Introduction to Aircraft Auxiliary Power Units

The aircraft auxiliary power unit (APU) is a compact, self-contained gas turbine engine that delivers essential power when the main engines are shut down. Found in the tail cone or rear fuselage of most commercial and many business jet aircraft, the APU enables ground operations without external ground power units or air conditioning carts. By providing electrical power, pneumatic bleed air for air conditioning and engine starting, and sometimes hydraulic pressure, the APU greatly improves operational flexibility and passenger comfort. Understanding the design, function, and required maintenance of APUs is critical for fleet operators, maintenance engineers, and pilots alike.

Modern aviation relies on APUs not only for ground convenience but also as a backup during flight in certain failure scenarios. For example, some aircraft can use their APU in flight to supply an emergency electrical source if both main generators fail. Because APUs operate under demanding conditions—frequent start cycles, high vibrations, and exposure to contaminants—they demand rigorous maintenance practices. This article provides an expanded look at APU functions, operational principles, maintenance best practices, common failure modes, and safety considerations.

What Is an Auxiliary Power Unit?

An auxiliary power unit (APU) is a small gas turbine engine that runs independently of the aircraft’s main engines. Typically housed in a fireproof compartment in the tail section (or occasionally in the fuselage or wing root), the APU comprises a power section (compressor, combustor, turbine) and accessory gearbox. The gearbox drives an electrical generator, a hydraulic pump (if fitted), and a load compressor that supplies bleed air. The APU is started by its own battery or via the aircraft’s main battery and uses a small starter motor.

The APU’s design prioritizes compactness, reliability, and low weight. While main engines are optimized for thrust, the APU is optimized for shaft power extraction and bleed air delivery. Most APUs operate at a constant high speed (typically 100% RPM) once started, with output modulated by fuel flow and inlet guide vanes. Modern APUs feature full-authority digital engine control (FADEC) for precise management of start sequences, speed, and load scheduling.

Key manufacturers include Honeywell (e.g., 131-9 series), Pratt & Whitney Canada (e.g., PW901A), and APIC (Auxiliary Power International Corporation, now part of Safran). Each model is tailored to a specific aircraft type, with variations in power output, bleed air capacity, and altitude start capability.

Historical Development

The first APUs appeared in the 1950s on large piston‑engine aircraft to provide cabin air conditioning on the ground. Early jet transports, like the Boeing 707 and Douglas DC‑8, used APUs primarily for electrical power. With the introduction of the Boeing 747 in 1970, APUs became standard on wide‑body aircraft to handle the increased electrical and pneumatic loads. Since then, APU technology has advanced significantly: today’s units are lighter, more fuel‑efficient, and produce lower emissions. The shift to more‑electric aircraft architecture is also influencing APU designs, with some newer APUs acting as electrical generators rather than pneumatic sources.

Primary Functions of APUs

APUs serve several critical roles throughout the aircraft’s operational cycle. Below is a detailed breakdown of each function.

Electrical Power Supply

The APU drives a generator that supplies 115 V AC or 28 V DC power to the aircraft’s electrical bus. While on the ground, this powers cockpit avionics, cabin lighting, galley equipment, in‑flight entertainment systems, and battery charging. In flight, the APU generator can serve as a backup source if main engine generators or the auxiliary power unit itself is already running. Some aircraft also use the APU generator as a primary power source during single‑engine taxi operations, reducing main engine running time and fuel burn.

Air Conditioning and Cabin Pressurization

Bleed air from the APU’s load compressor is ducted to the aircraft’s environmental control system (ECS). The ECS conditions the air (heating, cooling, dehumidifying) and delivers it to the cabin at the correct pressure. This is especially important on hot days when the cabin can quickly become uncomfortable without air conditioning. Pneumatic power from the APU also maintains differential pressure during ground pressurization checks.

