s tracked how fast CHTs will drop with various power reductions in his Cessna T310R. His observations were that it unusual to have CHTs drop at a rate of even 30 degrees per minute even with aggressive power reductions when ATC gives a slam-dunk approach.

In one of AVweb columnist John Deakin’s excellent articles on engine operation, he noted that when he waited 18 seconds to restart the engine of his Bonanza after running a tank dry, the CHTs only dropped 10 degrees.

In my opinion, It’s time to put the shock cooling myth to bed, so that pilots can worry about things that really are a risk to their safety and wallets—such as runway loss of control accidents. After all, with more than 25 percent of accidents that cause damage to the airplane and engine arising from loss of control on landing rollout it seems to me that rather than designing complex power reduction strategies to avoid a mythical risk of damaging an engine, we should be practicing crosswind landings to protect a real risk that actually does damage engines—and the airframes wrapped around them.

Rick Durden holds a CFII and ATP with type ratings in the Douglas DC-3 and Cessna Citation and is the author of The Thinking Pilot’s Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing it, Vols. 1 & 2.

On Saturday, January 6, 2018 at 8:40:13 AM UTC-5, wrote:
from SSA Clubs and Chapters:

Over the years I've come in contact with various club towplane operating procedures that made some attempt to address "shock cooling". These techniques often have the effect of substantially reducing the tow capability of a towplane during a busy day by increasing cycle time. Being the kind of person who always wants to know "why" when these kinds of facts are presented, I learned that there was no sound thermodynamic or metallurgical reasons for the practice. When I became our clubs Chief Tow Pilot and maintenance go-to guy I changed our procedures and worked with our towpilots to understand how our operational procedures might make our engines last longest yet yield the best tow service. Our procedure once the glider releases is to smoothly pull power to whatever setting is desired to get back to the pattern soonest and reduce mixture aggressively. If you belong to a club that has a prescribed "cool-down" procedure for towplane descents, you may be spending more on tow services than need be. The following from a more expert authority might be useful.
-----

Shock Cooling: Time To Kill The Myth

RICK DURDEN

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 Patrol. While he thought it was important to volunteer for a good group, he was ready to quit because of a screwy power reduction procedure imposed on the pilots by someone high up in the organization.. The procedure was ostensibly to prevent cylinder cracking due to shock cooling during descent after the glider released. However, the procedure he described took so long that, even if the glider did several minutes of soaring during its flight, it was on the ground well before the tow plane. As a longtime tow pilot, this struck me as ludicrous.

The anti-shock-cooling exercise required a series of small reductions in manifold pressure, each followed by flying around for a period of time before making the next, while the airplane descended slowly, burning lots of fuel. If shock cooling actually existed and caused cylinder cracking, it would probably be cheaper for the operation to have bought a bevy of cylinders and kept them on hand for replacement than pay for the fuel they were going through to avoid a phantasm.

I used to be astonished at how aviation myths, particularly when it came to engine operation, have such incredible staying power. Now, when I hear one spouted, I just shake my head in admiration of the influence of ignorance and belief over data. With some folks, the laws of physics, aerodynamics, metallurgy and thermodynamics are trumped by unwavering faith in their particular superstitions.

Nevertheless, when aviation superstitions get in the way of safe, efficient engine operation and addressing real risks of damage to engines, they need to be exposed for the nonsense they are, particularly when they are adversely affecting others—such as the glider operation that could only get off a few flights an hour. Such practices, especially when they are taught as fact to new pilots, only perpetuate the foolishness.

The widely respected Daniel Patrick Moynihan put it eloquently: “Everyone is entitled to his own opinion, but not his own facts.�

There is absolutely no hard evidence that making a large power reduction will cause cracking of the cylinders of a horizontally opposed piston aircraft engine. Because people like examples, we’ll start with a few: Bob Hoover regularly shut down and feathered the engines on his Aero Commander Shrike during airshows—going from max power to none—and never cracked a cylinder. That’s consistent with what skydiving and glider tow operators have known for decades—their engines hit TBO without much in the way of cylinder problems, even though they descend rapidly at low power settings. Flight schools, with their repeated touch and goes, don’t go through cylinders at a disproportionate rate.

Let’s look at the numbers involved in engine cooling, starting with the small role that the cylinder fins play. Only about 12 percent of the heat generated by combustion departs from the engine via the cooling fins. The biggest proportion, 44 percent, goes out the tailpipe. Eight percent, almost as much as is handled by the cooling fins, is dissipated through the oil. Most of the rest is dissipated via the big, metal prop bolted to the crankshaft.

The engine manufacturer that has published data on the potential for shock-cooling damage—Lycoming—said to avoid the risk of damage, pilots should limit CHT reduction in flight to 50 degrees F per minute. The good news is that, even assuming such a rate of cooling will damage an engine—Lycoming said that damage potential existed only if done "consistently"—it’s nearly impossible to cool an engine that fast in flight even by shutting it down. In an article written by Kas Thomas more than 20 years ago and reprinted in AVweb, he went through the published test data—which showed that cutting engine power by half only reduces CHT by 10 percent or so. That kind of CHT drop isn’t capable of trashing cylinders—and isn’t anywhere close to the CHT change that occurs in the opposite direction on takeoff—shock heating, so to speak. And there’s never been any data to indicate that the massive shock heating during takeoff harms the cylinders.

Thomas also pointed out that flying through rain reduces CHTs by nearly as much as a 50 percent power reduction. There’s no history of airplanes regularly flown through rain having to constantly replace cylinders.

