Shock heating is the situation that exists when a
pilot almost instantly applies full power to the
engines as he begins his takeoff. It comes as quite
a shock to the engines. Remember, the majority of
engine failures take place during power changes -
takeoff and landing. The pilot uses gear, flaps,
speed brakes, gradual power reductions, and 1200F to
1400F EGT mixture settings to control shock cooling
during rapid descents. RAM suggests the same degree
of attention when applying takeoff power.
A rapid advance to takeoff power can be as hard or
harder on the engines as shock cooling. Even at fast
idle while the aircraft is taxied to the runway, the
internal cylinder temperatures are only warm.
Numerous engine components are not yet up to the
heat and pressures of full power. The oil may be
warm, but it?s still cool enough to be at a higher
than normal pressure. This situation sends
misleading feedback pressures to the internal engine
control systems. It takes time for the engine
systems to perform their function and to adjust for
the heat.
Bursting into full takeoff power can be compared to
spilling an entire pot of hot coffee on your legs
vs. one small cup. Although both have the same
temperature, the full pot of coffee contains a
tremendous quantity of heat. Damage results anytime
anything is forced to absorb a large quantity of
heat quickly - rather than gradually. RAM acceptance
and test pilots use the following takeoff technique
to help smooth their transition to higher cylinder
temperatures and system pressures.
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Always Assert Your Pilot In Command Authority to
Ensure The Safe Operation Of Your Aircraft And
Passengers.
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When lined up on the runway centerline for
takeoff, hold the brakes and smoothly advance
the throttles to 27-30 inches of manifold
pressure. Allow the MP to stabilize 3-5 seconds.
During that time, scan the engine instruments.
Verify that they are normal.
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Release the brakes and smoothly advance the
throttles to takeoff power manifold pressure.
Remember, oil pressures and feedback signals to
the controllers are adjusting as the
temperatures rise. Pilots should allow engine
systems time to function.
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Again, scan the instruments and make any
necessary final throttle adjustments.
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Should an immediate takeoff be required upon
arrival at the runway, the same procedures can
be followed during the takeoff roll instead of
holding the brakes - if runway length is
suitable: partial power, stabilize, then full
power.
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As you takeoff, cross checking fuel flow, RPM,
MP and EGT is essential. It enables you to
recognize and react to critical or premature
instrument readings in a timely manner. Plus, it
helps you remember the exact numbers when later
communicating with your aircraft mechanic. Your
aircraft mechanic needs to know specifics in
order to effectively troubleshoot.
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Of further benefit, there should be no more
surging and overboosting when using the above
technique for smoother throttle advancement.
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Most importantly, smooth power increases will
allow the cylinder combustion chambers to
experience a more constant and orderly
transition to the high temperature range of
their design limits.
