Panel
Position
Beacon
Starter
Battery
+ 60- 60
-30 +300
A M P
Ammeter
Avionics relay
(Optional Avionics)
Trim
Instr
Lndg Lt
Radio
Xpdr
Clutch
Alternator switch
Avionics master switch
Starter relayBattery relay
Left magnetos
ADVRET
Right magnetos ADV
Battery switch
Clutch actuator (internal
limit switches shown in
full disengage position)
Alternator control unit
Mag switch
Off
Both
Starting vibrator
Alternator
+
F1F2
Starter switch
M/R gearbox
press switch
Release
Hold
Engage
Clutch switch
Figure 4-24. An electrical system schematic like this sample is included in most POHs. Notice that the various bus bar accessories are
protected by circuit breakers. However, ensure that all electrical equipment is turned off before starting the engine. This protects sensitive
components, particularly the radios, from damage that may be caused by random voltages generated during the starting process.
a heat source, such as an exhaust manifold, before it enters
the carburetor. [Figure 4-23] Refer to the RFM (see Chapter
5, Rotorcraft Flight Manual) for the specific procedure as to
when and how to apply carburetor heat.
Fuel Injection
In a fuel injection system, fuel and air are metered at the fuel
control unit but are not mixed. The fuel is injected directly
into the intake port of the cylinder where it is mixed with
the air just before entering the cylinder. This system ensures
a more even fuel distribution between cylinders and better
vaporization, which in turn promotes more efficient use of
fuel. Also, the fuel injection system eliminates the problem
of carburetor icing and the need for a carburetor heat system.
Electrical Systems
The electrical systems, in most helicopters, reflect the
increased use of sophisticated avionics and other electrical
accessories. [Figure 4-24] More and more operations in
today’s flight environment are dependent on the aircraft’s
electrical system; however, all helicopters can be safely flown
without any electrical power in the event of an electrical
malfunction or emergency.
Helicopters have either a 14- or 28-volt, direct-current
electrical system. On small, piston powered helicopters,
electrical energy is supplied by an engine-driven alternator
by means of a belt and pulley system similar to that of an
automobile. These alternators have advantages over older-
style generators as they are lighter in weight, require lower
maintenance, and maintain a uniform electrical output even
at low engine rpm. (As a reminder, think of volts or voltage
as the measure of electrical pressure in the system, analogous
to pounds per square inch in water systems. Amperes is the
measure of electrical quantity in the system or available. For
example, a 100-amp alternator would be analogous to a 100
gallon per hour water pump.)
Turbine-powered helicopters use a starter/generator system.
The starter/generator is permanently coupled to the accessory
gearbox. When starting the engine, electrical power from the
battery is supplied to the starter/generator, which turns the
engine over. Once the engine is running, the starter/generator
is driven by the engine and then functions as a generator.
Current from the alternator or generator is delivered through a
voltage regulator to a bus bar. The voltage regulator maintains
the constant voltage required by the electrical system, by
regulating the output of the alternator or generator. An over-
voltage control may be incorporated to prevent excessive
voltage, which may damage the electrical components. The
bus bar serves to distribute the current to the various electrical
components of the helicopter.
A battery is used mainly for starting the engine. In addition,
it permits limited operation of electrical components, such
as radios and lights, without the engine running. The battery
is also a valuable source of standby or emergency electrical
power in the event of alternator or generator failure.
An ammeter (or load meter) is used to monitor the electrical
current within the system. The ammeter reflects current
flowing to and from the battery. A charging ammeter
indicates that the battery is being charged. This is normal
after an engine start since the battery power used in starting
is being replaced. After the battery is charged, the ammeter
should stabilize near zero since the alternator or generator is
supplying the electrical needs of the system.
