Posted for Walter Atkinson <walter@advancedpilot.com>:

 We believe shock cooling to be a myth.  That said, there are two issues.  1)
how can you shock cool something that's not hot to start with? Keep CHTs under
380 and you can't cool 'em off fast enough to hurt anything, and 2) Look at
all of the years we have pulled the mixture in twins during training with no
known ill effects.
 
 We *think* we know where this OWT came from.  When the first TC'd piston
twins hit the market (Twin Cessnas) we were not accustomed to flying for
extended periods in the flight levels.  Even in the summer, the fuel would
cool off to below zero temps while at altitude.  The pilot would descend
rapidly to pattern altitude with low FF whereupon he would shove the mixture
to full rich before landing as per the POH.  This big blast of very cold fuel
would hit the side of the warm intake chamber and the results were cracks in
the intake housing.  We never saw CYLINDER or head problems, we saw intake
area cracks.  As soon as pilots quite shoving the mixture to full rich, the
problem disappeared...  it also disappeared when the misguided idea of
reducing power in 1"MP increments every two minutes slowed the rate of descent
enough that the pilot was altering the mixture slowly instead of all at once.
 There were never any reports in NA airplanes.  Why?
 
 Shock cooling cylinders from low power descents does not exist.  Heck, I can
leave 17k feet at 90% power, descend at  over 1000 fpm at very low power and
not have CHTs change more than 70-80 degrees over 15 minutes.  How can you
shock cool something that's not hot to start with? <g>
 
 This, as with most OWT's started with an accurate observation but an
inaccurate causal assignment.
 
 Walter