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 ignition and start energy.
- Compare battery and AC starts.
- Describe start sequence.
- Explain start/generate transition.
- Diagnose start faults.
1. Purpose and system role
The system rotates the APU, provides ignition and coordinates fuel until combustion becomes self-sustaining. The starter-generator then changes from motor to generator.
2. Architecture and energy flow
The ignition unit, high-voltage lead and igniter create spark in the combustor.
Battery start uses batteries and start-power electronics; AC start uses the aircraft AC network and SGCU conversion/control.
Both paths converge at the starter-generator and gearbox.
3. Major components
Ignition unit/lead/plug. Deliver high-energy spark. 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, electrical integrity, grounding and actual light-off. The technician should separate the command path from the actual physical response before replacing the component.
Starter-generator. Motors then generates. 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 current, torque, speed rise and generate-mode output. The technician should separate the command path from the actual physical response before replacing the component.
SGCU. Controls starter-generator modes and protection. 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 start enable, current control, communication and fault data. The technician should separate the command path from the actual physical response before replacing the component.
Start Power Unit. Supports battery-start energy conversion. 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 battery voltage/current and SPU status. The technician should separate the command path from the actual physical response before replacing the component.
ECB permissions. Sequence flap, fuel, ignition and protection. 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 inhibits, first-fault step and command timing. The technician should separate the command path from the actual physical response before replacing the component.
4. Normal operation
Step 1: Master on
ECB powers and commands intake/fuel preparation.
Step 2: Start command
SGCU/SPU energize starter-generator.
Step 3: Ignition/fuel
Scheduled spark and fuel create EGT rise.
Step 4: Acceleration
ECB monitors speed/EGT/oil and removes starter assist.
Step 5: Available/generate
Stable speed permits aircraft service connection.
5. Control, monitoring and protection
A commanded ignition does not prove a spark; a crank does not prove fuel or ignition.
Start succeeds from one power source but not the other is high-value isolation evidence.
6. Failure modes and maintenance reasoning
- No crank: Permission, power, SGCU/SPU, contactor, SG or seizure.
- Crank/no light: Ignition, fuel or combustion airflow.
- Light/slow acceleration: Fuel schedule, drag, starter transition or weak gas path.
- Hot start: Excess fuel, weak airflow, slow acceleration or EGT sensing.
- No generate after start: SGCU/generator/distribution or drive issue.
7. Interfaces with other aircraft systems
- ATA 24 batteries, AC network and generator distribution.
- ECB and DMM sequence data.
- Fuel and ignition systems.
- Intake flap and power section.
- Automatic start after severe electrical loss.
8. Practical scenarios
AC start works, battery start fails
Focus on batteries, SPU and battery start control.
No crank with high current
Mechanical drag or starter-generator/gearbox seizure is plausible.
Normal start but generator contactor stays open
Investigate generate-mode control and electrical network.
9. Technician takeaways
- Separate upstream power paths from common mechanical path.
- Preserve speed/EGT timing.
- Avoid repeated starts after wet/hot abort.
- Availability and generator connection are not the same.
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?