An ammeter showing a discharge means the electrical load
is exceeding the output of the alternator or generator, and the
battery is helping to supply electrical power. This may mean
the alternator or generator is malfunctioning, or the electrical
load is excessive. An ammeter displays the load placed on the
alternator or generator by the electrical equipment. The RFM
(see page 5-1) for a particular helicopter shows the normal
load to expect. Loss of the alternator or generator causes the
load meter to indicate zero.
Electrical switches are used to select electrical components.
Power may be supplied directly to the component or to a
relay, which in turn provides power to the component. Relays
are used when high current and/or heavy electrical cables are
required for a particular component, which may exceed the
capacity of the switch.
Circuit breakers or fuses are used to protect various electrical
components from overload. A circuit breaker pops out when
its respective component is overloaded. The circuit breaker
may be reset by pushing it back in, unless a short or the
overload still exists. In this case, the circuit breaker continues
to pop, indicating an electrical malfunction. A fuse simply
burns out when it is overloaded and needs to be replaced.
Manufacturers usually provide a holder for spare fuses in the
event one has to be replaced in flight. Caution lights on the
instrument panel may be installed to show the malfunction
of an electrical component.
Hydraulics
Most helicopters, other than smaller piston-powered
helicopters, incorporate the use of hydraulic actuators to
overcome high control forces. [Figure 4-25] A typical
hydraulic system consists of actuators, also called servos,
on each flight control, a pump which is usually driven by
the main rotor transmission and a reservoir to store the
hydraulic fluid. Some helicopters have accumulators located
on the pressure side of the hydraulic system. This allows for
a continuous fluid pressure into the system. A switch in the
cockpit can turn the system off, although it is left on under
normal conditions. When the pilot places the hydraulic
switch/circuit breaker into the on position, the electrical
power is being removed from the solenoid valve allowing
Servo actuator,
lateral cyclic
Servo actuator,
longitudinal cyclic
Servo actuator,
collective
Rotor control
Pressure regulator valve
Pump
Solenoid valve
Pilot input
Filter
Quick disconnects
RESERVOIR
Vent
Scupper drain
Pressure
Return
Figure 4-25. A typical hydraulic system for helicopters in the light to medium range.
hydraulic fluid to enter the system. When the switch/circuit
breaker is put in the off position, the solenoid valve is now
de-energized and closes, which then allows the pilot to
maintain control of the helicopter with the hydraulic fluid in
the actuators. This is known as a failsafe system. If helicopter
electrical power is lost in flight, the pilot is still able to
maintain control of the hydraulic system. A pressure indicator
in the cockpit may also be installed to monitor the system.
When making a control input, the servo is activated and
provides an assisting force to move the respective flight
control, thus reducing the force the pilot must provide. These
boosted flight controls ease pilot workload and fatigue. In
the event of hydraulic system failure, a pilot is still able to
control the helicopter, but the control forces are very heavy.
In those helicopters in which the control forces are so high that
they cannot be moved without hydraulic assistance, two or
more independent hydraulic systems may be installed. Some
helicopters are designed to use their hydraulic accumulators
to store hydraulic pressure for an emergency, allowing for
uninterrupted use of the controls for a short period of time
following a hydraulic pump failure. This gives you enough
time to land the helicopter with normal control.
Stability Augmentations Systems
Some helicopters incorporate a stability augmentation system
(SAS) to help stabilize the helicopter in flight and in a hover.
The original purpose and design allowed decreased pilot
workload and lessened fatigue. It allowed pilots to place an
aircraft at a set attitude to accomplish other tasks or simply
stabilize the aircraft for long cross-country flights.
Force Trim
Force trim was a passive system that simply held the cyclic
in a position that gave a control force to transitioning airplane
pilots who had become accustomed to such control forces.
The system uses a magnetic clutch and springs to hold the
cyclic control in the position where it was released. The
system does not use sensor-based data to make corrections,
but rather is used by the pilot to “hold” the cyclic in a desired
position. The most basic versions only apply to the cyclic
requiring the pilot to continue power and tail rotor inputs.
