RECIPROCATING ENGINES
Fuel is delivered to the cylinders by either a carburetor
or fuel injection system.
CARBURETOR
In a carburetor system, air is mixed with vaporized fuel as
it passes through a venturi in the carburetor. The metered
fuel/air mixture is then delivered to the cylinder intake.
Carburetors are calibrated at sea level, and the correct
fuel-to-air mixture ratio is established at that altitude
with the mixture control set in the FULL RICH posi-
tion. However, as altitude increases, the density of air
entering the carburetor decreases while the density of
the fuel remains the same. This means that at higher
altitudes, the mixture becomes progressively richer. To
maintain the correct fuel/air mixture, you must be able
to adjust the amount of fuel that is mixed with the
incoming air. This is the function of the mixture con-
trol. This adjustment, often referred to as “leaning the
mixture,” varies from one aircraft to another. Refer to
the FAA-Approved Rotocraft Flight Manual (RFM) to
determine specific procedures for your helicopter. Note
that most manufacturers do not recommend leaning hel-
icopters in-flight.
Most mixture adjustments are required during changes of
altitude or during operations at airports with field eleva-
tions well above sea level. A mixture that is too rich can
result in engine roughness and reduced power. The rough-
ness normally is due to spark plug fouling from exces-
sive carbon buildup on the plugs. This occurs because
the excessively rich mixture lowers the temperature inside
the cylinder, inhibiting complete combustion of the fuel.
This condition may occur during the pretakeoff runup at
high elevation airports and during climbs or cruise flight
at high altitudes. Usually, you can correct the problem by
leaning the mixture according to RFM instructions.
If you fail to enrich the mixture during a descent from
high altitude, it normally becomes too lean. High
engine temperatures can cause excessive engine wear
or even failure. The best way to avoid this type of situ-
ation is to monitor the engine temperature gauges regu-
larly and follow the manufacturer’s guidelines for
maintaining the proper mixture.
CARBURETOR ICE
The effect of fuel vaporization and decreasing air pres-
sure in the venturi causes a sharp drop in temperature
in the carburetor. If the air is moist, the water vapor in
the air may condense. When the temperature in the car-
buretor is at or below freezing, carburetor ice may form
on internal surfaces, including the throttle valve.
[Figure 5-10] Because of the sudden cooling that takes
place in the carburetor, icing can occur even on warm
days with temperatures as high as 38°C (100°F) and
the humidity as low as 50 percent. However, it is more
likely to occur when temperatures are below 21°C
(70°F) and the relative humidity is above 80 percent.
The likelihood of icing increases as temperature
decreases down to 0°C (32°F), and as relative humidity
increases. Below freezing, the possibility of carburetor
icing decreases with decreasing temperatures.
Although carburetor ice can occur during any phase of
flight, it is particularly dangerous when you are using
reduced power, such as during a descent. You may not
notice it during the descent until you try to add power.
Indications of carburetor icing are a decrease in engine
r.p.m. or manifold pressure, the carburetor air tempera-
ture gauge indicating a temperature outside the safe
operating range, and engine roughness. Since changes
in r.p.m. or manifold pressure can occur for a number
of reasons, it is best to closely check the carburetor air
temperature gauge when in possible carburetor icing
conditions. Carburetor air temperature gauges are
marked with a yellow caution arc or green operating
arcs. You should refer to the FAA-Approved Rotorcraft
Flight Manual for the specific procedure as to when
and how to apply carburetor heat. However, in most
cases, you should keep the needle out of the yellow arc
or in the green arc. This is accomplished by using a car-
buretor heat system, which eliminates the ice by
To Engine
Incoming Air
Ice
Ice
Venturi
Fuel/Air□
Mixture
Ice
Figure 5-10. Carburetor ice reduces the size of the air pas-
sage to the engine. This restricts the flow of the fuel/air
mixture, and reduces power.
routing air across a heat source, such as an exhaust
manifold, before it enters the carburetor. [Figure 5-11].
