Chapter IV
Feathering Systems
Propeller feathering systems are used on most
multi-engine aircraft to reduce the drag created bya windmilling propeller if an engine should fail. To
be able to feather, the propeller must have a great
enough blade angle range to achieve an approximate 90° blade angle. Also, the governor
must be designed so that the constant-speed
operation can be overidden at anytime to feather
the propeller.
A means of unfeathering in flight is required in
the system and this may require additional systemcomponents. These additional components may
include accumulators, electric oil pumps. and oilflow control valves.

A. Basic Feathering System
The basic constant-speed operating components of a feathering governor are the same as
those used in a constant-speed governor. In mostfeathering governors, the pilot valve and the governor oil passages are designed so that oil pressure
is directed to the propeller to decrease thepropeller blade angle (underspeed condition) andoil pressure is released from the propeller to increase the blade angles (overspeed condition).
The primary design difference between a constant-speed and a feathering governor is the addi-

Figure 4-2. The lift rod engages the pilot valve tofeather the propeller.

Figure 4-1. The feather angle is 90°to the plane ofrotation whereas the constant-speed blade angle Is
between 10 and 25° to the plane of rotation.

tion of a lift rod connected to the pilot valve tomechanically lift the pilot valve to the overspeed
position when it is desired to feather the propeller.
The lift rod raises the pilot valve when the cabinpropeller control is moved full aft. When the pilotvalve is raised, all of the oil pressure in thepropeller is released and the blades move to thefeather angle by the centrifugal force on the blade
counterweights and some other force, such assprings, depending on the propeller design.
The full aft movement of the cabin propeller
control usually requires additional effort or asideways or outward movement to enter thefeather position. This is to prevent accidental
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TACHOMETERMANIFOLD
PRESSURE (6FUEL FLOW

COUNTERWEIGHTENGINESHAFT

CYLINDERMIXTURES
POWERPLANTCONTROLPEDESTALPISTON

THROTTLESPROPELLERS
HUBASSEMBLY

CENTRIFUGAL
LATCH

PISTO

Figure 4-3. The cabin propeller control may have to be
moved sideways to go Into the feather range on thecontrol quadrant.

feathering of the propellers during constant-speed
adjustment.
When unfeathering the cabin propeller control
is moved forward into the constant-speed range.
This lowers the lift rod far enough so that it
disengages the pilot valve and the governor returns to constant-speed operation (underspeedcondition before the engine restarts) and allows
the engine to be restarted.
The engine is usually restarted by supplying oilpressure to the propeller through the governor,
which causes the blade angle to decrease and theFIXEDLATCH (PLATELATCHDISENGAGEDLATCHENGAGED

Figure 4-5. A version of the centrifugal latch
mechanism used to prevent a feathering propeller
from feathering when the engine is shut down on
the gorund.

engine to windmill. Once the engine is windmilling,
the ignition and fuel for the engine is turned on
according to the particular aircraft operations
manual and the system returns to normal operation.

B. Feathering PropellersFEATHERINGCONSTANT-SPEED

Figure 4-4. A feathering McCauley propeller compared
to a McCauley constant-speed propeller.

The propellers used with feathering systems arebasically the same as constant-speed propellers
except for the additional blade angle range and alonger nose cylinder to accommodate the greater
blade angle range. These propellers use oil pressure from the governor to decrease the propeller
blade angles and counterweights, springs, and airpressure in various combinations to increase theblade angles and feather the propellers.
Most feathering propellers incorporate a centrifugally operated latch mechanism which locks
the blades at a low angle when the engine is at idle

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COURTESY OF WOODWARD GOVERNOR CO.

CYLINDER TYPE

BRACKET
NUT
BRACKETBOLTACCUMULATORHOSE ASSEMBLY
FILLER VALVE
SCREW

CLAMP
SCREW
CLAMP
INTAKE MANIFOLD
NUT
LINE ASSEMBLYUNION
PROPELLER GOVERNOR

Figure 4-6. The accumulator system In a Cessna 310.

on the ground. These latches prevent the blades
from feathering when the engine is shut down onthe ground. In flight, centrifugal force holds thelatches out and they do not interfere with theconstant-speed or feathering operation.

