speed is reached, the flyweights move the fuel valve in the closing
direction until the proper steadystate fuel flow is reached.
b. Manual (emergency) control system. When the governor
control switch in the cockpit is moved from the automatic position to
the manual (emergency), a valve is actuated in the fuel control, and
fuel is redirected to the manual system metering valve. The throttle
in a helicopter is of the motorcycle twistgrip type. When the
governor is in the automatic position the throttle is rolled full
open and left there, with the fuel control making all fuelflow
changes automatically. If the automatic fuel control fails, the
pilot switches to the emergency mode and takes manual control of the
throttle, which is mechanically linked to the manual metering valve.
The manual throttle control has no compensation for altitude or
temperature, and it has no protection against an engine overspeed.
Keep in mind that so far the discussion has been on
principles of operation, and any specific fuel control may differ.
2.4.
FUEL PUMP
Main fuel pressure pumps for gas turbine engines generally
have one or two geartype, positivedisplacement, highpressure
elements. Each of these elements discharges fuel through a check
valve to a common discharge port. Thus, if one element fails, the
remaining element continues to supply sufficient fuel for engine
operation. On some engines, the fuel pump is built in to the fuel
control. However, on other engines the fuel pump may be a separate
component.
2.5.
STARTINGFUEL SYSTEM
Fuel flows through an external line from the fuel control to
the startingfuel solenoid. During the starting sequence, the pilot
actuates the startfuel solenoid switch in the cockpit. The solenoid
actuates the valve to the open position, then fuel flows through an
external line to the startfuel manifold. The startfuel nozzles are
attached to the manifold; the number of nozzles varies according to
engine design. The nozzles introduce atomized fuel in the combustion
chamber during the starting sequence. After the engine has attained
a specified speed, the main fuel starts to flow automatically. After
the engine is running on the main fuel system, the start fuel system
is shut off. A starting fuel system is shown in Figure 2.3.
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