Figure 12-7. Propeller blade angle.
As the airspeed increases after lift-off, the load on the engine is lightened because of the small blade angle. The governor senses this
and increases the blade angle slightly. Again, the higher blade angle, with the higher speed, keeps the blade AOA with respect to the
relative wind small and efficient.
For climb after takeoff, the power output of the engine is reduced to climb power by decreasing the manifold pressure and increasing
the blade angle to lower engine rpm. At the higher (climb) airspeed and the higher blade angle, the propeller is handling a greater
mass of air per second at a lower slipstream velocity. This reduction in power is offset by the increase in propeller efficiency. The
blade AOA is again kept small by the increase in the blade angle with an increase in airspeed.
At cruising altitude, when the airplane is in level flight, airspeed increases, and less power is required. Consequently, the pilot uses
the throttle to reduce manifold pressure and uses the propeller control to reduce engine rpm. The higher airspeed and higher blade
angle enable the propeller to handle a still greater mass of air per second at still smaller slipstream velocity. At normal cruising
speeds, propeller efficiency is at or near maximum efficiency.
Blade Angle Control
Once the rpm settings for the propeller are selected, the propeller governor automatically adjusts the blade angle to maintain the
selected rpm. It does this by using oil pressure. Generally, the oil pressure used for pitch change comes directly from the engine
lubricating system. When a governor is employed, engine oil is used and the oil pressure is usually boosted by a pump that is
integrated with the governor. The higher pressure provides a quicker blade angle change. The rpm at which the propeller is to operate
is adjusted in the governor head. The pilot changes this setting by changing the position of the governor rack through the flight deck
propeller control.
On some constant-speed propellers, changes in pitch are obtained by the use of an inherent centrifugal twisting moment of the blades
that tends to flatten the blades toward low pitch and oil pressure applied to a hydraulic piston connected to the propeller blades which
moves them toward high pitch. Another type of constant-speed propeller uses counterweights attached to the blade shanks in the hub.
Governor oil pressure and the blade twisting moment move the blades toward the low pitch position, and centrifugal force acting on
the counterweights moves them (and the blades) toward the high pitch position. In the first case above, governor oil pressure moves
the blades towards high pitch and in the second case, governor oil pressure and the blade twisting moment move the blades toward
low pitch. A loss of governor oil pressure, therefore, affects each differently.
Governing Range
The blade angle range for constant-speed propellers varies from about 11.5° to 40°. The higher the speed of the airplane, the greater
the blade angle range. [Figure 12-8]
Figure 12-8. Blade angle range (values are approximate).
The range of possible blade angles between high and low blade angle pitch stops define the propeller’s governing range. As long as
the propeller's blades operate within the governing range and not against either pitch stop, a constant engine rpm is maintained.
However, once the propeller blades reach their pitch-stop limit, the engine rpm increases or decreases with changes in airspeed and
propeller load similar to a fixed-pitch propeller. For example, once a specific rpm is selected, if the airspeed decreases enough, the
propeller blades reduce pitch in an attempt to maintain the selected rpm until they contact their low pitch stops. From that point, any
further reduction in airspeed causes the engine rpm to decrease. Conversely, if the airspeed increases, the pitch angle of the propeller
blades increase until the high pitch stop is reached. The engine rpm then begins to increase.
Constant-Speed Propeller Operation
The engine is started with the propeller control in the low pitch/high rpm position. This position reduces the load or drag of the
propeller and the result is easier starting and warm-up of the engine. During warm-up, the propeller blade changing mechanism is
operated slowly and smoothly through a full cycle. This is done by moving the propeller control (with the manifold pressure set to
produce about 1,600 rpm) to the high pitch/low rpm position, allowing the rpm to stabilize, and then moving the propeller control
back to the low pitch takeoff position. This is done for two reasons: to determine whether the system is operating correctly and to
circulate fresh warm oil through the propeller governor system. Remember the oil has been trapped in the propeller cylinder since the
last time the engine was shut down. There is a certain amount of leakage from the propeller cylinder, and the oil tends to congeal,
especially if the outside air temperature is low. Consequently, if the propeller is not exercised before takeoff, there is a possibility that
the engine may over-speed on takeoff.
