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
- Explain every operating phase.
- Connect loads to fuel/EGT.
- Describe cooldown.
- Explain autostart and flight logic.
- Use phase-specific diagnosis.
1. Purpose and system role
A sequence-based understanding shows which systems are active when a fault occurs. It is the fastest way to narrow ATA 49 troubleshooting.
2. Architecture and energy flow
Before crank, the ECB checks permissions and commands flap/fuel preparation.
During start, electrical torque, ignition, fuel, airflow and oil must develop in the correct order.
After availability, electrical and pneumatic loads are separate; shutdown removes loads before cooldown unless protection/emergency overrides it.
3. Major components
Master/start controls. Initiate normal sequence. 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 acceptance, inhibit and fault lights. The technician should separate the command path from the actual physical response before replacing the component.
Starter-generator path. Creates initial rotation. 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 source, current, SGCU/SPU and mechanical response. The technician should separate the command path from the actual physical response before replacing the component.
Fuel/ignition. Establish combustion. 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 EGT rise, drains, smoke and acceleration. The technician should separate the command path from the actual physical response before replacing the component.
Generator/bleed controls. Connect aircraft services. 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 separate demand, contactor/valve and load response. The technician should separate the command path from the actual physical response before replacing the component.
Cooldown/flap close. Manages thermal stop and aerodynamic closure. 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 load removal, timing, speed decay and position feedback. The technician should separate the command path from the actual physical response before replacing the component.
4. Normal operation
Step 1: Pre-crank
Permissions, flap and fuel preparation.
Step 2: Crank/light
Starter torque, ignition and scheduled fuel.
Step 3: Acceleration
ECB evaluates speed/EGT/oil and removes starter assist.
Step 4: Available/load
Generator/bleed demand increases fuel and temperature.
Step 5: Normal stop
Loads removed, cooldown, fuel cutoff and flap closure.
Step 6: Emergency/protective stop
Immediate fuel removal and stored event.
Step 7: Autostart
Aircraft command initiates start during severe electrical loss.
5. Control, monitoring and protection
The same message can have different causes depending on the phase.
Some protective shutdowns are inhibited in flight while warning remains.
6. Failure modes and maintenance reasoning
- Before-crank abort: Permissions, flap, emergency/fire state, power or sensor validity.
- Crank abort: Start power or mechanical drag.
- Light-off abort: Fuel/ignition/air.
- Load-triggered fault: Output system or insufficient core margin.
- Shutdown fault: Load removal, cooldown, fuel stop or flap closure.
7. Interfaces with other aircraft systems
- Aircraft electrical power/source logic.
- Fuel feed and fire/emergency status.
- Bleed demand and generator connection.
- Display/warning/CMS.
- Ground/flight and autostart logic.
8. Practical scenarios
Fault before rotation
Do not start with ignition/fuel parts; inspect permissives and power command.
Fault only when bleed selected
Pneumatic load/control is the active change.
Fault after master off
Focus on cooldown, shutdown and flap closure.
9. Technician takeaways
- Always record the failure phase.
- Availability is not the same as connected output.
- Load response exposes marginal power.
- Normal cooldown is a protection function.
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?
