Propeller

Hamilton Standard 14RF-19 

 

Each engine has a four bladed Hamilton Standard propeller with full feathering and reversing capabilities. The prop is driven by the power turbine through the propeller gearbox (PGB). The PGB steps down the high power turbine rpm to a speed compatible with propeller operation. The blades are composite with electrical deicing capability and integral erosion sheaths. The blade angle is controlled hydromechanically by a propeller control unit/PCU which uses oil to move the pitch change mechanism inside the prop hub via a beta/oil transfer tube. The pitch change mechanism consists of a pitch-lock screw, metering valve, and actuator piston. 

The major components of the PCU:
• An HP oil pump which boosts PGB oil pressure for blade angle control
• A governor which controls blade angle for constant-speed propeller operation. During beta/reverse operation, propeller blade angle is controlled by the PCU according to power lever position
• A servo piston assembly which controls operation of the pitch change mechanism in the propeller hub
• Two feather valves: one that is mechanically operated by the condition lever, and one that is electrically
opened by the PUMP switch or autocoarsen system 

 

 

 

Blade angle change is accomplished by using HP oil pressure as there are no springs or counterweights opposing oil pressure. Aerodynamic forces on the blade disc maintain a continuous decreasing pitch force. A loss of HP oil pressure or a blade angle change not synchronized with the PCU immediately results in a fixed-pitch propeller via mechanical
locking of the pitch-lock screw.

An electric feathering pump externally mounted on the PGB housing provides an alternate source of HP oil pressure for propeller feathering. An emergency reservoir located in the PGB sump provides oil exclusively for use by the electric feathering pump. The feathering pump is supplied from it's respective Battery Bus and is controlled by the PUMP switch located on the overhead panel. An overspeed governor driven separately by the PGB serves as a backup to the PCU governor to limit propeller overspeeding.

 

Propeller Operation

During normal constant speed governing, the constant speed governor maintains the propeller rpm set by the condition lever within a 1150-1396 rpm range by modulating blade angle in response to power and airspeed variations.During flight as power and airspeed progressively decrease, the PCU continually reduces blade angle in order to maintain the selected propeller rpm. When the blade angle decreases to about 17°, the PCU maintains a constant blade angle according to PLA, and no longer maintains the propeller rpm set by the condition lever. This manual control of blade angle, extending from 40° PLA to FLT IDLE, provides additional overspeed protection and control of propeller drag (during periods of low power and airspeed) by limiting low blade angle. This mode of operation becomes apparent during final approach as propeller rpm decreases below that selected by the condition lever. If airspeed is sufficiently increased (windmill action increased), then the PCU resumes normal governing at the set rpm. On Ground After landing, the power levers can be selected to GRD IDLE for deceleration. Each engine has independent reverse capability. In this position, the propeller develops significant drag due to low blade angle. If the power lever is moved from GRD IDLE towards REV the blade angle progressively decreases to maximum reverse blade angle. Below flight idle, the power lever controls propeller blade
angle through the PCU for taxi and reverse thrust control. Ground beta mode is indicated by illumination of the green BETA light on the flight status panel. During ground beta operation, the DECU maintains 1040 propeller rpm, and increases to 1200 rpm as blade angle decreases to -10° at maximum reverse. 

Feathering 

Prop feathering is accomplished by electrically or mechanically opening a feather valve inside the PCU. The propeller can be feathered by:
• Moving the condition lever to FEATHER (which opens the mechanical feather valve)
• Selecting the related PUMP switch to MAN FEATHER (which electrically opens the feather solenoid valve and activates the electric feather pump)
• The autocoarsen system (which electrically opens the feather solenoid valve)
 

There are yellow alignment marks are painted on the propeller spinner and leading edge of each propeller blade root. During preflight inspection, these marks must be checked that they align with each other to ensure that the propeller can fully feather.