In fact, the real shock cooling comes at the end of the flight when you pull the mixture to idle cutoff and the CHTs drop at more than 100 degrees per minute right away—yet every engine goes through that sort of shock cooling and manages to survive it.

In the last 20 years, graphic engine monitors have become common in general aviation—and the data they provide further support conclusions reached before they were around regarding the minor effect of big power changes. Many monitors are set to alarm if the CHTs show a drop at a rate of more than 60 degrees per minute. Pilots are discovering that it’s nearly impossible to hit that rate without slamming the throttle shut and diving—which isn’t comfortable for anyone in the airplane. Mike Busch, A&P and principal of Savvy Aircraft Maintenance Management, told me during a conversation at an AOPA Fly-In that he’s tracked how fast CHTs will drop with various power reductions in his Cessna T310R. His observations were that it unusual to have CHTs drop at a rate of even 30 degrees per minute even with aggressive power reductions when ATC gives a slam-dunk approach.

In one of AVweb columnist John Deakin’s excellent articles on engine operation, he noted that when he waited 18 seconds to restart the engine of his Bonanza after running a tank dry, the CHTs only dropped 10 degrees..

In my opinion, It’s time to put the shock cooling myth to bed, so that pilots can worry about things that really are a risk to their safety and wallets—such as runway loss of control accidents. After all, with more than 25 percent of accidents that cause damage to the airplane and engine arising from loss of control on landing rollout it seems to me that rather than designing complex power reduction strategies to avoid a mythical risk of damaging an engine, we should be practicing crosswind landings to protect a real risk that actually does damage engines—and the airframes wrapped around them.

Rick Durden holds a CFII and ATP with type ratings in the Douglas DC-3 and Cessna Citation and is the author of The Thinking Pilot’s Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing it, Vols. 1 & 2.

I don't see a recommendation as to the procedure the writer thinks should be used.
We run 4 cylinder Lycoming engines in Cub and Pawnee. Our procedure is to reduce power smoothly after release while establishing a descent air speed of 2300 rpm.
We fly a profile in descent and through the pattern that holds that condition until base leg where speed and throttle are progressively reduced to glide condition on final. We try for a little over 2 minutes from release to final power reduction. CHT normally has dropped from 230C to a little under 200 by that time. We don't mess with mixture. It is just one more thing to forget before the next launch. Our typical turn time is about 7-1/2 minutes for the Pawnee(180 HP) pulling a 2-33 to 2000 feet.
We have found one cracked cylinder in the last 10 years over about 15000 tows.
FWIW
UH

As Uncle Hank said:
"I don't see a recommendation as to the procedure the writer thinks should be used."

I'm all ears and would consider a different descent procedure, but I use a similar procedure as Hank and not cracked a cylinder in decades. That's how my father (Captain Fritz Compton / A&P) taught me in 1968 as well as advice from mechanics and engine overhaulers. Seems that the owner of the towplane has the final say.

So if "shock-cooling" is a myth, so be it. I'll stick to my descent procedure until convinced otherwise.
If I buy into that shock-cooling is a myth will the writer of the article pay for a cracked cylinder replacement? Give me a new procedure / reason to change!

A real concern for me is a potential midair with an aircraft below release altitude when aggressively diving the towplane down after release, so my towpilots make a slow descent, looking outside. What's the hurry? As my Dad (who flew the US Mail for Eastern Air Lines starting in 1939) said, "Take it slow son, we ain't flyin' the mail, just gliders."

Then again, y'all do what you want . . . my way isn't the only way.

Well, since it was published and on the internet, it must be true. Right?

Lasham Gliding Society do many thousands of aerotows each year, using
Robin DR400, Super Cub and Pawnee. There used to be a steady incidence
of cracked cylinders. After instituting a standard engine handling
procedure
this stopped, and saved a lot of expence and tug downtime. The procedure
doesn't significantly increase turnround time; in the recent Europeans held

at Lasham, 60 Competition launches to 2000ft took typically 30 minutes.

The club has a comprehensive tow pilot manual, which you can find at

https://tinyurl.com/y7w9otoa

and engine handling on descent is section 2.21

On Sunday, January 7, 2018 at 3:38:34 PM UTC-5, Steve Leonard wrote:
Well, since it was published and on the internet, it must be true. Right?

Steve - you can't believe everything you read on the internet! Remember - that's how WW-I got started!
Uli
'Uli'

Really? Al Gore had not invented the Internet back then...sheesh, he wasn't even born yet!
Don't believe me? Look it up on the Internet, he stated he invented the Internet......!



LOL......
Sigh.......

A good laugh is almost as good as an hour in the air.Â* Thanks!

On 1/7/2018 5:03 PM, Charlie M. (UH & 002 owner/pilot) wrote:
Really? Al Gore had not invented the Internet back then...sheesh, he wasn't even born yet!
Don't believe me? Look it up on the Internet, he stated he invented the Internet......!



LOL......
Sigh.......

--
Dan, 5J

I "believe" a part of this is......steep bank, pull stick, keep upper front hinge of cowling on horizon, drop like a "homesick brick". Part of this makes airflow ACROSS the cowling inlets, thus dropping airflow through the engine.

This is NOT a "dive towards the ground"..,,,,

Yes, Hank can elaborate if he feels the need to......

Yep Charlies, that was I was explaining in my earlier post, hard slip= slow speed, large rate of decent and disrupted airflow into cowl hence keeps the cht up. Works liike a champ but half the guys out there flying have ever done a hard slip and even less use it on a regular basis.
Dan