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The accessory gearbox-mounted fuel pump is comprised of three separate pumps
contained within a single housing. The engine-driven fuel pump provides highpressure
fuel at a flow rate that exceeds the requirements of the engine at any power
setting. Fuel pump pressure is used to generate motive flow for the scavenge and main
ejectors of the aircraft fuel system.
Fuel/Oil Heat Exchanger and Fuel Filter
A fuel/oil heat exchanger is used to warm engine fuel and cool engine oil. The fuel
temperature at the fuel filter is indicated on the EICAS status page. A fuel filter is used
to remove solid contaminants from the fuel. If the filter becomes clogged, the fuel
bypasses the filter to ensure continued operation. A clogged filter is indicated by the
L(R) FUEL FILTER caution EICAS message.
Fuel Control Unit (FCU)
The fuel control unit is a hydromechanical metering device that supplies fuel in
response to mechanical inputs from the thrust levers. In addition, the FCU controls and
actuates the VG inlet guide vanes and stator vanes of the engine compressor.
During start and at low power, the FCU hydromechanically schedules the fuel.
POWER PLANT
For Training Purposes Only
May 03
18-5
P I LOT T R A I N I N G GU I D E
Engine Fuel System - Schematic
Figure 18-2
LEGEND
Motive Fuel
Supply
FUEL
CONTROL
UNIT
AIRCRAFT
FUEL
SYSTEM
LH ENG
FIRE
PUSH
Fuel
Shutoff
Valve
N1 SPEED
CONTROL
Fuel
Low-Pressure
Switch
Fuel
Temp
Sensor
Filter
with P
Indicator
Bypass
Valve
Metered Fuel
Fuel Shutoff
Valve
Mechanical
Connection
Electrical
Connection
Engine-Driven
Pump Element
Temperature
Probe
Note: Left Engine Illustrated
Fuel/Oil Heat
Exchanger
Fuel Flow
Meter
FUEL TEMP
ENGINE
BULK
80 80
20
P604_18_006
N2
FF
OIL PRESS 82
3500 3500
10
L FUEL FILTER
L FUEL LO PRESS
Engine-Driven
Pump
VARIABLE
GEOMETRY
ACTUATOR
POWER PLANT
18-6 For Training Purposes Only
May 03
P I LOT T R A I N I N G GU I D E
Electronic Control Unit (ECU)
When at high thrust settings, the engine is controlled by an electronic control unit
(ECU) which works in tandem with the FCU. The ECU is also referred to as the N1
control amplifier. The ECU (or N1 control amplifier) trims the FCU fuel output to
maintain a N1 speed schedule for a given thrust lever setting.
Fuel System Operation
N2 Speed Control (N1 < 79%)
At low power settings (N1 below 79%), the FCU hydromechanically controls the N2
speed. In N2 mode, the FCU adjusts N2 speed so that matched movement of the thrust
levers produces nearly matched N2 rpm for the engines. N1 speeds, fuel flows, or ITT
indications may differ between engines.
N1 Speed Control (N1 > 79%)
At takeoff, climb and cruise power settings (N1 above 79%), the N1 control amplifier
controls the engine N1 rpm. The amplifier trims the FCU fuel output to achieve the
desired N1 rpm.
With the ENG SPEED switches selected to ON, automatic switchover from N2 to N1
speed governing occurs at 79% N1. Matched movement of the thrust levers produces
nearly matched N1 rpm and nearly matched thrust between the engines.
If an ENGINE SPEED switch is moved from ON to OFF at high
power settings, the engine will revert to N2 speed control. A rapid
increase in engine acceleration will occur and an
overtemperature limit may be exceeded.
ENGINE CONTROL Panel
Figure 18-3
NOTE
TEST
2
TE
ARM
TEST OFF
3
TEST
1 TEST
APR VIB
OFF
ON
ENG
SPEED
LH RH
ENGINE CONTROL
A
OFF OFF
P604_18_020
ENG SPEED Switches
O
POWER PLANT
For Training Purposes Only
May 03
18-7
P I LOT T R A I N I N G GU I D E
Automatic Performance Reserve System
Description
During takeoff, the APR system monitors the N1 rpm of both engines. If a significant
loss of N1 rpm is sensed on one engine, the APR system automatically increases the
thrust of the remaining engine to the APR thrust rating.
APR activation does not override thrust lever input to the FCU nor does it restrict
movement of the thrust lever.
Operation
The APR system is armed during takeoff when the APR switch is selected to ARM
and both engines’ N1 rpm is above 79% (N1 speed mode). This is indicated by the APR
ARM advisory EICAS message. During a normal takeoff, the advisory message is
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