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 distri-
bution in the cylinders and better vaporization, which
in turn, promotes more efficient use of fuel. Also, the
fuel injection system eliminates the problem of carbu-
retor icing and the need for a carburetor heat system.
TURBINE ENGINES
The fuel control system on the turbine engine is fairly
complex, as it monitors and adjusts many different
parameters on the engine. These adjustments are done
automatically and no action is required of the pilot
other than starting and shutting down. No mixture
adjustment is necessary, and operation is fairly simple
as far as the pilot is concerned. New generation fuel
controls incorporate the use of a full authority digital
engine control (FADEC) computer to control the
engine’s fuel requirements. The FADEC systems
increase efficiency, reduce engine wear, and also
reduce pilot workload. The FADEC usually incorpo-
rates back-up systems in the event of computer failure.
ELECTRICAL SYSTEMS
The electrical systems, in most helicopters, reflect the
increased use of sophisticated avionics and other elec-
trical accessories. More and more operations in today’s
flight environment are dependent on the aircraft’s elec-
trical 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-cur-
rent electrical system. On small, piston powered
helicopters, electrical energy is supplied by an engine-
driven alternator. 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 r.p.m.
[Figure 5-12]
Turbine powered helicopters use a starter/generator
system. The starter/generator is permanently coupled
to the engine gearbox. When starting the engine, elec-
trical 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 is then used 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
Avionic□
Bus□
Bar
Avionic□
Bus Avionics Relay
On
Off
Avionics Master□
Switch
Lights
Panel□
Position□
Beacon
Trim□
Instr□
Lndg Lt□
Radio□
Xpdr□
Clutch
Ammeter
Mag Switch
Left□
Magneto
Right□
Magneto
Starter□
Relay Engine□
Starter
Bus Bar
Battery□
Relay
Battery
Battery□
Switch
Starter□
Switch
M/R Gearbox□
Press Switch
Release
Hold
Engage
Clutch□
Switch
Alternator
Alternator□
Switch
Alternator□
Control Unit
Clutch Actuator□
(Internal Limit Switches□
Shown in Full□
Disengage Position)
24V
– +
F1F2
–
+
Starting□
VibratorR
Both
L L
Off Ret
Adv
Adv
(Optional Avionics)
Figure 5-12. An electrical system scematic like this sample is
included in most POHs. Notice that the various bus bar
accessories are protected by circuit breakers. However, you
should still make sure all electrical equipment is turned off
before you start the engine. This protects sensitive compo-
nents, particularly the radios, from damage which may be
caused by random voltages generated during the starting
process.
Filter To Carb
Carb Heat□
Collector
Manifold□
Pipe
Door
Filter To Carb
Carb Heat□
Collector
Manifold□
Pipe
Door
Heated Air
Carb Heat Off
Carb Heat On
Figure 5-11. When you turn the carburetor heat ON, normal
air flow is blocked, and heated air from an alternate source
flows through the filter to the carburetor.
incorporated to prevent excessive voltage, which may
damage the electrical components. The bus bar serves
to distribute the current to the various electrical com-
ponents of the helicopter.
A battery is mainly used 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 loadmeter 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 sta-
bilize near zero since the alternator or generator is
supplying the electrical needs of the system. A dis-
charging ammeter 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 mal-
functioning, or the electrical load is excessive. A
loadmeter displays the load placed on the alternator
or generator by the electrical equipment. The RFM
for a particular helicopter shows the normal load to
expect. Loss of the alternator or generator causes the
loadmeter to indicate zero.
Electrical switches are used to select electrical compo-
nents. 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 com-
ponent, 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 elec-
trical 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 instru-
ment 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 5-13] A typi-
cal hydraulic system consists of actuators, also called
Pressure
Return
Supply
Scupper□
Drain
Vent Reservoir
Pump
Pressure□
Regulator□
Valve
Quick□
Disconnects
Filter
Solenoid□
Valve
Servo□
Actuator,□
Lateral□
Cyclic
Servo□
Actuator,□
Fore and□
Aft□
Cyclic
Servo□
Actuator,□
Collective
Pilot□
Input
Rotor□
Control
Figure 5-13. A typical hydraulic system for helicopters in the light to medium range is shown here.
igation capabilities, such as VOR, ILS , and GPS
intercept and tracking, which is especially useful in
IFR conditions. 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 anti-
torque 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 con-
trols when they receive control commands from a cen-
tral 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 cock-
pit that allows you to select the desired functions, as
well as engage the autopilot.
