Jay Honeck wrote:
As previously noted (in the thread about Paul's wife getting scared),
Mary and I had virtually stopped doing this kind of flying for fear of
harming our (very expensive) engine. A lively debate ensued as to
whether or not repeated high-to-low-to-high power applications would
wear out your engine any faster than would normal operations.

I eventually agreed that gradual power changes would not unduly harm
an air-cooled engine, and vowed that I would endeavor to practice this
most-important skill on our next flight. And we did.

We were on a flight back from Galesburg, IL when I started the
procedure, and very gradually began a power reduction whilst in cruise
flight at 3500 feet. I took a full minute to reduce the power to
idle, watching our (newly reinstalled) JPI EDM-700 engine analyzer for
signs of stress.

As RPMs dropped below 1000, the "shock-cooling alarm" suddenly went
off, flashing its dire warnings that EGTs had dropped beyond (and
faster) than recommended limits. (I can't remember what the threshold
is for that alarm -- it's preset.)

When I practiced in my Skylane and also in the club Arrow, I retarded
the throttle smoothly in probably 2-3 seconds. I didn't worry about
shock cooling and never saw any signs of distress in either the O-470 or
the O-360.

The issue with shock cooling isn't the rate of cooling per se, but
rather stress induced by differential cooling. Most engines see far
higher temperature differentials during start-up than they do during
cooldown. Jay, have you timed your engine heat up rate? It would be
interesting to watch how fast your engine heats up from say a 50 degree
cold start and then compare that to the cool-down rate when you pull the
throttle for engine out practice. I'm assuming this would be fairly
trivial with your engine analyzer.

I'm not sure what your normal operating temps are, but I assume it is a
much larger delta between ambient and your normal operating temps than
between your normal operating temps and the temps you see during a glide
at idle. I also suspect that the rate of heat-up during take-off is at
least as high as the rate of cooling during an idle glide. However, it
would be very interesting to see the data if you are inclined to collect
it some day.

Matt