How to Maintain Auxiliary Power Units

Core APU Components

While specific configurations will vary by manufacturer and aircraft type, most modern APUs share a common set of assemblies.

Compressor Section

The compressor draws ambient air into the APU and increases its pressure for efficient combustion, typically containing:

• An inlet case and inlet guide vanes to channel airflow
• An impeller and–in many units–axial compressor stages that progressively raise air pressure
• Bleed-air ports to extract pneumatic output

Combustor

Within the combustor, fuel and compressed air are mixed and ignited to generate a high-energy gas stream that drives the turbine. Standard sub-components include:

• A burner can or annular combustion liner to house the combustion process
• Fuel nozzles that meter and atomize fuel
• Igniters
• Combustion transition ducts

Turbine Section

The turbine converts thermal energy into rotational power for the accessory gearbox, utilizing:

• A turbine rotor
• Turbine stators or nozzle guide vanes, which direct airflow at optimal angles
• An exhaust collector to channel spent gases out of the aircraft

Accessory Gearbox (AGB)

The accessory gearbox distributes mechanical power from the turbine shaft to the APU’s supporting systems through:

• Drive gears and idler gears
• Bearings that support rotating shafts while reducing friction
• Drive splines that couple the gearbox to the main shaft

Fuel System

The fuel system delivers metered fuel at the correct pressure and flow rate to support stable combustion, mostly relying on:

• A fuel pump and distribution lines that carry fuel to the nozzles
• Filters for contamination control

Electrical Generator

The generator provides electrical output for aircraft subsystems with the help of:

• A Stator and rotor windings
• A generator control unit (GCU) that regulates voltage and frequency
• A drive coupling to interface the generator with the AGB

Lubrication System

To protect bearings, gears, and rotating assemblies from high loads and temperatures, APUs include:

• An oil pump connected to a reservoir or tank
• Air- or fuel-managed oil coolers
• Seals, scavenge lines, and pressure lines to circulate oil

Maintenance Pointers

A methodical approach to maintenance is imperative for keeping APUs functioning with no major issues. Keep in mind that the manual provided by the original manufacturer of your APU model will be your best reference for service intervals, operation parameters, and inspection requirements. However, maintenance will usually follow schedules like:

• Daily or Pre-Flight Checks: During these frequent checks, operators should verify proper oil levels and observe for any signs of leaks, unusual residue, or loose access panels. Additionally, observing start-up behavior and shutdown characteristics will help catch any anomalies.
• Flight-Hour or Cycle-Based Inspections: Typical intervals include inspections at 150, 300, or 600 hours depending on manufacturer recommendations. These checks may include more in-depth internal examinations, filter replacements, and accessory system testing.

As far as component-specific considerations, technicians tend to carry out the following tasks to preserve APU performance:

Compressor

• Remove surface contamination like dirt, soot, or oil residue from the inlet case, impeller, and axial stages.
• Inspect for erosion, nicks, tip wear, or general deformation on compressor vanes.
• Monitor for stall or surge indicators, including abnormal vibration or fluctuating compressor discharge pressure.

Combustor

• Clean and check the flow of fuel nozzles to ensure proper atomization.
• Check burner cans, liners, and transition ducts for cracks, hotspots, and carbon buildup that could compromise combustion.
• Replace worn igniter plugs.
• Confirm the condition and seating of combustion seals to maintain appropriate pressure parameters and prevent leakage into adjacent sections.

Turbine Section

• Perform borescope inspections to identify blade cracks, trailing-edge wear, or foreign object damage.
• Keep an eye out for signs of thermal distress like discoloration or warping across all components in this section.

Fuel System

• Inspect and replace fuel filters at manufacturer-recommended intervals to avoid flow restriction and contamination.
• Test fuel pump output pressure.
• Look over manifolds and fuel lines for leaks, chafing, and secure fittings to prevent either fuel starvation or oversupply.

Electrical Generator

• Confirm that voltage, frequency, and load-carrying capability all remain within specification through testing.
• Conduct insulation resistance checks on stator and rotor windings to identify potential breakdowns or moisture ingress.

Lubrication System

• Perform oil changes and replace filters per manufacturer recommendations.
• Verify lubrication pump output and system pressure for proper circulation through all supply and scavenge lines, as well as check for any signs of leaks or degradation.
• Check that oil coolers are regulating temperature appropriately to avoid lubricant breakdown.

Typical Wear Indicators

Along with consistent maintenance, being vigilant of symptoms that point to issues in the APU can prevent costly and dangerous failures. Some warning signs to address include:

• Unexpected shutdowns, suggesting issues with fuel delivery or lubrication pressure.
• Abnormal vibrations, which are often linked to rotor imbalance, bearing deterioration, or compressor blade damage.
• Elevated exhaust gas temperature that signals combustion inefficiency or turbine degradation.
• Oil leaks or rising oil consumption, pointing to seal wear or lubrication system issues.
• Sluggish starting performance that may indicate ignition, gearbox, or bleed-air system complications.

Procure Reliable APUs and Associated Parts from Integrated Enterprises

The key takeaway is that keeping APUs in good condition requires comprehensive checks of all subsystems at regular, manufacturer recommended intervals and whenever poor performance arises. Dedicating time to prevent issues in an APU is always more cost-effective than repairs, but if you find yourself needing high-quality replacement components or entire APUs, we have you covered on Integrated Enterprises.