An airplane equipped with a constant-speed propeller has better takeoff performance than a similarly powered airplane equipped with
a fixed-pitch propeller. This is because with a constant-speed propeller, an airplane can develop its maximum rated horsepower (red
line on the tachometer) while motionless. An airplane with a fixed-pitch propeller, on the other hand, needs to accelerate down the
runway to increase airspeed and aerodynamically unload the propeller so that rpm and horsepower can steadily build up to thei r
maximum. With a constant-speed propeller, the tachometer reading should come up to within 40 rpm of the red line as soon as full
power is applied and remain there for the entire takeoff. Excessive manifold pressure raises the cylinder combustion pressures,
resulting in high stresses within the engine. Excessive pressure also produces high-engine temperatures. A combination of high
manifold pressure and low rpm can induce damaging detonation. In order to avoid these situations, the following sequence should be
followed when making power changes.
⦁ When increasing power, increase the rpm first and then the manifold pressure
⦁ When decreasing power, decrease the manifold pressure first and then decrease the rpm
The cruise power charts in the AFM/POH should be consulted when selecting cruise power settings. Whatever the combinations of
rpm and manifold pressure listed in these charts —they have been flight tested and approved by engineers for the respective airframe
and engine manufacturer. Therefore, if there are power settings, such as 2,100 rpm and 24 inches manifold pressure in the power
chart, they are approved for use. With a constant-speed propeller, a power descent can be made without over-speeding the engine.
The system compensates for the increased airspeed of the descent by increasing the propeller blade angles. If the descent is too rapid
or is being made from a high altitude, the maximum blade angle limit of the blades is not sufficient to hold the rpm constant. When
this occurs, the rpm is responsive to any change in throttle setting.
Although the governor responds quickly to any change in throttle setting, a sudden and large increase in the throttle setting causes a
momentary over-speeding of the engine until the blades become adjusted to absorb the increased power. If an emergency demanding
full power should arise during approach, the sudden advancing of the throttle causes momentary over-speeding of the engine beyond
the rpm for which the governor is adjusted.
Some important points to remember concerning constant speed propeller operation are:
⦁ The red line on the tachometer not only indicates maximum allowable rpm; it also indicates the rpm
required to obtain the engine’s rated horsepower.
⦁ A momentary propeller overs-peed may occur when the throttle is advanced rapidly for takeoff. This is
sually not serious if the rated rpm is not exceeded by 10 percent for more than 3 seconds.
⦁ The green arc on the tachometer indicates the normal operating range. When developing power in this
ange, the engine drives the propeller. Below the green arc, however, it is usually the windmilling propeller
that powers the engine. Prolonged operation below the green arc can be detrimental to the engine. On
takeoffs from low elevation airports, the manifold pressure in inches of mercury may exceed the rpm. This
is normal in most cases, but the pilot should always consult the AFM/POH for limitations.
⦁ All power changes should be made smoothly and slowly to avoid over-boosting and/or over-speeding.
Turbocharging
The turbocharged engine allows the pilot to maintain sufficient cruise power at high altitudes where there is less drag, which means
faster true airspeeds and increased range with fuel economy. At the same time, the powerplant has flexibility and can be flown at a
low altitude without the increased fuel consumption of a turbine engine. When attached to the standard powerplant, the turbocharger
does not take any horsepower from the engine to operate; it is relatively simple mechanically, and some models can pressurize the
cabin as well.
The turbocharger is an exhaust-driven device that raises the pressure and density of the induction air delivered to the engine. It
consists of two separate components: a compressor and a turbine connected by a common shaft. The compressor supplies pressurized
air to the engine for high-altitude operation. The compressor and its housing are between the ambient air intake and the induction air
manifold. The turbine and its housing are part of the exhaust system and utilize the flow of exhaust gases to drive the compressor.
