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position, whereupon the propeller will usually begin
to windmill. (On some airplanes, an assist from the
electric starter may be necessary to initiate rotation
and completely unfeather the propeller.) If fuel and
ignition are present, the engine will start and run.
For airplanes used in training, this saves much electric
starter and battery wear. High oil pressure from
the propeller governor recharges the accumulator
just moments after engine rotation begins.
PROPELLER SYNCHRONIZATION
Many multiengine airplanes have a propeller synchronizer
(prop sync) installed to eliminate the annoying
“drumming” or “beat” of propellers whose r.p.m. are
close, but not precisely the same. To use prop sync, the
propeller r.p.m. are coarsely matched by the pilot and
the system is engaged. The prop sync adjusts the r.p.m.
of the “slave” engine to precisely match the r.p.m. of
the “master” engine, and then maintains that relationship.
The prop sync should be disengaged when the
pilot selects a new propeller r.p.m., then re-engaged
after the new r.p.m. is set. The prop sync should always
be off for takeoff, landing, and single-engine operation.
The AFM/POH should be consulted for system
description and limitations.
A variation on the propeller synchronizer is the propeller
synchrophaser. Prop sychrophase acts much
like a synchronizer to precisely match r.p.m., but the
synchrophaser goes one step further. It not only
matches r.p.m. but actually compares and adjusts the
positions of the individual blades of the propellers in
their arcs. There can be significant propeller noise and
vibration reductions with a propeller synchrophaser.
From the pilot’s perspective, operation of a propeller
synchronizer and a propeller syncrophaser are very
similar. A synchrophaser is also commonly referred to
as prop sync, although that is not entirely correct
nomenclature from a technical standpoint.
As a pilot aid to manually synchronizing the
propellers, some twins have a small gauge mounted
in or by the tachometer(s) with a propeller symbol
on a disk that spins. The pilot manually fine tunes
the engine r.p.m. so as to stop disk rotation, thereby
synchronizing the propellers. This is a useful backup
to synchronizing engine r.p.m. using the audible
propeller beat. This gauge is also found installed
with most propeller synchronizer and synchrophase
systems. Some synchrophase systems use a knob for
the pilot to control the phase angle.
FUEL CROSSFEED
Fuel crossfeed systems are also unique to multiengine
airplanes. Using crossfeed, an engine can draw fuel
from a fuel tank located in the opposite wing.
On most multiengine airplanes, operation in the crossfeed
mode is an emergency procedure used to extend
airplane range and endurance in OEI flight. There are
a few models that permit crossfeed as a normal, fuel
balancing technique in normal operation, but these are
not common. The AFM/POH will describe crossfeed
limitations and procedures, which vary significantly
among multiengine airplanes.
Checking crossfeed operation on the ground with a
quick repositioning of the fuel selectors does nothing
more than ensure freedom of motion of the handle. To
actually check crossfeed operation, a complete, functional
crossfeed system check should be accomplished.
To do this, each engine should be operated from its
crossfeed position during the runup. The engines
should be checked individually, and allowed to run at
moderate power (1,500 r.p.m. minimum) for at least 1
minute to ensure that fuel flow can be established from
the crossfeed source. Upon completion of the check,
each engine should be operated for at least 1 minute at
moderate power from the main (takeoff) fuel tanks to
reconfirm fuel flow prior to takeoff.
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Ch 12.qxd 5/7/04 9:54 AM Page 12-5
12-6
This suggested check is not required prior to every
flight. Infrequently used, however, crossfeed lines are
ideal places for water and debris to accumulate unless
they are used from time to time and drained using their
external drains during preflight. Crossfeed is ordinarily
not used for completing single-engine flights when
an alternate airport is readily at hand, and it is never
used during takeoff or landings.
COMBUSTION HEATER
Combustion heaters are common on multiengine
airplanes. A combustion heater is best described as
a small furnace that burns gasoline to produce
heated air for occupant comfort and windshield
defogging. Most are thermostatically operated, and
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