C. Unfeathering Systems
To unfeather a propeller, oil pressure must be
supplied to the propeller so that the blades will
rotate to a lower angle and start to windmill theengine. There are four methods which may be used
to accomplish unfeathering: developing oil pressure by rotating the engine with the engine starter;
supplying oil pressure to the propeller from anaccumulator; using an electric pump to supply oilpressure to the governor; and, directing oil fromanother engine propeller system to apply pressure
to the feathered propeller.
1. Engine StarterTo unfeather the propeller in some aircraft thestarter motor for the engine is used to rotate theFigure 4-7. Two types of unfeathering accumulators.

engine and the governor to develop oil pressure
which is directed to the propeller through theunderspeed condition of the governor anddecreases the propeller blade angle. As the blade
angles decrease, the airflow from the aircraft
moving through the air will cause the propeller tostart to rotate. As the engine and propeller start torotate, the oil pressure from the governor will
increase to the normal governor pressure and thepropeller-governor system will return to constant
speed operation.
2. Accumulator Systems
Many aircraft incorporate an accumulator in
the propeller system to be used when unfeathering the propeller. The accumulator is normallylocated in the engine compartment and is connected to the governor by a flexible or a rigid oilline.
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OIL LINE TO GOVERNOR
AIR VALVESEALSNITROGEN
AT100 PSIPISTONNO OIL CHARGENITROGENCOMPRESSEDTO 300 PSIOIL CHARGE
OF 300 PSI

Figure 4-9. The governor is set to decrease thepropeller blade angle when unfeathering with anaccumulator.

OIL CHARGED ACCUMULATOR

Figure 4-8. When the accumulator is charged with oil
the air in the accumulator Is compressed to thesame pressure as the oil.

The accumulator may be of the ball-diaphragm or the piston-cylinder type. In either
case, an air charge of about 100 psi will be on
one side of the diaphragm or piston when theengine is shut down on the ground and the otherside will be connected to the governor oil pressure line for the propeller through the oil line
mentioned above.
During normal constant-speed operation, thegovernor oil pressure that is applied to thepropeller is also directed to the oil side of theaccumulator to charge the accumulator with oil atgovernor oil pressure. Since the governor oil pressure is two to three hundred psi, the oil pressurein the accumulator will compress the air in theaccumulator to the same pressure as the oil andthereby store a charge of oil at governor oil pressure.
If the propeller is feathered, the oil in the accumulator is trapped by a check valve in thegovernor or by a control valve attached to thegovernor.
To unfeather the propeller, the cabin propeller
control is moved forward. This sets the governorpilot valve to direct oil to the propeller (governorunderspeed condition) and the check valve orcontrol valve is opened and the oil pressure in theaccumulator is released into the propeller. The oilpressure moves the propeller to a lower angle, thepropeller starts to windmill, and the engine can berestarted.

Once the system returns to constant-speed
operation, the accumulator is again charged andis set for the next unfeathering operation.

Unfeathering 011 Pumps
A few aircraft use an electrically operated oil pumpto supply oil pressure for unfeathering the propeller.
In this unfeathering system an oil line from theengine oil sump is connected to an electric oil pumplocated in the engine compartment. The output sideof the pump is connected to the propeller oil line
between the governor and the propeller.
The oil pump operation is controlled from thecabin by a toggle switch or push button. When thepilot desires to unfeather the propeller, thepropeller control lever in the cabin is moved into
the constant-speed range and the cabin
unfeathering switch is operated until the oil pressure from the electric oil pump causes the blades
to move to a low enough angle that the propeller
starts to windmill. The switch is then released andthe normal engine restart procedure is followed.
During this operation, oil pressure from theelectric pump is also applied to the governor, butwith the governor in the underspeed condition. No
significant amount of oil pressure is lost throughthe governor.

Unfeathering Crossfeed System
A few aircraft designs use a crossfeed device totap oil pressure from an operating constant-speed
system and direct it to the feathered propeller.
To unfeather with a crossfeed system, thepropeller control lever for the feathered engine isplaced in the full forward position (this may varyfor some aircraft) and the crossfeed valve in the18
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