[Figure 12-9]
Figure 12-9. Turbocharging system.
The turbine has the capability of producing manifold pressure in excess of the maximum allowable for the particular engine. In order
not to exceed the maximum allowable manifold pressure, a bypass or waste gate is used so that some of the exhaust is diverted
overboard before it passes through the turbine.
The position of the waste gate regulates the output of the turbine and therefore, the compressed air available to the engine. When the
waste gate is closed, all of the exhaust gases pass through and drive the turbine. As the waste gate opens, some of the exhaust gases
are routed around the turbine through the exhaust bypass and overboard through the exhaust pipe.
The waste gate actuator is a spring-loaded piston operated by engine oil pressure. The actuator, which adjusts the waste gate position,
is connected to the waste gate by a mechanical linkage.
The control center of the turbocharger system is the pressure controller. This device simplifies turbocharging to one control: the
throttle. Once the desired manifold pressure is set, virtually no throttle adjustment is required with changes in altitude. The controller
senses compressor discharge requirements for various altitudes and controls the oil pressure to the waste gate actuator, which adjusts
the waste gate accordingly. Thus the turbocharger will maintain the manifold pressure called for by the throttle setting.
Ground Boosting Versus Altitude Turbocharging
Altitude turbocharging (sometimes called “normalizing”) is accomplished by using a turbocharger that maintains maximum allowable
sea level manifold pressure (normally 29 –30 "Hg) up to a certain altitude. This altitude is specified by the airplane manufacturer and
is referred to as the airplane’s critical altitude. Above the critical altitude, the manifold pressure decreases as additiona l altitude is
gained. Ground boosting, on the other hand, is an application of turbocharging where more than the standard 29 inches of manifold
pressure is used in flight. In various airplanes using ground boosting, takeoff manifold pressures may go as high as 45 "Hg.
Although a sea-level manifold pressure setting and maximum rpm can be maintained up to the critical altitude, the engine may not be
developing sea-level power. Because the turbocharged induction air is heated by compression, lower induction air density causes a
loss of engine power. Maintaining the equivalent horsepower output requires a somewhat higher manifold pressure at a given altitude
than if the induction air were not compressed and heated by turbocharging. If, on the other hand, the system incorporates an automatic
density controller, which automatically positions the waste gate so as to maintain constant air density to the engine, a near equivalent
to sea-level horsepower output results.
Operating Characteristics
First and foremost, all movements of the power controls on turbocharged engines should be slow and smooth. Aggressive or abrupt
throttle movements increase the possibility of over-boosting. Carefully monitor engine indications when making power changes.
When the waste gate is open, the turbocharged engine reacts the same as a normally aspirated engine when the rpm is varied. That is,
when the rpm is increased, the manifold pressure decreases slightly. When the engine rpm is decreased, the manifold pressure
increases slightly. However, when the waste gate is closed, manifold pressure variation with engine rpm is just the opposite of the
normally aspirated engine. An increase in engine rpm results in an increase in manifold pressure, and a decrease in engine rpm results
in a decrease in manifold pressure.
Above the critical altitude, where the waste gate is closed, any change in airspeed results in a corresponding change in manifold
pressure. This is true because the increase in ram air pressure with an increase in airspeed is magnified by the compressor resulting in
an increase in manifold pressure. The increase in manifold pressure creates a higher mass flow through the engine, causing higher
turbine speeds and thus further increasing manifold pressure.
When running at high altitudes, aviation gasoline tends to vaporize prior to reaching the cylinder. If this occurs in the portion of the
fuel system between the fuel tank and the engine-driven fuel pump, an auxiliary positive pressure pump may be needed in the tank.
Since engine-driven pumps pull fuel, they are easily vapor locked. A boost pump provides positive pressure, which pushes the fuel
and reduces the tendency to vaporize.
