TM 1-1510-218-10
2-70
panel indicate a fully armed system. The annunciator
panel
lights
are
green
and
are
placarded
1
AUTOFEATHER (left eng) and 2 AUTOFEATHER
(right eng). The system will remain inoperative as long
as either power levers is retarded below 90% N1
position, unless TEST position of the AUTOFEATHER
switch is selected to disable the power lever limit
switches. The system is designed for use only during
takeoff and landing and should be turned off when
establishing cruise climb. During takeoff or landing,
should the torque for either engine drop to an
indication between 16-21 %, the autofeather system
for the opposite engine will be disarmed. Disarming is
confi rmed when the AUTOFEATHER light of the
opposite engine becomes extinguished. If torque
drops further, to a reading between 9-14%, oil is
dumped from a servo of the affected propeller allowing
a feathering spring to move the blades into feathered
position. Feathering also causes the AUTOFEATHER
light of the feathered propeller to extinguish. At this
time both annunciator AUTOFEATHER lights are
extinguished, the propeller of the defective engine has
feathered, and the propeller of the operative engine
has been disarmed from the autofeathering capability.
Only manual feathering control remains for the second
propeller.
b. Propeller
Autofeather
Switch.
Autofeathering is controlled by an AUTOFEATHER
switch on the overhead control panel, Figure 2-15.
The three position switch, placarded ARM / OFF /
TEST, is spring-loaded from TEST to OFF. The ARM
position is used only during takeoff and landing. At
ARM, if engine torque drops below 16-21%, two
torque-sensing switches of the affected engine are
actuated by loss of torque pressure. Switch actuation
applies current through an autofeather relay, to a
corresponding dump valve, causing the release of oil
pressure which held an established pitch angle on the
blades of the affected propeller. Following the release
of oil pressure, feathering movement is accomplished
by the feathering springs assisted by centrifugal force
applied to the blade shank counterweights. The TEST
position enables the pilot to check readiness of the
autofeather systems, below 88% to 92% N1, and is for
ground checkout purposes only. Chapter 8 contains
normal operating information.
c. Autofeather Lights. Two green lights on the
caution/advisory
annunciator
panel,
placarded
AUTOFEATHER #1 and #2, when illuminated indicate
that the autofeather system is armed. Both lights will
be
extinguished
if
either
propeller
has
been
autofeathered or if the system is disarmed by retarding
a power lever. Autofeather circuits are protected by
one
5 -ampere
circuit
breaker,
placarded
AUTOFEATHER, located on the overhead circuit
breaker panel, Figure 2-16.
2-52. PROPELLER GOVERNORS.
Two governors, a constant speed (primary)
governor, and an overspeed governor, control the
propeller RPM. The constant speed governor,
mounted on top of the reduction housing, control the
propeller through its entire range. The propeller
control lever operates the propeller by means of this
governor. If the constant speed governor should
malfunction and request more than 2000 RPM, the
overspeed governor activates at 2080 RPM and
dumps oil from the propeller to keep the RPM from
exceeding approximately 2080 RPM. A solenoid,
actuated by the PROP GOV TEST C D T1 and
GOVERNOR TEST / ON T2 switch located on the
overhead control panel, is provided for resetting the
overspeed governor to approximately 1830 to 1910
RPM for test purposes. If the propeller sticks or
moves too slowly during a transient condition causing
the propeller governor to act too slowly to prevent an
overspeed condition, the power turbine governor,
contained within the constant speed governor housing,
acts as a fuel topping governor. When the propeller
RPM reaches 2120, the fuel topping governor limits
the fuel flow to the gas generator, thereby reducing the
power driving the propeller. During operation in the
reverse range, the power turbine governor is reset to
approximately 95% propeller RPM before the propeller
reaches a negative pitch angle. This ensures that the
engine power is limited to maintain a propeller RPM
somewhat less than that of the constant speed
governor setting. The constant speed governor
therefore, will always sense an underspeed condition
and direct oil pressure to the propeller servo piston to
permit propeller operation in Beta and reverse ranges.
2-53. PROPELLER TEST SWITCHES.
Two-position propeller governor test switches on
the overhead control panel, Figure 2-15, are provided
for operational test of the propeller systems. The
switches control test circuits for the corresponding
propeller. In the TEST position, the switches are used
to test the function of the corresponding overspeed
governor. Refer to Chapter 8 for test procedures.
Propeller test circuits are protected by one 5-ampere
circuit breaker, placarded PROP GOV, located on the
overhead circuit breaker panel.
2-54. PROPELLER SYNCHROPHASER SYSTEM.
a. Description C D . The propeller synchro-
phaser automatically matches the RPM of the right
propeller (slave propeller) to that of the left propeller,
(master propeller) and maintains the blades of one
propeller at a predetermined relative position with the
blades of the other propeller. To prevent the right
propeller from losing excessive RPM if the left