With the force trim on or in use, the pilot can override the
system by disengaging the system through the use of a force
trim release button or, with greater resistance, can physically
manipulate the controls. Some recent basic systems are
referred to as attitude retention systems.
Active Augmentation Systems
So-called actual augmentation systems use electric
actuators that provide input to the hydraulic servos. These
servos receive control commands from a computer that
senses external environmental inputs, such as wind and
turbulence. SAS complexity varies by manufacturer but can
be as sophisticated as providing three-axis stability. That is,
computer-based inputs adjust attitude, power and aircraft
trim for a more stabilized flight.
Once engaged by the pilot, these actual systems use a
multitude of sensors, from stabilized gyros to electro-
mechanical actuators, which provide instantaneous inputs
to all flight controls without pilot assistance. As with all
SASs, they may be overridden or disconnected by the pilot
at any time. Helicopters with complex Automatic Flight
Control Systems (AFCS) and autopilots normally have
a trim switch referred to as “beeper trim.” This switch is
used when minor changes to the trim setting are desired.
Stability augmentation systems reduce pilot workload by
improving basic aircraft control harmony and decreasing
disturbances. These systems are very useful when the pilot
is required to perform other duties, such as sling loading and
search-and-rescue operations. Other inputs such as heading,
speed, altitude, and navigation information may be supplied
to the computer to form a complete autopilot system.
Autopilot
Helicopter autopilot systems are similar to stability
augmentation systems, but they have additional features. An
autopilot can actually fly the helicopter and perform certain
functions selected by the pilot. These functions depend on
the type of autopilot and systems installed in the helicopter.
The most common functions are altitude and heading hold.
Some more advanced systems include a vertical speed or
indicated airspeed (IAS) hold mode, where a constant rate
of climb/descent or IAS is maintained by the autopilot. Some
autopilots have navigation capabilities, such as very high
frequency (VHF) OmniRange Navigation System (VOR),
Instrument Landing System (ILS), and global positioning
system (GPS) intercept and tracking, which is especially
useful in instrument flight rules (IFR) conditions. This is
referred to as a coupled system. An additional component,
called a flight director (FD), may also be installed. The FD
provides visual guidance cues to the pilot to fly selected
lateral and vertical modes of operation. The most advanced
autopilots can fly an instrument approach to a hover without
any additional pilot input once the initial functions have
been selected.
The autopilot system consists of electric actuators or servos
connected to the flight controls. The number and location of
these servos depends on the type of system installed. A two-
axis autopilot controls the helicopter in pitch and roll; one
servo controls fore and aft cyclic, and another controls left
and right cyclic. A three-axis autopilot has an additional servo
connected to the antitorque pedals and controls the helicopter
in yaw. A four-axis system uses a fourth servo which controls
the collective. These servos move the respective flight
controls when they receive control commands from a central
computer. This computer receives data input from the flight
instruments for attitude reference and from the navigation
equipment for navigation and tracking reference. An autopilot
has a control panel in the cockpit that allows the pilot to
select the desired functions, as well as engage the autopilot.
For safety purposes, an automatic disengagement feature
is usually included which automatically disconnects the
autopilot in heavy turbulence or when extreme flight attitudes
are reached. Even though all autopilots can be overridden
by the pilot, there is also an autopilot disengagement button
located on the cyclic or collective which allows pilots to
completely disengage the autopilot without removing their
hands from the controls. Because autopilot systems and
installations differ from one helicopter to another, it is very
important to refer to the autopilot operating procedures
located in the RFM.
Environmental Systems
Heating and cooling the helicopter cabin can be accomplished
in different ways. The simplest form of cooling is by ram air.
Air ducts in the front or sides of the helicopter are opened or
closed by the pilot to let ram air into the cabin. This system
is limited as it requires forward airspeed to provide airflow
and also depends on the temperature of the outside air. Air
conditioning provides better cooling, but it is more complex
and weighs more than a ram air system.
