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 oil flow.
- Explain filtration and bypass.
- Describe scavenge and deaeration.
- Interpret oil protection.
- Diagnose heat, pressure, quantity and debris.
1. Purpose and system role
The oil system lubricates and cools bearings, gears and the starter-generator, removes oil from sumps, separates air, rejects heat and provides mechanical-health evidence.
2. Architecture and energy flow
A tank stores oil and a gearbox-driven pressure pump supplies components through filters and regulation.
Scavenge stages return oil from sumps; an air-oil separator removes entrained air.
A thermal bypass and air-cooled heat exchanger manage temperature; sensors monitor pressure, temperature and quantity.
3. Major components
Oil tank/level sensor. Stores and reports usable quantity. 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 service condition, leakage trend, sensor plausibility and drain-down. The technician should separate the command path from the actual physical response before replacing the component.
Pressure pump/regulator. Provides controlled supply pressure. 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 pressure trend, drive condition, leakage and filter state. The technician should separate the command path from the actual physical response before replacing the component.
Filters/bypass. Protect components while preserving flow. 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 debris, differential pressure and bypass indication. The technician should separate the command path from the actual physical response before replacing the component.
Scavenge pumps/separator. Return and deaerate oil. 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 foaming, oil mist, sump pooling and quantity instability. The technician should separate the command path from the actual physical response before replacing the component.
Cooler/thermal bypass. Reject heat and manage warm-up/restriction. 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 cooling airflow, blockage, valve function and temperature trend. The technician should separate the command path from the actual physical response before replacing the component.
4. Normal operation
Step 1: Start
Pressure must establish as shaft and gearbox accelerate.
Step 2: Run
Supply/scavenge maintain lubrication across load.
Step 3: Cooling
Compartment airflow passes the cooler and heat exits through eductor/exhaust.
Step 4: Shutdown
Scavenge and heat-soak behavior affect level and next start.
5. Control, monitoring and protection
Ground and flight protection can differ to balance APU protection against backup-source availability.
Oil debris and physical leakage may provide earlier evidence than a shutdown message.
6. Failure modes and maintenance reasoning
- Low pressure: Low quantity, pump/drive, regulator, filter, leakage or sensor.
- High temperature: Cooling loss, cooler/bypass, high load, low quantity or internal distress.
- Low quantity: Leak, consumption, incorrect service or sensor.
- Debris: Gear/bearing/pump wear.
- Foaming/oil mist: Scavenge, separator, vent or seal issue.
7. Interfaces with other aircraft systems
- Accessory gearbox drives pumps/separator.
- Starter-generator lubrication.
- Cooling intake/eductor/exhaust.
- Drains and seal cavities.
- ECB/DMM protection and indication.
8. Practical scenarios
High oil temperature only with bleed
Check load and cooling-air margin.
Quantity falls but compartment is dry
Inspect drains, exhaust/oil mist and separator.
Filter bypass plus metal
Preserve debris and evaluate internal mechanical wear.
9. Technician takeaways
- Pressure, temperature, quantity and debris are separate evidence.
- Bypass preserves flow but reduces protection.
- Interpret quantity in the specified condition.
- Do not erase debris evidence.
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?
