Questions for the engineers:

Not the pilots, the engineers. Powerplant engineers, specifically. (I am not an aviation powerplant expert, so bear with me).

I was flying today. The air temp was a balmy -4 degrees F at the surface….Neg 10 at altitude. Farking cold for the preflight, especially when I spilled avgas all over my hand.

The flight instructor warmed me on taxi to “be careful and make sure my power adjustments, especially power reductions, were done slowly and smoothly so as to prevent “shock cooling” of the engine”. (ETA: Apparently “shock cooling” causes issues with the heads and cylinders…cracking and/or other structural issues)

Now, as I said, I am not an aviation powerplant engineer, nor any sort of powerplant engineer…..I have, however, an engineering degree, and I have lots of thermal management experience under my belt….further, I used to service all sorts of air cooled small engines when I was in college for rent money.

Therefore the theory of “Shock Cooling” an aviation engine simply by closing the throttle quickly sounds fishy to me. Shutting down an already overheated powerplant? Maybe. Immersing it in water (or filling the cowling with a firehose?) maybe…but probably not.  But a gas engine that is still running should not be able to be cooled fast enough to damage it simply by slowing down the engine. What happens to an engine in flight when the airplane flies through a rain shower? Mist? heavy rain? snow?  All these should lead to significant cooling of the engine in a very short time. Water conducts heat better, plus evaporation. Faster cooling. Yet I don’t hear of Lycomings or Continentals (or Rotaxes (Rotaxi?)) suddenly shedding cylinders or valves because of rain.

I would think shutting the engine off after use would do more damage (due to sudden changes in cooling vs heat) than closing the throttle suddenly.  Or unloading the prop with quick attitude changes (say climb to level to dive)….And, of course, the sudden dimensional changes in temp due to starting the engine at all……

Again:  not an aviation powerplant expert. But have more experience than most on thermal issues.

Does anyone with real knowledge have some input? Is there something to it, or izzit just some myth carried forward by folks who are pilots but not engineers?

5 thoughts on “Questions for the engineers:

  1. You DO get some carb icing issues without using carb heat if you reduce power quickly, but I've never heard of 'shock cooling'…

  2. Yeah, carb icing is a different issue. That's all venturi effect. I'm talking cylinder and head structural issues.

  3. Unfortunately, I'm an engineer and pilot, just not the "right kind" of engineer…

    Those cylinders have an aluminum head tightly coupled to a steel cylinder sleeve. The theory is that the head gets most of it's heat from combustion and conduction of exhaust gases. Aluminum loses heat faster than steel, thus when the heat source is removed, the head shrinks faster than the steel parts, and then cracks start. I assume your CFI is concerned with the high delta-T at the cold temperature; CHT can run 325-400F on a non-turbo airplane, so the temp difference is 330-405F, versus 285-360 on a "standard" day.

    When they crack, it's common to see it happen between a spark plug and the cylinder sleeve, as well as between a plug and an exhaust stud. They show up around exhaust valve guides too. Once cracked, they're getting replaced at $850 for a small Continental, or $1200+ for a Lycoming. That's just a cylinder assembly, labor will cost a few hundred more on top of that.

    The guys that really sweat shock cooling are the ones running big motors fed by turbochargers; those struggle with high head temperatures.

    If it's -4 out, I'd be far more concerned with preheat before starting if the airplane was stored outdoors. Anything below 20F requires it, or it will tear things up quick. I usually won't start it under 28-30F without heat.

  4. I am a mechanical engineer, have been a pilot since I was 16, from a family of pilots, and have spent some considerable time researching this story. An excellent technical resource is the Pelican's Perch series of articles at avweb.com – these also include references to source material.

    In short, this is another of those good habits that are stressed during training and in practice. No, it will generally not harm the piston engine of a light training aircraft to close the throttle suddenly. The habit might save your engine someday.

    However, if you end up flying a turbo-charged piston aircraft at very high altitude someday, and you've been pushing it near the limit for a while, and then slam the throttle shut and do a high speed descent, you could over stress a cylinder head. Especially if it has a small flaw to begin with.

    This issue traces its roots back to WWII and the dawn of civil transport aviation. Large radial piston engines had to be operated economically and reliably. This was one of the techniques that helped with that. Those engines were not as reliable or well fabricated as our modern piston engines.

    WWII piston aircraft operated at or near the very edge of what material science of the time could do would be more likely to throw heads if treated like this. Very high temperatures (for aluminum) or whatever else they were made of, very cold air at high altitude, and sudden power changes.

    Basically, if you habitually and suddenly cool the engine from a very high power setting and in very cold air, you might have a problem someday but it is very unlikely in a modern engine. A naturally aspirated engine cannot make enough power at high altitudes (as the air is also thin as well as cold) to get hot enough to cool suddenly enough.

    It is good airmanship to make those power changes smoothly and slowly in any case. Any sudden change in power settings stresses all the engine components and the noisemaker is the only thing keeping you up there…

  5. My dad told me many times to not"ram" my car around, and it would run longer. So I drive a little bit like an old man, and my cars have never burned oil or shed U-joints early. We have a Grasshopper with a Kawasaki twin engine(water cooled)that we bought new in 2006. We mow about 10 acres of lawn with it, and run it up against the governor while working it. We keep the radiator screen clean, and when done mowing, we idle back to the garage and let it run slow for a couple minutes. I do it for the valves. I want them to cool down before I switch off. No way to know if it has made a difference, but after twelve seasons of mowing, the engine runs like new. David (I am on the Mrs' computer.)

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