Educational familiarization only. This original TechOpsBase lesson does not reproduce manufacturer pages, proprietary figures, maintenance procedures, numerical limits or controlled task data. Actual aircraft work requires current approved data, correct effectivity, operator procedures, authorization, safety controls and supervision.
Resource profile
- Aircraft: Airbus A350 family
- ATA: 49 - Airborne Auxiliary Power
- Audience: Enthusiasts, students, junior technicians and professionals
- Level: Intermediate-to-advanced
- Status: Draft pending technical review
Learning objectives
- Trace fuel from aircraft to combustor.
- Explain FCU control and filtration.
- Describe staged manifolds.
- Explain servo fuel.
- Diagnose no-light, hot-start and leakage.
1. Purpose and system role
The APU fuel system supplies clean pressurized fuel, meters it for every operating phase, distributes it to the combustor and removes it immediately when shutdown is commanded.
2. Architecture and energy flow
Aircraft ATA 28 provides the inlet feed and dedicated pump/valve logic.
The FCU pumps, filters, regulates and meters fuel under ECB command.
Primary and secondary manifolds stage flow to atomizers, while conditioned fuel also powers pneumatic actuators.
3. Major components
Inlet low-pressure sensing. Reports inadequate aircraft feed at the APU. In normal service, the component must perform its role while the ECB or related aircraft system monitors command, feedback or the effect it produces. A defect may be electrical, mechanical, pneumatic, fuel-powered or caused by installation. Useful maintenance evidence includes actual pressure, aircraft pump status, switch/wiring and restriction. The technician should separate the command path from the actual physical response before replacing the component.
FCU pumps. Raise pressure for metering and actuation. In normal service, the component must perform its role while the ECB or related aircraft system monitors command, feedback or the effect it produces. A defect may be electrical, mechanical, pneumatic, fuel-powered or caused by installation. Useful maintenance evidence includes pressure, noise, contamination and drive condition. The technician should separate the command path from the actual physical response before replacing the component.
Filter/bypass. Protects precision components while preserving flow. In normal service, the component must perform its role while the ECB or related aircraft system monitors command, feedback or the effect it produces. A defect may be electrical, mechanical, pneumatic, fuel-powered or caused by installation. Useful maintenance evidence includes differential pressure, debris and bypass status. The technician should separate the command path from the actual physical response before replacing the component.
Metering/shutdown elements. Schedule fuel and stop it immediately. In normal service, the component must perform its role while the ECB or related aircraft system monitors command, feedback or the effect it produces. A defect may be electrical, mechanical, pneumatic, fuel-powered or caused by installation. Useful maintenance evidence includes command, leakage, hot/no-light start and post-shutdown flow. The technician should separate the command path from the actual physical response before replacing the component.
Manifolds/atomizers. Stage and atomize combustion fuel. In normal service, the component must perform its role while the ECB or related aircraft system monitors command, feedback or the effect it produces. A defect may be electrical, mechanical, pneumatic, fuel-powered or caused by installation. Useful maintenance evidence includes restriction, uneven combustion, smoke and drains. The technician should separate the command path from the actual physical response before replacing the component.
4. Normal operation
Step 1: Start
Low-flow scheduling supports light-off and acceleration.
Step 2: Governed run
Fuel changes to maintain speed under load.
Step 3: Pneumatic control
Servo fuel moves IGV, SCV and bleed actuators.
Step 4: Shutdown
Metering closes and shutdown solenoid removes fuel.
5. Control, monitoring and protection
A low-pressure message crosses ATA 28/49 and must not be assigned to the FCU automatically.
Multiple air-actuator faults can reveal common servo-fuel pressure or FCU control.
6. Failure modes and maintenance reasoning
- No fuel/no light: Feed, FCU pump, metering, shutdown state or control permission.
- Excess fuel/hot start: Metering, schedule, airflow or slow mechanical acceleration.
- Filter restriction/bypass: Contamination and reduced downstream protection.
- Manifold/nozzle restriction: Poor light-off, uneven temperature and smoke.
- Shutdown leakage: Fuel drains, smoke or fire risk after stop.
7. Interfaces with other aircraft systems
- ATA 28 APU pump, feed valves and crossfeed.
- ECB fuel schedule and shutdown command.
- Power-section combustion.
- IGV/SCV/bleed servo actuation.
- Drain system and fire/emergency shutdown.
8. Practical scenarios
Crank and fuel at drain but no EGT
Ignition becomes a strong focus; avoid repeated wet starts.
Low fuel pressure with pump running
Separate actual delivered pressure from sensor/wiring and local restriction.
Three air actuators report faults
Check FCU servo fuel and common control before three replacements.
9. Technician takeaways
- Fuel is both combustion energy and actuator power.
- Filter bypass preserves flow, not cleanliness.
- Use phase-specific evidence.
- Confirm aircraft feed before condemning FCU.
Maintenance boundary
This resource teaches architecture, operating logic and troubleshooting reasoning. It excludes task steps, torque values, test limits, servicing quantities, start thresholds and dispatch decisions. The APU contains hot surfaces, rotating machinery, high-energy start circuits, pressurized fuel and oil, automatically moving components and fire-system interfaces. Use current approved AMM/TSM/WDM data and all required isolation procedures.
Review prompts
- What service should the subsystem provide?
- Which component creates the output and which component controls it?
- Which sensor or feedback proves the result?
- What is the command-versus-response evidence?
- Which ground/flight protection logic applies?
- Which other ATA system supplies or receives the command?
- What physical evidence should be preserved before reset?
- What heat, rotation, pressure, electrical or fire-boundary hazard remains?
