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
- Identify controls/pages.
- Interpret speed/EGT together.
- Separate availability from outputs.
- Use messages as evidence.
1. Purpose and system role
The APU page and warning system summarize gas-turbine condition, intake state and aircraft service availability. Detailed CMS/ECB data is needed for diagnosis.
2. Architecture and energy flow
Master/start controls initiate normal operation.
Generator and bleed controls connect separate services.
Fire/external controls initiate emergency shutdown.
3. Major components
Speed sensing. Supports control and overspeed 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 redundancy, wiring and phase behavior. The technician should separate the command path from the actual physical response before replacing the component.
EGT thermocouples. Measure hot-section response. 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 agreement, trend and sensor plausibility. The technician should separate the command path from the actual physical response before replacing the component.
Inlet temperature sensing. Supports protection and performance. 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 ground/flight protection behavior. The technician should separate the command path from the actual physical response before replacing the component.
System page. Shows speed, EGT, flap, availability, generator and bleed. 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 missing value, incorrect state and timing. The technician should separate the command path from the actual physical response before replacing the component.
Warning/CMS. Reports start fault, shutdown and maintenance status. 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 first fault and detailed channel data. The technician should separate the command path from the actual physical response before replacing the component.
4. Normal operation
Step 1: Start
Speed/EGT relationship identifies light and acceleration.
Step 2: Available
Stable speed permits output requests.
Step 3: Load
Generator/bleed states and temperature response are observed.
Step 4: Shutdown
Flap and fault messages continue after rotation stops.
5. Control, monitoring and protection
Availability does not prove generator or bleed connection.
Ground and flight sensor-fault effects may differ.
6. Failure modes and maintenance reasoning
- Sensor disagreement: Protection reduction or false message.
- Start fault: Many possible sequence causes.
- Flap moving/open after stop: Actuator/feedback or obstruction.
- Master fault light: Summary, not a failed-switch diagnosis.
7. Interfaces with other aircraft systems
- ECB/DMM/CMS.
- ATA 24 generation.
- ATA 21/36 bleed.
- ATA 28 fuel.
- ATA 26 fire controls.
8. Practical scenarios
Stable APU but generator absent
Use electrical page/SGCU evidence.
One EGT lost in flight
Review redundancy and protection logic.
9. Technician takeaways
- Record before cycling master.
- Use trends and operating phase.
- Confirm physical state.
- Do not replace by message name alone.
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?
