Shock Cooling: Time To Kill The Myth

There's no evidence that big power reductions damage horizontally opposed aircraft engines by "shock-cooling" them. It's time to trash the myth so pilots can focus on real risks to their airplanes and engines.


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. Much 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.


  1. Once again Rick, thank you. And for many more ‘coffee cup’ discussions that we have in our hangars I suggest reading one or both of your books. I did and can tell anyone there is a lot to learn on those pages !!!

  2. I would ask the same question – how else do cylinders usually crack if not as a result of shock cooling? I think I’ll continue to treat the engine nicely when descending off tow!

  3. I used to fly a C-206 and we did cut the mixture, opened the throttle fully and descended like a rock, we never cracked any cylinders. Just had to remember to close the throttle before letting the fuel back on again.
    Also, if you look at snow mobiles and of road bikes, when they runs red hot and then being cooled down with snow or water poured directly on the cylinder, they never have any cracks.
    However, at redline cylinder temp there is a significant loss of structural strength in aluminum heads at the same time as there is the highest pressure, and maybe even detonation. Easy to do by applying full power, climb at low speed and rpm, lean to best power and forget to open the cooling flaps on a hot day.
    So, my opinion, (not scientifically verified) is that you damage the cylinders by mismanagement of the cyl temperature on the way up, not down
    BTW, my car engine will not go to pieces when I start it at -40, which might be considered as shock heating.

  4. What could lead to cracked cylinders? Variations in casting, machining or assembly process. There’s quite a few parts that make up the system, stacking up those tolerances may lead to slow deterioration that one day may lead to a crack.

  5. I will say the writer, while having good intent, does not have a good handle on this issue.
    My argument starts with the premise that the US Army (and then the Air Force) and the Navy, both operated Piston Engine aircraft from the start through the early 1980’s (Vietnam saw a great deal of piston engine aircraft by all services with the last phased out about 1982?)
    So, you have hundreds of different makes a models (to include many Cessna’s and Navions) in the respective inventories of all branches.
    Much of our knowledge and best practices in the operations of Piston Engines comes from How Many Thousands of hours of military use; as well as Airline Use?
    In this day and age, anyone with access to Facebook can deride the learned opinions of scientist who spent years studying viruses and pestilence and derive a following of equally undereducated individuals.
    As a pilot of over 50 years my advice is: Who pays the bills? Then the Pilot follows the directions of the guy who writes the check. Now, the writer does well to site guidance from engine manufacturers, and I would certainly recommend the pilot take Lycoming’s (or Continental’s) guidance to the guy who writes the checks and help them become educated.
    But take Cheap Talk with a grain of salt (large grains at that) and dig deep into a multitude of sources and press your argument to the owners of the aircraft.

    • The only book the pilot/owner should be buying and studying is the POH and the Engine Manufacture Owners Manual respectively. Usually in many many cases when you come in hot and close the throttle, along with the gear warning indicator, you will likely hear backfiring out the exhaust (ever had that happen guess why) causing potential, unnecessary, and preventable damage to the augmenter, the muffler, and exhaust valves, where cracks often occur.
      If Mr. A&P/IA Mike Bush selling website clicks says so, as proven in some comments above, unwitting readers blindingly think it must be true. (click here for $).
      Apparently, if your cylinder fails it must be the manufactures fault, or excessive heat (what ever that is)(might should have cooled it down gradually), and not the way pilot is abusing his motor.
      I call BS on this so called “opinion” because that obviously is all it is and absolutely not based on any science of metallurgy, homogeneous chemistry compounds, or metal alloy whatsoever. I do however agree it is time to kill the myth. The myth “NO DAMAGE” can possibly, or “EVER WILL BE DONE” by rapid temperature changes in the aircraft engine (not a ground based motor bike motor, or vehicle where you can coast to the side of the road). Be advised, the only possible damage being done is attempting to sell books based on opinions (click here for $).
      I am a very experience pilot with thousands of hours as my dad, my uncle, my brother and I were all born and raised on the airfield. What this myth (opinion) is doing is providing unnecessary income for your mechanic who if is worth his salt will tell you not to practice such nonsense. Ever heard “take care of your motor and it will take care of you?”. After all you are the one who is going to be paying the invoice! I doubt when you do damage anyone selling these alternative opinions in the guise of facts will be writing your mechanic the check from income derived from book sales or website clicks…. You practice this proven ill fated technique and one day after you blow your augmenter apart or shorten the life of your high performance engine you will remember this conversation. I also agree with the statement “you may reference your own opinion but not your own facts”, so believe what you will.
      There will be no ongoing dialog concerning this matter as I will not entertain any attempted retort. -END-

      • Here it is over a year later and no one has challenged you. Bravo. Your opinion reigns supreme.

        Learning is best achieved by having conversations with those with whom you disagree, and exchange relevant supporting information and data for comparison and analysis.

        I am aware of much data collected in a contemporary engine test cell that supports assertions on aircraft engine operation that are much more recent than the POH and engine manufacturer guidance that has mostly not evolved from their original issues many decades ago. The contemporary test data and the purveyors of what the data shows are not encumbered with legal departments concerned with liability concerns about changing the “book” from what was originally approved by the FAA to something more relevant, even if it’s a more accurate reflection of contemporary research findings. Those encumbered with such departments have nothing to gain by making changes to the way it’s always been, even if updates would make the material accurate and in line with the the analysis of improved data sets available to them. There have been some exceptions, but generally speaking no change is a good change in the eye of the original manufacturer. We won’t discuss the marketing angle as that is a totally different conversation.

        I haven’t done the historical research, beyond the guidance in my POH and engine manual, to reveal the science and data that led to the recommended operating procedures in those documents. And when I speak of science, I refer to true science that starts with a hypothesis and then sets out to collect data and determine whether the data invalidates the hypothesis or supports (not proves!!) the hypothesis. I presume it exists, and would like to review it, but the research isn’t juice worth the squeeze to me to look for it, in light of the contemporary data. But if someone could point me to it I’d be most appreciative for the learning I could achieve.