APU: Independent Power and Air

When your aircraft sits at the gate with engines off, what keeps the lights on, the air conditioning running, and the computers powered? When you need to start engines without ground support, what provides the compressed air? The answer lies in a remarkable piece of engineering tucked away in the tail: the Auxiliary Power Unit (APU). The APU solves a fundamental problem—how to make your aircraft completely self-sufficient for power generation, whether electrical or pneumatic, independent of engines or ground equipment.

How the APU Works

Think of the APU as a small jet engine dedicated entirely to serving your aircraft's needs. This single-shaft gas turbine operates on the same principles as your main engines but with a different mission. Instead of producing thrust, it drives an accessory gearbox that powers an electrical generator and produces high-pressure bleed air for pneumatic systems.

The APU's design reflects its dual purpose. The gas turbine core generates mechanical shaft power that drives the electrical generator through the accessory gearbox, while simultaneously producing bleed air from its load compressor. This means one compact unit can replace ground power units and air carts, making your aircraft truly independent.

Normally, the operating speed is 100 % RPM, but the system intelligently adjusts to 99 % when providing air conditioning in moderate temperatures (-18 °C to 35 °C). This slight reduction optimizes performance for air conditioning loads while maintaining adequate electrical generation. Outside this temperature range, the APU returns to 100 % speed to ensure maximum capability.

The Electronic Control Box: The APU's Brain

The Electronic Control Box (ECB) serves as the APU's full-authority digital controller, managing every aspect of operation from start to shutdown. This sophisticated computer sequences the start process, continuously monitors critical parameters like speed and temperature, and makes split-second decisions about system protection.

During startup, the ECB orchestrates a precise sequence: it opens the air intake flap, activates the fuel system, engages the starter, and initiates ignition at exactly the right moment. Throughout operation, it monitors exhaust gas temperature, oil pressure and temperature, and bleed air parameters. If anything goes wrong, the ECB can execute an automatic shutdown faster than any pilot response.

The ECB also manages the Inlet Guide Vanes (IGVs), which control bleed air flow. Using a fuel-pressure-powered actuator, it adjusts these vanes based on aircraft operational needs, ensuring optimal bleed air delivery whether you're starting engines or running air conditioning packs.

APU Systems Integration

The APU's fuel system connects to the left fuel feed line, with tank pumps normally providing the required pressure. However, the system includes its own APU fuel pump that automatically activates when tank pump pressure is unavailable—crucial during battery-only operations when main fuel pumps might be offline.

The independent oil system handles both lubrication and cooling, ensuring the APU can operate reliably throughout its speed range. This self-contained lubrication system means the APU doesn't depend on any engine or aircraft oil systems.

For air intake, an electrically operated flap directs external air to the compressor inlet. The ECB controls this flap's operation, ensuring it's fully open before starter engagement and properly closed during shutdown sequences.

Starting and Shutdown Sequences

APU starting follows a logical progression. When you press the MASTER switch, electrical power flows to the APU system and the ECB performs its power-up test. The air intake flap opens, the fuel isolation valve opens, and the APU fuel pump activates if no fuel tank pump is running. After pressing START, the system waits for the intake flap to fully open before energizing the starter.

Ignition begins 1.5 s after starter engagement, with the starter disengaging at 55 % RPM or 60 % on some aircraft. The ON light extinguishes when the APU reaches 95 % RPM, and the AVAIL light illuminates when the APU exceeds 95 % or 2 s after reaching this speed—confirming the APU is ready to supply power.

Manual shutdown is equally systematic. When you press the MASTER switch to initiate shutdown, the system considers the current bleed air usage. If APU bleed air was supplying aircraft systems, the APU continues running for a cooling period of 60 to 120 s before beginning shutdown. The air inlet flap closes when the APU speed drops to 7 % RPM.

Automatic Protection Systems

The APU incorporates extensive automatic shutdown protection, recognizing that it often operates unattended. Automatic shutdowns occur for: fire detection (ground only), air inlet flap closure, overspeed, slow start or failure to accelerate, EGT overtemperature, flame loss or reverse flow, low oil pressure or high oil temperature, and various sensor or component failures.

When an automatic shutdown occurs, the ECAM displays "APU AUTO (EMER) SHUT DOWN." For non-emergency shutdowns, the air inlet flap remains open for 15 min after APU speed drops below 7 % (this timer starts at liftoff when airborne).

Fire protection deserves special attention. On the ground, if the APU fire detection system activates, the sequence is completely automatic: fire warnings activate in the cockpit and nose gear bay, the APU automatically shuts down, and the fire extinguisher discharges without crew intervention. This automation ensures fire suppression even when the cockpit is unattended.

Operational Capabilities and Limitations

The APU operates throughout the normal flight envelope, providing backup electrical power and air conditioning assistance in flight. For engine starting, it can provide bleed air up to 20 000 ft. For air conditioning and pressurization, limits depend on pack configuration: single pack operation up to 22 500 ft, dual pack operation up to 15 000 ft.

Important limitation: APU bleed air cannot be used for wing anti-ice. This restriction exists because the APU's bleed air capacity, while adequate for air conditioning and engine starting, cannot meet the high flow requirements of wing anti-ice systems.

Starting limitations include a maximum of three consecutive start attempts with a 60-minute waiting period before new attempts. Maximum rotor speed is limited to 107 %. During refueling or defueling operations, if the APU fails to start or undergoes automatic shutdown, do not attempt a restart.

Electrical Integration

In the electrical system hierarchy, the APU generator integrates seamlessly with other power sources. When APU generator parameters are normal and external power isn't connected, the APU generator field activates and its line contactor closes. If engine generators aren't operating, the bus tie contactor automatically closes to supply both AC buses.

The system follows a clear priority: engine generators take precedence over APU generator, which takes precedence over external power. This hierarchy ensures the most reliable power source always supplies the aircraft while maintaining automatic switching between sources.

What This Means for You

Understanding your APU transforms it from a mysterious box in the tail to a sophisticated, reliable partner in aircraft operations. Its automatic protection systems and intelligent control mean you can trust it to operate safely while focusing on other flight duties. The systematic startup and shutdown sequences reflect careful engineering to ensure reliability and longevity.

The APU's independence makes your aircraft truly self-sufficient—no longer dependent on ground equipment for power or air. Whether starting engines at a remote airport or providing backup power during flight, the APU delivers the electrical and pneumatic power your aircraft needs, when it needs it, with remarkable reliability and sophistication.

our complete A320 APU guide – how it starts, what it powers, ECB control logic, automatic shutdown protection, and operational limits pilots need to know.