DOC AIRBUS | AMM A320FAPU FUEL CONTROL - DESCRIPTION AND OPERATION
** ON A/C NOT FOR ALL
** ON A/C NOT FOR ALL
1. General
The APU fuel control subsystem operates fully automatically. During APU operation, the FCU, which is governed by the ECB, meters the fuel dependent on the APU load condition. The flow divider distributes the fuel via both fuel manifolds to the fuel nozzles. During APU start-up/shut-down, the FCU increases/reduces the quantity of fuel delivered to the flow divider accordingly. During APU start, the flow divider routes fuel to the combustion chamber via the pilot fuel manifold only.
** ON A/C NOT FOR ALL The APU fuel control subsystem operates fully automatically. During APU operation, the FCU, which is governed by the ECB, meters the fuel dependent on the APU load condition. The flow divider distributes the fuel via both fuel manifolds to the fuel nozzles. During APU start-up/shut-down, the FCU increases/reduces the quantity of fuel delivered to the flow divider accordingly. During APU start, the flow divider routes fuel to the combustion chamber via the pilot fuel manifold only.
2. Component Location
** ON A/C NOT FOR ALL 
APU Fuel Control - Component Location| ------------------------------------------------------------------------------- |
| FIN | FUNCTIONAL DESIGNATION | PANEL|ZONE|ACCESS | ATA |
| | | | | DOOR | REF. |
| ------------------------------------------------------------------------------- |
| 8022KM FUEL CONTROL UNIT (FCU) 315 315AL 49-32-11 |
| 8024KM FLOW DIVIDER 315 315AL 49-32-12 |
3. System Description
A. Fuel Control Unit (FCU)
(1) The fuel control unit, together with the flow divider, schedule the fuel flow to the APU. The FCU also supplies high-pressure fuel to the bleed control valve and to the Inlet Guide Vane (IGV) actuator to give the necessary power to position the load compressor IGVs and the bleed control valve.
The FCU has an inlet section, a pump section and a control and metering section.
The inlet section includes:
The FCU has an inlet section, a pump section and a control and metering section.
The inlet section includes:
- an inlet fuel filter,
- a filter bypass valve,
- an inlet-filter pressure differential-indicator.
- a low-pressure fuel pump,
- a high-pressure fuel pump,
- a high-pressure relief valve.
- an actuator-pressure regulator,
- a servo valve,
- a constant delta P valve,
- a 3 way solenoid valve.
(2) The fuel which goes into the FCU passes through the centrifugal low pressure pump to the inlet fuel filter and into the high-pressure fuel pump. The output from the high-pressure fuel pump goes to the:
The filter bypass valve permits continued operation with a clogged filter.
- actuator pressure regulator,
- servo valve.
The filter bypass valve permits continued operation with a clogged filter.
B. Flow Divider
(1) The flow divider together with the FCU, schedule the fuel flow to the APU. The flow divider controls the fuel flow to the main and the pilot manifolds and drains the pilot manifold at APU shutdown.
(2) The flow divider has a housing assembly that contains:
- a pilot injector and purge valve,
- a main injector valve,
- a filter screen.
(3) Fuel from the fuel control is supplied to the inlet port of the fuel flow divider during APU starting and normal operation.
Fuel is supplied from the pilot fuel port (pilot injector valve at 1.38 bar (20 psid) open) to the pilot fuel manifold during engine starting and operation.
Fuel is supplied from the main fuel port (main injector valve at 13.8 bar (200 psid) open) to the main fuel manifold when the engine reaches approximately 20% speed during starting. Fuel continues to be supplied from this port during APU operation.
During engine shut down, fuel is purged from the pilot fuel nozzles and pilot manifold and flows from the purge port of the fuel flow divider to the exhaust by combustion chamber pressure.
Fuel is supplied from the pilot fuel port (pilot injector valve at 1.38 bar (20 psid) open) to the pilot fuel manifold during engine starting and operation.
Fuel is supplied from the main fuel port (main injector valve at 13.8 bar (200 psid) open) to the main fuel manifold when the engine reaches approximately 20% speed during starting. Fuel continues to be supplied from this port during APU operation.
During engine shut down, fuel is purged from the pilot fuel nozzles and pilot manifold and flows from the purge port of the fuel flow divider to the exhaust by combustion chamber pressure.
A. APU Fuel System
For a normal start, operation or a restart, the fuel pressure at the APU fuel inlet is:
For a normal start, operation or a restart, the fuel pressure at the APU fuel inlet is:
- minimum 1,03 bar (15 psi) absolute (to ensure at least 0,7 bar (10 psi), maximum 3,8 bar (55 psi).
(1) Set the MASTER SW 14KD to ON. When you push the APU START switch 2KA, the engine starts to turn. After approx. 1.5 sec. the ECB 59KD energizes the fuel solenoid valve to the open position. Fuel from the servo valve of the FCU flows through the pilot injector and purge valve of flow divider. From the flow divider it is sent to the pilot fuel nozzles. After another 1.5 sec. the ECB 59KD energizes the ignition system, combustion occurs and the engine continues to accelerate.
(2) At 55% engine speed, the ECB 59KD de-energizes the exiter and the starter motor 8 KA. The engine continues to accelerate under its own power to governed speed. The speed control function in the ECB 59KD transmit a signal to the FCU. The FCU then increases or decreases the fuel flow as necessary for the changes in load. This keeps the speed of the APU constant.
(3) As the engine accelerates, the ECB 59KD transmits a signal to the servo valve in the FCU to control the metered fuel flow. The fuel acceleration control in the ECB 59KD keeps the necessary fuel flow until the engine gets to governed speed. At low speed all pump flow goes to the pilot injectors.
The engine accelerates, the fuel flow increases to a set level and the main injector valve of the FDDVA opens. This permits more fuel to flow.
The engine accelerates, the fuel flow increases to a set level and the main injector valve of the FDDVA opens. This permits more fuel to flow.