One of the simplest methods of cooling a helicopter is to
remove the doors allowing air to flow through the cockpit
and engine compartments. Care must be taken to store the
doors properly, whether in a designed door-holding rack in
a hangar, or if it is necessary to carry them on the flight, in
the helicopter. When storing the doors, care must be taken
to not scratch the windows. Special attention should be paid
to ensuring that all seat belt cushions and any other loose
items are stored away to prevent ingestion into the main or
tail rotor. When reattaching the doors, proper care must be
taken to ensure that they are fully secured and closed.
Air conditioners or heat exchanges can be fitted to the
helicopter as well. They operate by drawing bleed air from
the compressor, passing it through the heart exchanger and
then releasing it into the cabin. As the compressed air is
released, the expansion absorbs heat and cools the cabin. The
disadvantage of this type of system is that power is required
to compress the air or gas for the cooling function, thus
robbing the engine of some of its capability. Some systems
are restricted from use during takeoff and landings.
Piston-powered helicopters use a heat exchanger shroud
around the exhaust manifold to provide cabin heat. Outside
air is piped to the shroud and the hot exhaust manifold heats
the air, which is then blown into the cockpit. This warm air
is heated by the exhaust manifold but is not exhaust gas.
Turbine helicopters use a bleed air system for heat. Bleed air
is hot, compressed, discharge air from the engine compressor.
Hot air is ducted from the compressor to the bleed air heater
assembly where it is combined with ambient air through
and induction port mounted to the fuselage. The amount of
heat delivered to the helicopter cabin is regulated by a pilot-
controlled bleed air mixing valve.
Anti-Icing Systems
Anti-icing is the process of protecting against the formation
of frozen contaminant, snow, ice, or slush on a surface.
Engine Anti-Ice
The anti-icing system found on most turbine-powered
helicopters uses engine bleed air. Bleed air in turbine engines
is compressed air taken from within the engine, after the
compressor stage(s) and before the fuel is injected in the
burners. The bleed air flows through the inlet guide vanes
and to the inlet itself to prevent ice formation on the hollow
vanes. A pilot-controlled, electrically operated valve on the
compressor controls the air flow. Engine anti-ice systems
should be on prior to entry into icing conditions and remain
on until exiting those conditions. Use of the engine anti-ice
system should always be in accordance with the proper RFM.
Airframe Anti-Ice
Airframe and rotor anti-icing may be found on some larger
helicopters, but it is not common due to the complexity,
expense, and weight of such systems. The leading edges of
rotors may be heated with bleed air or electrical elements to
prevent ice formation. Balance and control problems might
arise if ice is allowed to form unevenly on the blades. Research
is being done on lightweight ice-phobic (anti-icing) materials
or coatings. These materials placed in strategic areas could
significantly reduce ice formation and improve performance.
The pitot tube on a helicopter is very susceptible to ice and
moisture buildup as well. To prevent this, they are usually
equipped with a heating system that uses an electrical element
to heat the tube.
Deicing
Deicing is the process of removing frozen contaminant,
snow, ice, and/or slush from a surface. Deicing of the
helicopter fuselage and rotor blades is critical prior to starting.
Helicopters that are unsheltered by hangars are subject
to frost, snow, freezing drizzle, and freezing rain that can
cause icing of rotor blades and fuselages, rendering them
unflyable until cleaned. Asymmetrical shedding of ice from
the blades can lead to component failure, and shedding ice
can be dangerous as it may hit any structures or people that
are around the helicopter. The tail rotor is very vulnerable to
shedding ice damage. Thorough preflight checks should be
made before starting the rotor blades. If any ice was removed
prior to starting, ensure that the flight controls move freely.
While in flight, for those helicopters that have them, deicing
systems should be activated immediately after entry into an
icing condition.
Chapter Summary
This chapter discussed all of the common components,
sections, and systems of the helicopter. The chapter also
explained how each of them work with one another to make
flight possible.