Propeller Oil System

The PGB oil system is completely independent of the engine oil system and supplies oil to the PGB, PCU, and oil-to-fuel heater. The PGB sump serves as an oil tank and is equipped with an oil filler port and sight gage on the lower right side of the PGB for visual oil level checks. A one quart reservoir within the PGB sump is accessible only by the electric feathering pump for propeller feathering. Oil drawn from the PGB sump by the lubrication pump is directed through an oil-to-fuel heater and through an oil-to-air cooler located on the underside of the engine nacelle. The oil is filtered and monitored by temperature and pressure sensors which provide signals to the PROP OIL indicator. Low pressure alerting is activated at
one of two thresholds depending on the position of the power lever: less than seven psi when AFT of flight idle; less than 25 psi when forward of flight idle. On some aircraft, the low pressure thresholds are controlled according to flight/ground sensing by the WOW system. Low oil pressure triggers master warning alerting and flashing red ENG OIL PRESS on the CWP 


Propeller Control Failures
Loss of HP oil pressure to the pitch change mechanism (such as a beta/oil transfer tube failure) results in a propeller
rpm increase of approximately 30 rpm (the equivalent of one degree blade angle decrease). In such cases, blade angle becomes fixed pitch via the pitch-lock screw within the pitch change mechanism. The condition lever has no effect on propeller rpm and the propeller will not feather by any means. Consequently, a change in power or airspeed will cause a corresponding propeller rpm change. If excessive power or airspeed causes propeller rpm to reach approximately
1573, then the DECU recirculates engine fuel to prevent further Np overspeed.PCU Governor Failure A PGB drive failure to the PCU results in an immediate propeller overspeed (the constant speed governor responds to a detected underspeed). The overspeed governor, independently driven by the PGB, activates at approximately 1467 propeller rpm by dumping oil from the constant speed governor. Propeller rpm remains at approximately 1467 independent of the condition lever if
sufficient power or airspeed is present. Propeller feathering via the propeller governor remains available as this
function does not require speed sensing. 

Autocoarsen System 

The autocoarsen system is an automatic drag reduction system that is normally armed during takeoff and landing. The autocoarsen system is supplied from the Left Essential Bus and controlled by the AUTO COARSEN switch located on the PROPELLER panel. The system monitors various engine parameters in order to detect an engine failure. Depending on engine power, the system is armed in either a high or low power mode. The current mode is identified by illumination of the green AUTOCOARS HIGH or AUTOCOARS LOW light on the flight status panel. Autocoarsening is activated when any two of the monitored parameters for the existing mode decrease below established thresholds. When activated, full propeller feathering is accomplished automatically.

Low Power Mode

The low power mode is armed and the AUTOCOARS LOW light illuminates on the flight status panel when all of the
following conditions exist:
• One or both PLA <64°
• Ng >55% on both engines
• Starter/generator rpm >60% on both engines

Autocoarsen is activated if all of the following conditions exist:
• Ng <55% on the failed engine
• Starter/generator rpm <60% on the failed engine
• Ng >55% on the good engine

If a starter/generator failure occurs due to a sheared driveshaft, the AUTOCOARS LOW arm light will not illuminate. However, autocoarsening is activated if the  remaining low mode parameters decrease below established
thresholds. 

Uncoarsening occurs when:

• P3 increases >120 psi on the failed engine and
• Torque increases >50% on the failed engine
• Uncoarsening also occurs if the AUTOCOARSEN switch is selected OFF

High Power Mode

The high power mode is armed and the autocoarsen high light illuminates on the flight status panel if all of the following conditions exist on both engines:
• PLA>64°
• Torque >50%
• P3>120psi
 

Autocoarsen is activated if all of the following conditions exist:
• PLA >64° on both engines
• Torque >50% on the good engine
• Torque <50% on the failed engine
• Torque differential >25% between engines

• P3 <120 psi on the failed engine
 

As a backup feature, if a high power mode autocoarsen does not occur due to a system malfunction, then autocoarsen
occurs if all the following conditions exist:
• Ng <55% on the failed engine
• Starter/generator rpm <60% on the failed engine
• Ng >55% on the good engine
Uncoarsening occurs if:
• Torque increases >50% on the failed engine and
• P3 increases >120 psi on the failed engine 

•Uncoarsening also occurs if the AUTOCOARSEN switch is
selected OFF.

Normal computer functioning prevents simultaneous feathering of both propellers, and a lockout relay prevents simultaneous propeller feathering due to system malfunctions caused by lightning strikes or computer failure. Autocoarsen system malfunctions are indicated by master caution alerting with flashing amber AUTOCOARSEN light on the CWP and illumination of the following lights on the flight status panel:
• AUTO COARS LOW at high power
• AUTO COARS HIGH at low power
• AUTO COARS LOW and HIGH simultaneously