For safety purposes, an automatic disengage 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 disen-
gage button located on the cyclic or collective which
allows you to completely disengage the autopilot with-
out removing your hands from the controls. Because
autopilot systems and installations differ from one hel-
icopter to another, it is very important that you refer to
the autopilot operating procedures located in the
Rotorcraft Flight Manual.
ENVIRONMENTAL SYSTEMS
Heating and cooling for the helicopter cabin can be
provided in different ways. The simplest form of cool-
ing is ram air cooling. 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
servos, on each flight control, a pump which is usually
driven by the main rotor gearbox, and a reservoir to
store the hydraulic fluid. A switch in the cockpit can
turn the system off, although it is left on under normal
conditions. A pressure indicator in the cockpit may also
be installed to monitor the system.
When you make a control input, the servo is activated
and provides an assisting force to move the respective
flight control, thus lightening the force required by the
pilot. These boosted flight controls ease pilot workload
and fatigue. In the event of hydraulic system failure,
you are still able to control the helicopter, but the con-
trol forces will be very heavy.
In those helicopters where 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 use hydraulic accu-
mulators to store pressure, which can be used for a
short period of time in an emergency if the hydraulic
pump fails. This gives you enough time to land the hel-
icopter with normal control
STABILITY AUGMENTATIONS SYSTEMS
Some helicopters incorporate stability augmentations
systems (SAS) to aid in stabilizing the helicopter in
flight and in a hover. The simplest of these systems is a
force trim system, which uses a magnetic clutch and
springs to hold the cyclic control in the position where
it was released. More advanced systems use electric
servos that actually move the flight controls. These
servos receive control commands from a computer that
senses helicopter attitude. Other inputs, such as
heading, speed, altitude, and navigation information
may be supplied to the computer to form a complete
autopilot system. The SAS may be overridden or
disconnected by the pilot at any time.
Stability augmentation systems reduce pilot workload
by improving basic aircraft control harmony and
decreasing disturbances. These systems are very useful
when you are required to perform other duties, such as
sling loading and search and rescue operations.
AUTOPILOT
Helicopter autopilot systems are similar to stability
augmentations systems except 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 indicated airspeed is
maintained by the autopilot. Some autopilots have nav-
VOR—Ground-based navigation system consisting of very high fre-
quency omnidirectional range (VOR) stations which provide course
guidance.
ILS (Instrument Landing System)—A precision instrument approach
system, which normally consists of the following electronic components
and visual aids: localizer, glide slope, outer marker, and approach
lights.
GPS (Global Positioning System)—A satellite-based radio positioning,
navigation, and time-transfer system.
IFR (Instrument Flight Rules)—Rules that govern the procedure for
conducting flight in weather conditions below VFR weather minimums.
The term IFR also is used to define weather conditions and the type of
flight plan under which an aircraft is operating.
depends on the temperature of the outside air. Air con-
ditioning provides better cooling but it is more com-
plex and weighs more than a ram air system.
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, com-
pressed, discharge air from the engine compressor. Hot
air is ducted from the compressor to the helicopter
cabin through a pilot-controlled, bleed air valve.
ANTI-ICING SYSTEMS
Most anti-icing equipment installed on small helicopters
is limited to engine intake anti-ice and pitot heat systems.
The anti-icing system found on most turbine-powered
helicopters uses engine bleed air. The bleed air flows
through the inlet guide vanes to prevent ice formation on
the hollow vanes. A pilot-controlled, electrically operated
valve on the compressor controls the air flow. The pitot
heat system uses an electrical element to heat the pitot
tube, thus melting or preventing ice formation.
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 signifi-
cantly reduce ice formation and improve performance.