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Posted for Walter Atkinson <walter@advancedpilot.com>:
Bob:
There is a reported concern of the rapid expansion of the aluminum piston
inside of a steel barrel during "shock cooling." What about shock HEATING
during every takeoff where the piston would expand faster that the cylinder
barrel? This is just not a problem. If shock heating and shock cooling were
real issues, one would think that no jump plane, anywhere would make it to
TBO. The record reflects that they frequently do.
Let me relate the record of the engine in George Braly's Bonanza that had
been run at 80-90% power, LOP, and had undergone what most pilots would call a
shock cooling descent on almost every flight for 1000 hours, and then
subsequently on to TBO. The engine was torn down at the 1000 hour mark for
research purposes. He found:
1) No unusual cylinder wall scuffing at 1000 hours TIS, cross-hatching still
evident, the choke still there.
2) Compressions still good.
3) Measurements showed this 1000 hour engine still within *new* limits; not
*servicable* limits, but NEW limits.
4) The engine was re-assembled and is now approaching TBO. Still no cylinder
problems.
This is one of several engines we know to have been "shock cooled" repeatedly
with no ill effects. Mine and John Deakin's, among others. (A borescope of my
previous engine at 1000 hours was circulated to an number of mechanics for an
evaluation of how the health of the engine looked. Most estimated the engine
to have less than 150 hours on it. A few thought is was not even finished
with the breakin. This engine had been operated in the same way as the one
referenced above--high power, LOP, and "shock cooled" descents.)
Based on these facts, observations, and measurements, we believe
shock-cooling to be a myth. (As long as you're not at Red Line CHT and
chopping the throttle on a daily basis. That wouldn't be contributory to good
longevity.)
If my memory has incorrectly reported any of the engine tear-down specifics
on his engine, I trust George will correct me.
Walter
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