Engine Start Assistance

One of the most valuable APU functions is providing bleed air to the main engine air starters. By starting the APU first, the flight crew can initiate main engine starts without ground carts. The high‑volume bleed air spins the main engine’s turbine up to ignition speed. This capability dramatically reduces turnaround times and eliminates dependencies on airport ground equipment.

Hydraulic Power (Selected Models)

Some APUs (e.g., on the Airbus A380 and certain military transports) include a hydraulic pump mounted on the accessory gearbox. This pump can pressurize the aircraft’s hydraulic system for cargo door operation, landing gear retraction tests, or flight control checks while on the ground. However, most commercial aircraft rely on electric motor‑pumps or engine‑driven pumps for hydraulic power, with the APU supplying electricity to them instead.

In‑Flight Backup Power

Regulatory requirements (FAR 25.1351) mandate that transport category aircraft have an independent emergency power source. The APU meets this requirement on many types. If both main generators fail, the APU can be started in flight (if altitude permits) to restore electrical and pneumatic power. This feature is a vital safety net, especially during extended twin‑engine operations (ETOPS) where diversion airports may be far away.

APU Maintenance Tips and Best Practices

Proper APU maintenance is essential to maximize reliability, avoid unplanned ground times, and control costs. Below are expanded maintenance guidelines organized by system.

Routine Inspections

Visual inspections should be performed at every line maintenance check. Look for oil leaks at seals, gaskets, and drain tubes. Inspect the exhaust duct for cracks or soot deposits that indicate poor combustion. Check all electrical connectors for corrosion or loose wires. Pay special attention to the air intake and exhaust areas—foreign object debris (FOD) or bird nests can cause catastrophic damage. Schedule a borescope inspection of the compressor and turbine blades at manufacturer‑recommended intervals to detect erosion, fouling, or foreign object damage (FOD) early.

Oil and Filter Changes

APU lubricating oil operates under high temperature and high stress. Follow the Airframe Maintenance Manual (AMM) for oil brand, type, and level checks. Most APUs require oil level checks every 25 to 50 operating hours, with full oil changes every 500 to 1,000 hours depending on the model. Replace the oil filter at every oil change. A clogged or bypassed oil filter can lead to bearing failure. Use approved synthetic oils (e.g., Mobil Jet Oil II, BP Turbo Oil 2380) to ensure thermal stability and wear protection.

Fuel System Maintenance

The APU fuel system includes a fuel control unit (FCU), fuel manifold, nozzles, and drain valves. Contaminated fuel or water in fuel can cause nozzle coking, flameouts, or fuel control malfunctions. Replace fuel filters on schedule and drain water from the APU fuel filter bowl daily. Perform fuel nozzle flow checks and cleaning after major overhauls. Check for leaks around the FCU and fuel lines regularly, as fires are a serious risk.

Ignition and Start System

The APU starter motor and ignition exciter are subject to high electrical loads. Inspect starter brushes (if applicable) and commutator condition. Check ignition leads for chafing or arcing. Test the start sequence periodically—watch for abnormal trends like longer start times, higher exhaust gas temperature (EGT) peaks, or starter cut‑out failures. Such trends often indicate a degrading battery, starter motor, or fuel system.

Bleed Air System and Load Compressor

The load compressor and associated ducting must be checked for leaks, cracks, and proper valve operation. The bleed air valve (also called the APU bleed air shutoff valve) should be tested for leakage when closed and for correct flow when open. Inspect the surge control valves and inlet guide vane actuators for smooth movement. Bleed air leaks can cause overheating in the tail cone and damage surrounding structures.

Software and Electronic Controls

Modern APU FADEC systems receive periodic software updates from the manufacturer to improve performance, fix bugs, or address safety issues. These updates are typically installed during heavy maintenance checks. Ensure that the aircraft’s central maintenance computer and APU controller have matching software versions. Record software part numbers in the technical log. Additionally, verify that the APU’s electronic control unit (ECU) is free from fault history—clear nuisance messages after verifying the cause.

