Avoiding Shock Cooling in Quick Descent

O. Sami Saydjari wrote:
: My engine operating manual (for my Piper Turbo Arrow III) strongly
: discourages pulling the power back and doing a quick descent -- it warns
: of engine-killing shock cooling. Sounds reasonable to me...but it (and
: my airplane manual) does not really seem to say how best to do a fast
: descent when you have to.

: I inferred that the right thing to do might be to lower the prop speed
: to a minimum and ease back power as slowly as you can. Does that sound
: about right? How quickly can one expect to pull the throttle back and
: not risk shock cooling?

From all I've read on it, the shock-cooling thing is probably mostly a bunch
of crap. Unless you do radical configuration changes (e.g. 8000' climb at Vy then kill
the engine and glide back down), you probably won't exceed the Lycoming recommended CHT
change rate of 50 degrees per minute. EGT only affects CHT as a secondary effect....
most of the CHT results from the combination of power produced and cooling.
Something akin to (MP x RPM)/IAS.

I pretty much figure from a cruise, you can either point the nose down and
speed up, or reduce the engine power, but shouldn't do a lot of both. For a relatively
rapid cruise descent of 500 fpm, this seems to keep the CHT from moving more than 50
degrees per minute. Either pull 4-5" MP, nose over another 10-20 kt, or maybe a bit of
both (2-3" and 5-10 kt).

The big one (I believe) is keeping the mixture at cruise lean. Since the power
is reduced, you can't hurt the engine with it. Keep cruise lean until it coughs on the
way down, then fatten as necessary.

Now with my flame-suit properly donned, what does everyone else think?

-Cory

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