Heat Management
Turbocharged engines should be thoughtfully and carefully operated with continuous monitoring of pressures and temperatures.
There are two temperatures that are especially important —turbine inlet temperature (TIT) or, in some installations, exhaust gas
temperature (EGT) and cylinder head temperature. TIT or EGT limits are set to protect the elements in the hot section of the
turbocharger, while cylinder head temperature limits protect the engine’s internal parts.
Due to the heat of compression of the induction air, a turbocharged engine runs at higher operating temperatures than a non-
turbocharged engine. Because turbocharged engines operate at high altitudes, their environment is less efficient for cooling. At
altitude, the air is less dense and, therefore, cools less efficiently. Also, the less dense air causes the compressor to work harder.
Compressor turbine speeds can reach 80,000 –100,000 rpm, adding to the overall engine operating temperatures. Turbocharged
engines are also operated at higher power settings a greater portion of the time.
High heat is detrimental to piston engine operation. Its cumulative effects can lead to piston, ring, and cylinder head failure and place
thermal stress on other operating components. Excessive cylinder head temperature can lead to detonation, which in turn can cause
catastrophic engine failure. Turbocharged engines are especially heat sensitive. The key to turbocharger operation is effective heat
management.
Monitor the condition of a turbocharged engine with manifold pressure gauge, tachometer, exhaust gas temperature/turbine inle t
temperature gauge, and cylinder head temperature gauge. Manage the “heat system” with the throttle, propeller rpm, mixture, and
cowl flaps. At any given cruise power, the mixture is the most influential control over the exhaust gas/TIT. The throttle regulates total
fuel flow, but the mixture governs the fuel-to-air ratio. The mixture, therefore, controls temperature.
Exceeding temperature limits in an after-takeoff climb is usually not a problem since a full rich mixture cools with excess fuel. At
cruise, power is normally reduced and mixture adjusted accordingly. Under cruise conditions, monitor temperature limits closely
because that is when the temperatures are most likely to reach the maximum, even though the engine is producing less power.
Overheating in an en route climb, however, may require fully open cowl flaps and a higher airspeed.
Since turbocharged engines operate hotter at altitude than normally aspirated engines, they are more prone to damage from cooling
stress. Gradual reductions in power and careful monitoring of temperatures are essential in the descent phase. Extending the landing
gear during the descent may help control the airspeed while maintaining a higher engine power setting. This allows the pilot to reduce
power in small increments which allows the engine to cool slowly. It may also be necessary to lean the mixture slightly to eliminate
roughness at the lower power settings.
Turbocharger Failure
Because of the high temperatures and pressures produced in the turbine exhaust system, any malfunction of the turbocharger should
be treated with extreme caution. In all cases of turbocharger operation, the manufacturer’s recommended procedures should be
followed. This is especially so in the case of turbocharger malfunction. However, in those instances where the manufacturer’s
procedures do not adequately describe the actions to be taken in the event of a turbocharger failure, the following procedures should
e used.
Over-Boost Condition
If an excessive rise in manifold pressure occurs during normal advancement of the throttle (possibly owing to faulty operation of the
waste gate):
⦁ Immediately retard the throttle smoothly to limit the manifold pressure below the maximum for the rpm
and mixture setting.
⦁ Operate the engine in such a manner as to avoid a further over-boost condition.
Low Manifold Pressure
Although this condition may be caused by a minor fault, it is quite possible that a serious exhaust leak has occurred creating a
potentially hazardous situation:
⦁ Shut down the engine in accordance with the recommended engine failure procedures, unless a greater
emergency exists that warrants continued engine operation.
⦁ If continuing to operate the engine, use the lowest power setting demanded by the situation and land as
soon as practicable.
It is very important to ensure that corrective maintenance is undertaken following any turbocharger malfunction.