Operational Checks

Run the APU through a full start‑and‑load cycle at least once per maintenance visit. Monitor start time, EGT peak, and steady‑state speed. Then load the APU by drawing electrical power (turning on galley loads or using a load bank) and/or opening bleed air (selecting packs ON). Check that the load compressor speed remains stable. Listen for abnormal noises—whistling, grinding, or surging. After shutdown, let the APU cool down per the AMM to prevent thermal shock to bearings and seals.

Record Keeping

Maintain a detailed log of all APU events: starts, operating hours, cycles, fuel flow, oil consumption, and any unscheduled maintenance. Many operators use electronic engine monitoring systems (EEMS) to track trends. Use this data to predict component life and schedule overhauls. Keep copies of all airworthiness directives (ADs) and service bulletins (SBs) related to the APU model. Regulatory compliance requires traceability for every repair and modification.

Common APU Failure Modes and Troubleshooting

Understanding typical APU failures helps maintenance teams respond faster. Below are the most frequent issues:

  • Hard starts/starter failures: Often caused by weak batteries, sticking starter motor contacts, or low oil viscosity in cold weather. Solution: inspect battery voltage, clean starter contacts, and use winter‑grade oil if applicable.
  • Hot starts (EGT exceedance): Results from delayed ignition, incorrect fuel flow, or insufficient starter torque. Shut down immediately to avoid turbine damage. Investigate fuel nozzles, FCU calibration, and ignitor condition.
  • Oil leaks: Common at the oil filler cap, sump drain plug, or bearing seals. Small leaks can become large if left unchecked. Replace seals and torque fasteners to specification.
  • Bleed air valve failures: The valve may fail open (bleed air cannot be shut off) or fail closed (no bleed air output). Actuator failure, sticking poppet, or electrical fault. Clean or replace the valve and actuator.
  • FADEC fault codes: Many fault codes point to sensor failures (EGT thermocouple, speed probes, oil pressure switch). Swap sensors and update software per the troubleshooting chart.
  • Foreign object damage (FOD): Birds, debris, or ice entering the APU inlet can destroy compressor blades. Install inlet screen (if available) and inspect regularly.

Safety Considerations for APU Operation and Maintenance

APUs contain high‑speed rotating parts, flammable fuel, and hot exhaust. Adhere to these safety rules:

  • Grounding: Ensure the aircraft is properly grounded before starting the APU to prevent static discharge or spark ignition.
  • Personal protective equipment (PPE): Wear hearing protection (APUs are loud), safety glasses, and fire‑resistant clothing when working in the APU compartment.
  • Fire hazards: Keep a CO₂ or dry chemical fire extinguisher within reach. Never use water on an APU fire. Know the location of the APU fire switch in the cockpit and the ground‑level fire‑handle (if equipped).
  • Hot surfaces: Allow the APU to cool for at least 5 minutes after shutdown before touching any components. Exhaust duct temperatures can exceed 600°C (1100°F).
  • Lockout/tagout: Before performing maintenance, de‑energize the APU and install a lock on the start switch in the cockpit. Ensure the APU battery is disconnected.
  • Fuel isolation: Close the APU fuel shutoff valve at the wing tank or feed line before working on fuel components. Place a warning tag.
  • Confined space: The APU bay is a confined space with limited access. Ensure adequate ventilation and use a partner when entering.

External Resources for Further Reading

To deepen your understanding of APU systems and maintenance, consider the following authoritative sources:

Conclusion

The auxiliary power unit remains a cornerstone of modern aircraft ground operations and in‑flight backup capability. From providing electrical and pneumatic power to enabling rapid engine starts, the APU reduces reliance on ground support equipment and improves dispatch reliability. However, this sophisticated turbine requires diligent maintenance to keep it airworthy and cost‑effective. By following the inspection, oil, fuel, and electronic control best practices outlined in this article, operators can avoid many common failures, extend overhaul intervals, and maintain the highest safety standards. As aircraft electrical systems become more demanding, the APU’s role will continue to evolve—making ongoing training and adherence to manufacturer guidance more important than ever.