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WF — FUEL FLOW
TAMB — AMBIENT TEMPERATURE
TLA — THRUST LEVER ANGLE
AMBIENT PRESSURE
TLA (Secondary)
SURGE
BLEED
SYSTEM
HYDROMECHANICAL
FUEL CONTROL UNIT
N2
ITT
N1
AB
TT2
PT2
FUEL CONTROL LOGIC DIAGRAM
Figure 2-1
Pilot’s Manual
PM-126A 2-3
ENGINE FUEL AND CONTROL SYSTEM
The engine fuel and control system pressurizes fuel routed to the engine
from the aircraft fuel system, meters fuel flow, filters the fuel, heats
it as necessary to prevent filter icing, and delivers atomized fuel to the
combustion section of the engine. The system also supplies high-pressure
motive-flow fuel to the aircraft fuel system for jet pump operation.
The major components of the system are the thrust levers, the enginedriven
fuel pump, the hydromechanical fuel control unit, the Digital
Electronic Engine Control (DEEC), surge bleed control valve and the
fuel heater/oil cooler.
THRUST LEVERS
Two thrust levers, located on the upper portion of the pedestal, are operated
in a conventional manner with the full forward position being
maximum power. Stops at the IDLE position prevent inadvertent reduction
of the thrust levers to CUTOFF. The IDLE stops can be released
by lifting a finger lift on the outboard side of each thrust lever. Detents
are provided for CUTOFF, IDLE, Maximum Cruise (MCR), Maximum
Continuous Thrust (MCT), Takeoff (T/O) and Automatic Performance
Reserve (APR).
Primary Thrust Lever Angle (TLA) input to each DEEC is provided
through Rotary Variable Differential Transformers (RVDTs) located
within the thrust lever quadrant. Secondary TLA input is provided by
a control cable connecting each thrust lever to the corresponding engine’s
hydromechanical fuel control unit.
A flight director go-around button is installed in the left thrust lever
handle. An aural warning horn/voice mute button is installed in the
right thrust lever handle. A thrust reverser control lever is mounted
piggyback fashion on each thrust lever. Refer to THRUST REVERSERS
in this Section for a functional description of the thrust reverser levers.
The Engine Indicating (EI) display will illuminate a green MCR, MCT,
T/O or APR for the corresponding thrust lever detents.
Pilot’s Manual
2-4 PM-126A
ENGINE-DRIVEN FUEL PUMP
The engine-driven fuel pump provides high-pressure fuel to the engine
fuel control system as well as motive-flow fuel for operation of the aircraft
jet pumps. The pump consists of a low-pressure pump element,
high-pressure pump element, high-pressure relief valve, filter, filter bypass
valve, and motive-flow provisions.
The fuel pump is mounted to the accessory drive gearbox of the engine.
Fuel entering the first stage low-pressure element is pressurized to flow
through the fuel heater/oil cooler and filter. A second flow path for this
fuel is to the Auxiliary Motive Flow Pump (AMFP). The fuel from the
AMFP is used to operate the various jet pumps in the wing tanks. Fuel
that is supplied to the fuel heater/oil cooler and filter is passed on to
the pump high-pressure element. The high-pressure element provides
fuel at the fuel pressures required by the hydromechanical fuel control
unit. The high-pressure relief valve protects the fuel pump and hydromechanical
fuel control unit from extreme fuel pressure surges. A fuel
filter bypass valve begins to open at a pressure differential of 9 to 12 psi
(62 to 82 kPa) and allows flow of unfiltered fuel to the inlet of the highpressure
pump.
The following CAS illuminations are specific to the fuel pumps:
HYDROMECHANICAL FUEL CONTROL UNIT
The hydromechanical fuel control unit meters the required amount of
fuel to the engine combustor that corresponds to TLA, atmospheric and
engine operating conditions. The unit is mounted on the fuel pump and
contains the hydromechanical fuel metering section, thrust lever input
and position potentiometer, shutoff valve, and a mechanical governor.
The mechanical governor functions as an overspeed governor for the
high-pressure rotor. In addition, the mechanical governor provides
manual control when the DEEC is deactivated. When activated, the
DEEC controls fuel scheduling by means of a torque motor located
within the hydromechanical fuel control unit. The torque motor controls
the metering section of the hydromechanical fuel control unit.
CAS Color Description
FUEL PRESS LOW Red Fuel pressure is low at the associated (L or
R) engine’s fuel pump inlet.
FUEL FILTER White The engine or wing fuel filter, on the associated
(L or R) side, is becoming clogged.
　
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