The International Cessna^® 170 Association

I'm personally not convinced that "super cooling" in flight is a valid theory, but it's accepted as valid by more people than not. (I'm not arguing against smooth, gentle engine operation.)
I can imagine that bending the paper-clip will break it with fewer strokes if bent back-and-forth rapidly (due to weakening from heat build-up) than if bent back-and-forth more slowly, but either method will eventually break it.... I'm not convinced by such example that rapid cooling has the same weakening effect as rapid heat build-up...if anything it seems to indicate that cooler is less weakening.
Similarly, I'm not convinced that rapid cooling shrinks cylinders against hot, expanded pistons thereby increasing wear. It seems to me that steel cylinders might expand/shrink more slowly than aluminum pistons (a good argument for slow engine warm-ups... and a counter-argument on the cooling-down side) and that, in any case, the moment the throttle is reduced those pistons receive the benefit of much cooler induction air saturated with vaporized fuel. Geez....is this a seque or what?

What am I doing up at this hour!

George,

I always thought super cooling was the process of lowering the temperature of a liquid, such as water, below its freezing point without solidification. (Until something comes along, like an airplane, that disturbs it and causes it to freeze instantly.)

If you're talkng about "shock" cooling of an engine by reducing the throttle setting too quickly, I've never heard the argument of increased wear due to the cylinder shrinking around the piston. Your argument that the piston will cool more quickly than the cylinder seems valid.

The main argument I've heard aginst rapid throttle reduction is that the aluminum cylinder heads tend to cool unevenly, which has the potential of causing cracks in the head. Reducing the throttle more slowly allows the less efficiently cooled areas of the head to "catch up" with those with better cooling.

Miles

Yep. No argument from me that wild temp excursions are probably not good for cylinders (or gearboxes of geared engines, or turbos, etc.). It's the piston-seizing theory that was recently postulated to me that I don't believe.

When I find myself in a discussion or debate on the subject of shock cooling, and I find myself some how starting to buy into the logic, I remind myself of all those hours of instruction time accumulated on the venerable Cessna 150. I remind myself how little trouble we had with those engines despite how badly they were treated by students. I have a friend who will start reducing throttle 100 NM from hsi destination to avoid shock cooling the cylinders on his C-210 - drives me absolutely nuts!

I have found this debate to be endless and always will be!

The guys at Tanis heaters have done some pretty extensive testing of engine temps in cold weather and they indicate if you keep the power anywhere in the green arc cylinder head shock cooling will not exist. Flying in cold weather for the last 23 years in 170s, 180s, 185s, and beavers has taught me you don't ever want to drop power below the green arc anyway until touch down. All it takes is staying ahead of the airplane by planning ahead. You can put full flaps down on downwind and carry power all the way to the ground at the correct airspeed.

Hilltop, sounds like you buy into the same logic that I do. I always make my approaches w/power, most of the engine outs on approach that I have heard about happened when the throttle got pulled back to idle. I know an instructor that has been around these small airplanes for years and he is quiet a good instructor (older and humble)... but he is constantly harangueing on other pilots when he sees them drag in an approach, this is one thing that I just don't see eye to eye with him on. Everyone has there own beliefs.

I try to concentrate on smooth transitions, there is no reason to chop the throttle as a matter of habit, just as there is no reason to instantaneously firewall the throttle every time you need power. I don't think these habits are quiet as critical in our engines as say a TGIO 550, or a radial, but since I am paying the nickel for rebuild, I am going to try to avoid any rapid changes in the temperatures of my engine as much as possible. I do try to "baby" my engine, not from a standpoint of rpm, but from a standpoint of smooth adjustments to power.
David

David- I agree entirely with what you said. Aircraft engines are made for steady-state operation, not stop-and-go. If the engine has run well for the entire flight I'm going to trust it to run well through the approach to landing. Has anyone ever seen anybody shoot an ILS or any other instrument approach at idle power? I don't think so if they had a choice. I fly a stabilized approach before every landing whether it is to a gravel bar, beach, lake, glacier, or international airport. That requires holding power somewhere above idle, ideally in the green arc. My whole point was to share that in cold weather, power-on approaches should be used and power-off approaches should be avoided in order to maintain cylinder head temps in the green, but it also applies to all approaches if you are interested in keeping your engine healthy as long as possible.
Richard