Retractable Landing Gear
The primary benefits of being able to retract the landing gear are increased climb performance and higher cruise airspeeds due to a
decrease in drag after gear retraction. Retractable landing gear systems may be operated either hydraulically or electrically or may
employ a combination of the two systems. Warning indicators are provided in the flight deck to show the pilot when the wheels are
down and locked and when they are up and locked or if they are in intermediate positions. Systems for emergency operation are also
provided. Due to the complexity of a retractable landing gear system, the pilot should adhere to specific operating procedures and
should not exceed any operating limitations.
Landing Gear Systems
An electrical landing gear retraction system utilizes an electrically-driven motor for gear operation. The system is basically an
electrically-driven jack for raising and lowering the gear. When a switch in the flight deck is moved to the UP position, the electric
motor operates. Through a system of shafts, gears, adapters, an actuator screw, and a torque tube, a force is transmitted to the drag
strut linkages. Thus, the gear retracts and locks. Struts are also activated that open and close the gear doors. If the switch is moved to
the DOWN position, the motor reverses and the gear moves down and locks. Once activated, the gear motor continues to operate until
an up or down limit switch on the motor’s gearbox is tripped.
A hydraulic landing gear retraction system utilizes pressurized hydraulic fluid to actuate linkages to raise and lower the gear. When a
switch in the flight deck is moved to the UP position, hydraulic fluid is directed into the gear up line. The fluid flows through
sequenced valves and downlocks to the gear actuating cylinders. A similar process occurs during gear extension. The pump that
pressurizes the fluid in the system can be either engine-driven or electrically-powered. If an electrically-powered pump is used to
pressurize the fluid, the system is referred to as an electrohydraulic system. The system also incorporates a hydraulic reservoir to
contain excess fluid and to provide a means of determining system fluid level.
Regardless of its power source, the hydraulic pump is designed to operate within a specific range. When a sensor detects excessive
pressure, a relief valve within the pump opens, and hydraulic pressure is routed back to the reservoir. Another type of relief valve
prevents excessive pressure that may result from thermal expansion. Hydraulic pressure is also regulated by limit switches. Each gear
has two limits switches—one dedicated to extension and one dedicated to retraction. These switches de-energize the hydraulic pump
after the landing gear has completed its gear cycle. In the event of limit switch failure, a backup pressure relief valve activates to
relieve excess system pressure.
Controls and Position Indicators
Landing gear position is controlled by a switch on the flight deck panel. In most airplanes, the gear switch is shaped like a wheel in
order to facilitate positive identification and to differentiate it from other flight deck controls.
Landing gear position indicators vary with different make and model airplanes. Some types of landing gear position indicators utilize
a group of lights. One type consists of one green light to indicate when the landing gear is down and an amber light to indicate when
the gear is up. [Figure 12-10] Another type consists of a group of three green lights, which illuminate when the landing gear is down
and locked. [Figure 12-10] Still other systems incorporate a red or amber light to indicate when the gear is in transit or unsafe for
landing. [Figure 12-11] When the lights use a “press to test” feature, the bulbs are often interchangeable. Integrated electronic
displays may also indicate gear position on a portion of the screen without any dedicated lights.
Other types of landing gear position indicators consist of tab- type indicators with markings “UP” to indicate the gear is up and
locked, a display of red and white diagonal stripes to show when the gear is unlocked, or a silhouette of each gear to indicate when it
locks in the DOWN position.
Landing Gear Safety Devices
Most airplanes with a retractable landing gear have a gear warning horn that sounds when the airplane is configured for landing and
the landing gear is not down and locked. Normally, the horn is linked to the throttle or flap position and/or the airspeed indicator so
that when the airplane is below a certain airspeed, configuration, or power setting with the gear retracted, the warning horn sounds.
Accidental retraction of a landing gear may be prevented by such devices as mechanical downlocks, safety switches, and ground
locks. Mechanical downlocks are built-in components of a gear retraction system and are operated automatically by the gear
retraction system. To prevent accidental operation of the downlocks and inadvertent landing gear retraction while the airplane is on
the ground, electrically-operated safety switches are installed.