Page 22 - March 25

engine can simply be shut down and the airplane flown
with the good engine to a safe landing. Yet, the dastardly
effects of a windmilling prop are still present. If the
offending windmilling prop is not feathered, the excessive
drag of the windmilling prop can cause huge aerodynamic
problems. It can even cause a King Air to lose control if
improperly flown.
A Piper Meridian, M500 and M600 have the identical
engine as many of the King Air 200s. The only difference
between the two engines is the King Air has the PT6-
42, and the Meridian has the PT6-42A. That “A” means
the fuel control for the Meridian has provisions for the
MOR. Otherwise, the two engines are identical. Both are
susceptible to a power rollback.
A power rollback occurs when there is a loss of Py
pressure. Py pressure is modified P3 air, and P3 air is
bleed air tapped from the engine’s compressor section.
P3 air is bled (or ported) from the engine and used for
all sorts of important in-flight functions. It is used to
pressurize the cabin, “poof the boots,” create vacuum
for pressurization control, and P3 air also is sent to the
fuel control unit.
In the FCU, this P3 air is downgraded slightly and
called Px air, and then it is modified again slightly and
called Py air. This air is used to control the fuel that
goes into the engine.
Fuel flow in the FCU is ultimately controlled by the
metering valve, which is like the valve that is on the hand
wand of a water pressure sprayer you might use to clean
your home’s siding, deck or concrete. You control the
valve with your hand in a pressure sprayer but not in your
PT6-powered King Air. Beechcraft knew a pilot could not
be trusted to control the metering valve directly. They
knew that a pilot might cause a flameout or a compressor
stall if the metering valve were mishandled.
A flameout could occur if the metering valve were
allowed to close too quickly and not allow enough fuel to
flow to keep the flame burning in the combustion section
of the engine. A flameout results in an engine failure, a
bad potential for any regime of flight.
A compressor stall occurs when the metering valve
opens too quickly, allowing fuel to be dumped into the
engine at a higher rate than the compressor can provide
compressed air. If you want the engine to speed up, you
must also make the compressor turn faster. If not, the
compressor will stall and the excessive pressures can
destroy an engine internally.
To prevent both a flameout and a compressor stall,
Pratt & Whitney devised an ingenious system: They
married the compressor’s speed with the movement of
the metering valve. The engine uses bleed air (Py air)
from the compressor section to move the metering valve.
22
29
6
20 •
­ KING AIR MAGAZINE MARCH 2025