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The chill factor - rapid descents
We have all heard about it and talked about it - shock
cooling and the problem with rapid descents. Novice pilots
throttling back and pointing the nose down after misjudging
their distance and altitude to their destination airport. ATC
calling us and asking for our 'best possible' descent rate to a
new assigned altitude.
Shock cooling is unique to air-cooled engines in aircraft.
Aircraft are one of the few moving vehicles that can keep up a
high rate of speed with little or no power for a long duration.
During a rapid descent our air-cooled engines may cool down
anywhere from 100 to 250F in a matter of a few minutes. The
problem is that this rapid cool down is usually uneven and leads
to warpage of the cylinder heads and even the cylinder barrels.
But shock cooling isn't the only enemy - there is also shock
heating to consider. Once the engine has cooled down a rapid
increase in power will also heat the cylinder heads up unevenly
further aggravating the huge temperature differentials that tend
to exist in air-cooled heads. As the power is increased the
exhaust side of the head rapidly heats up while the intake side
remains relatively cool. This uneven heating introduces the
kinds of stresses that lead to cylinder head cracking and loss
of valve seats.
Water cooling controls the temperature
A water cooled engine doesn't have a shock cooling or
heating problem. Since a thermostat controls the coolant
temperature the engine remains at a stable temperature during
all phases of flight. Both shock cooling and heating become a
problem of the past. Furthermore because a water cooled
cylinder head maintains a T6 hardness it is far more capable
of resisting cracking due to quick changes in power. But even those
quick changes in power don't result in tremendous changes in
cylinder head temperature since the coolant quickly absorbs the
higher heat output.
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