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the fuel tank allows the pilot to drain water and sediment
that may have collected in the tank. A fuel vent
prevents the formation of a vacuum in the tank, and an
overflow drain allows for fuel to expand without rupturing
the tank. A fuel quantity gauge located on the
pilot’s instrument panel shows the amount of fuel
measured by a sensing unit inside the tank. Some
gauges show tank capacity in both gallons and pounds.
The fuel travels from the fuel tank through a shut-off
valve, which provides a means to completely stop fuel
flow to the engine in the event of an emergency or fire.
The shut-off valve remains in the open position for all
normal operations.
Most non-gravity feed fuel systems contain both an
electric pump and a mechanical engine driven pump.
The electrical pump is used to maintain positive fuel
pressure to the engine pump and also serves as a
backup in the event of mechanical pump failure. The
electrical pump is controlled by a switch in the cockpit.
The engine driven pump is the primary pump that supplies
fuel to the engine and operates any time the
engine is running.
Afuel filter removes moisture and other sediment from
the fuel before it reaches the engine. These contaminants
are usually heavier than fuel and settle to the bottom
of the fuel filter sump where they can be drained
out by the pilot.
Some fuel systems contain a small hand-operated pump
called a primer. A primer allows fuel to be pumped
directly into the intake port of the cylinders prior to
engine start. The primer is useful in cold weather when
fuel in the carburetor is difficult to vaporize.
ENGINE FUEL CONTROL SYSTEM
The purpose of the fuel control system is to bring outside
air into the engine, mix it with fuel in the proper
proportion, and deliver it to the combustion chamber.
Throttle
Low Level

Warning

Light
Vent
Fuel Quantity

Gauge
Mixture

Control
Fuel

Shutoff
Primer
Tank
Shut-off

Valve
Carburetor
Fuel

Strainer
Primer Nozzle

at Cylinder
Figure 5-9. A typical gravity feed fuel system, in a helicopter
with a reciprocating engine, contains the components
shown here.
Stationary

Swash

Plate
Pitch

Link
Rotating

Swash

Plate
Control

Rod
Figure 5-8. Collective and cyclic control inputs are transmitted
to the stationary swash plate by control rods causing it to
tilt or to slide vertically. The pitch links attached from the
rotating swash plate to the pitch horns on the rotor hub
transmit these movements to the blades.
5-7
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 position.
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 control.
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 helicopters
in-flight.
Most mixture adjustments are required during changes of
altitude or during operations at airports with field elevations
well above sea level. Amixture that is too rich can
result in engine roughness and reduced power. The roughness
normally is due to spark plug fouling from excessive
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
　
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