***Shock cooling is a myth, except for what Walter is about to tell you. ***
OK, here goes.
No one had ever heard of shock cooling until the pressurized, turbocharged twin Cessnas came along in the 1970s. There are no maintenance reports of the hard to manage supercharged radials ever having shock cooling problems. We had 65+ years of aviation with no reports of shock cooling.
So where did this come from?
In the 70s, these new turbo twins began flying at much higher altitudes than their NA brethren. So what, you might ask? Well, the fuel in the wings at altitude was becoming super-cooled, cold-soaked to very low temps. No one routinely had flown that high before. Since they were pressurized, and rapid depressurization was not a problem for passengers, the pilots were descending rapidly from altitude to land and, guess what they did next? They followed the POH's recommendations to go full rich in the pattern before landing. This shoved a large amount of very COLD fuel onto the intake walls of the warm cylinder. This resulted in cracks forming--in the INTAKE area, NOT in the cylinders. In any case, this required cylinder replacement.
Pilots who ignored the POH recommendation to go full rich, and left the mixture leaned, or added mixture very slowly when approaching the pattern, did not suffer these cracks.
Soooo, LAMEs and A&Ps made the CORRECT OBSERVATION that these cracks followed rapid descents, but then assigned the WRONG cause as to the phenomenon, resulting in pilots doing something really silly----reducing power 1" per minute or some such insanity and starting their descents more than 100 miles out. It did nothing to address the problem where the cylinder was concerned since there was no problem with the cylinder in the first place, but did allow the fuel to warm up a bit before it was thrown agains the intake walls.
You can descend as rapidly as you want, with the power as low as you want, with CHTs plummeting and as long as you do not throw super-cooled fuel rapidly against the intake walls, you will not have "shock cooling" problems.
This is a perfect example of pilots, LAMEs and A&Ps making a valid observation, yet assigning improper causality and creating another aviation Old Wives Tale.
If the rapid change in CHT was the problem and inconsistent heating or cooling, there would be frequent cylinder failures as we fly into cold rain (of which there is no record) and parachute droppers would never make TBO (which they regularly do) or we would suffer shock heating on every takeoff (which we don't).
OK, all of that said, there are those who are convinced (without data) that shock cooling is real and will continue to recommend unnecessary actions or take unnecessary actions like the above to avoid a problem that simply has never existed.
My flame suit is on in preparation to receive incoming from those with no data.
