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 exhaust flow.
- Explain thermal movement.
- Inspect joints/supports.
- Diagnose noise and heat.
1. Purpose and system role
The exhaust system conveys turbine gas and cooling air overboard, reduces noise and protects the tailcone from heat while accommodating relative movement.
2. Architecture and energy flow
The flexible coupling isolates APU/muffler movement.
The surrounding structure supports muffler mass.
Insulation and fire seals limit heat transfer; the low-point drain removes fluid.
3. Major components
Bellows. Accommodates alignment and thermal growth. 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, distortion, contact and soot. The technician should separate the command path from the actual physical response before replacing the component.
Muffler/acoustic liner. Routes gas and attenuates noise. 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 tone change, dents and internal damage. The technician should separate the command path from the actual physical response before replacing the component.
Clamps/joints. Contain hot gas. 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 engagement, flange condition and leakage evidence. The technician should separate the command path from the actual physical response before replacing the component.
Supports. Carry muffler load while allowing growth. 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 looseness, cracking and over-constraint. The technician should separate the command path from the actual physical response before replacing the component.
Insulation. Reduces compartment temperature. 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, compressed or fluid-soaked blanket. The technician should separate the command path from the actual physical response before replacing the component.
4. Normal operation
Step 1: Start/run
Hot gas and pulsation load the assembly.
Step 2: Load change
Temperature and flow change expansion.
Step 3: Shutdown
Heat soak continues after fuel stops.
5. Control, monitoring and protection
The ECB does not directly monitor every joint or blanket.
Noise, heat, soot and vibration are important.
6. Failure modes and maintenance reasoning
- Gas leak: Heat/soot and fire-boundary damage.
- Misalignment: Bellows/support stress and vibration.
- Liner damage: Noise and possible flow disturbance.
- Blocked drain: Fluid near hot structure.
7. Interfaces with other aircraft systems
- Power section exhaust.
- Cooling eductor flow.
- Fire seals.
- Tailcone structure and drains.
8. Practical scenarios
New vibration after exhaust work
Inspect alignment, support and contact first.
High compartment heat with normal oil
Inspect gas leakage and insulation.
9. Technician takeaways
- Allow full cooling.
- Do not force alignment.
- Inspect hidden inner/adjacent damage.
- Record soot/heat before cleaning.
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?
