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
- Differentiate engine and cooling airflow.
- Explain flap command and feedback.
- Describe plenum supply.
- Connect eductor flow to oil cooling.
- Diagnose duct, seal and actuator faults.
1. Purpose and system role
The intake system supplies low-distortion outside air to the engine and load compressor while a separate cooling duct supplies the compartment and oil cooler.
2. Architecture and energy flow
The aerodynamic flap closes flush when the APU is not required and opens to a commanded position for ground or flight demand.
The engine duct feeds a common inlet plenum and screen before flow divides to the two compressors.
A separate cooling duct, oil cooler and eductor/exhaust path form the accessory-cooling circuit.
3. Major components
Air-intake flap. Controls inlet area and aircraft outer-skin 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 command/feedback, hinge friction, obstruction, ice and seal condition. The technician should separate the command path from the actual physical response before replacing the component.
Flap actuator. Moves the flap under ECB command. 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 power, wiring, actuator force and mechanical freedom. The technician should separate the command path from the actual physical response before replacing the component.
Position feedback. Confirms flap state to the ECB. 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 disagreement, adjustment, connector and sensor faults. The technician should separate the command path from the actual physical response before replacing the component.
Engine-intake duct. Provides low-loss airflow and acoustic treatment. 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 cracks, loose acoustic material, contamination and seal leakage. The technician should separate the command path from the actual physical response before replacing the component.
Cooling duct/eductor. Moves compartment air through the oil cooler. 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, damaged duct, cooler blockage and exhaust-flow dependence. The technician should separate the command path from the actual physical response before replacing the component.
4. Normal operation
Step 1: Master on
ECB commands flap opening and checks position.
Step 2: Start
Plenum air supports engine and load-compressor inlet.
Step 3: Run/load
Flow must remain stable under electrical and pneumatic demand.
Step 4: Shutdown
Flap closes after the scheduled sequence and speed decay.
5. Control, monitoring and protection
Correct command with no movement points to actuator or obstruction; no command may indicate an unmet start permission.
Engine-intake restriction affects start and bleed; cooling restriction mainly affects oil/compartment temperature.
6. Failure modes and maintenance reasoning
- Flap failed closed: Start inhibited/aborted or reduced airflow.
- Flap failed open: Post-shutdown indication and aerodynamic exposure.
- Duct restriction: Slow start, high EGT or weak bleed output.
- Cooling restriction: High oil/compartment temperature with otherwise normal performance.
- Seal/fire blanket damage: Hidden fire-boundary degradation.
7. Interfaces with other aircraft systems
- ECB start and shutdown logic.
- Power section and load compressor.
- Oil cooler and exhaust eductor.
- Fire seals/intake blanket.
- Aircraft outer skin and tailcone structure.
8. Practical scenarios
Physical flap closed but page says moving
Focus on feedback, adjustment, wiring or ECB input.
Normal start but high oil temperature
Inspect cooling duct, cooler and eductor rather than engine-intake flow.
Slow start plus low bleed output
Look for common inlet/plenum restriction or weak power section.
9. Technician takeaways
- Treat the two airflow paths separately.
- Inspect acoustic material and foreign objects.
- Never infer physical flap position from indication alone.
- Restore fire seals after duct work.
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?
