Debunking the Shock Cooling Myth

We are running 2 Lycoming 235 Pawnees (one wide deck & one narrow deck) and a Continental L-19. We went through a spate of cracked cylinders (3 - In two different aircraft as I recall) about 4 years ago but I am not sure it had anything really to do with cool down procedures. Then the problem went away with no good explanation. Our method is to hold 2000 - 2100 rpm for a minute after glider release while descending then make a normal landing. I suspect that there are other issues involved in cylinder cracking: quality control in cylinder manufacture, torque variations in cylinder installation, localized vibration caused by cylinder/piston/ring fit, front vs rear cylinders (they cool differently), valve adjustment, and plain bad luck.
ROY - GBSC Boston (where it is really cold this morning!)

When confronted with the bugaboo of cylinder shock cooling, my response,
even before reading the subject article, has always been: "What about
flying through rain?"Â* There's never been a cogent reply to that.

As to my procedure before retiring from towing, I simply reduced power
(throttle and prop) as necessary to arrive on downwind, at pattern speed
as quickly as possible, from wherever the glider released.Â* It's
difficult to tell a dozen glider pilots waiting for a tow, to "Slow
down, we're not hauling the mail"...Â* BTW, I'm not aware of any cracked
cylinders in 28 years of towing.

PS - I really enjoyed the original article and, from reading the
responses, see that there are folks out there who still cling to the myth.

On 1/6/2018 8:03 AM, Roy B. wrote:
We are running 2 Lycoming 235 Pawnees (one wide deck & one narrow deck) and a Continental L-19. We went through a spate of cracked cylinders (3 - In two different aircraft as I recall) about 4 years ago but I am not sure it had anything really to do with cool down procedures. Then the problem went away with no good explanation. Our method is to hold 2000 - 2100 rpm for a minute after glider release while descending then make a normal landing. I suspect that there are other issues involved in cylinder cracking: quality control in cylinder manufacture, torque variations in cylinder installation, localized vibration caused by cylinder/piston/ring fit, front vs rear cylinders (they cool differently), valve adjustment, and plain bad luck.
ROY - GBSC Boston (where it is really cold this morning!)

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Dan, 5J