Aviation glossary
What is an Auxiliary Power Unit (APU)?
Aircraft systems · Aviation technology · APU · Ground operations · Redundancy
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Definition
An Auxiliary Power Unit (APU) is a compact turbine engine installed in aircraft, primarily to provide electrical power and bleed air for systems operation independent of the main engines. It enhances aircraft autonomy, safety, and operational efficiency, especially during ground phases and as a backup in select in-flight situations.
Auxiliary Power Unit (APU) – Aviation Glossary
Definition and Core Purpose
An Auxiliary Power Unit (APU) is a compact, self-contained gas turbine engine installed on most modern aircraft, providing electrical power and pneumatic (bleed) air independently of the main propulsion engines. Its core purpose is to enable the operation of aircraft systems, such as avionics, lighting, air conditioning, and engine starters, during ground operations, pre-flight, and select in-flight scenarios, without external support equipment.
APUs are an essential component for operational autonomy, supporting system functionality during pre-flight preparation, passenger boarding, maintenance, and engine start procedures. Their gas turbine design is valued for high reliability, rapid power delivery, and an excellent power-to-weight ratio.
Key Features:
- Independence: Operates autonomously, reducing reliance on airport ground power or air carts.
- Redundancy: Provides backup power and air for critical systems in the event of main engine or generator failure.
- Safety: Equipped with fire detection, suppression, and automatic shutdown for abnormal conditions.
- Regulatory Compliance: Designed according to ICAO, FAA, and EASA standards for environmental, safety, and operational performance.
Primary Functions and Technical Principles
Electrical Power Generation
The APU’s integrated generator supplies alternating current (AC) power, typically 115V at 400 Hz, to energize:
- Avionics and flight instruments
- Cabin and exterior lighting
- Passenger comfort systems (galley, entertainment)
- Aircraft maintenance and ground servicing equipment
Some APUs also deliver direct current (DC) power (28V) for specific systems, either directly or through transformer-rectifier units (TRUs).
Technical Notes:
- The generator is driven by the APU’s main shaft.
- Output is automatically regulated for voltage and frequency stability.
- Power can be distributed to all or select electrical buses as needed.
Pneumatic (Bleed) Air Supply
The APU’s compressor delivers high-pressure, high-flow bleed air to:
- Environmental Control System (ECS): For cabin air conditioning and pressurization.
- Engine Start System: Powers the air turbine starter to spin main engines before ignition.
- Occasionally, Anti-Ice Systems: Supplies bleed air for wing or engine anti-icing in some aircraft.
Parameters:
- Typical bleed air output: 250–500 lbs/min at 30–45 psi
- System includes pressure regulation, temperature control, and non-return valves for safety
Hydraulic Power (in Select Aircraft)
Some APUs, mainly in large commercial or military aircraft, drive hydraulic pumps for ground operation of:
- Flight control surfaces
- Landing gear
- Cargo doors
Aircraft Installation and Operational Deployment
Physical Location
The APU is typically located in the tail cone or rear fuselage to:
- Mitigate noise and vibration in passenger areas
- Isolate the unit from fuel tanks and critical systems
- Simplify maintenance access
Some smaller aircraft may locate the APU in an engine nacelle, wing root, or landing gear bay.
Typical Operational Scenarios
Ground Operations:
- Pre-flight: Starts before passenger boarding for system and cabin preparation.
- Maintenance: Powers electrical and pneumatic systems for checks and repairs.
- Engine Start: Provides bleed air to spin main engines for ignition.
Remote Operations:
- Critical at airports lacking ground power or air carts
In-Flight Operations:
- In APUs certified for in-flight use, provides emergency backup for electrical and pneumatic systems
Transition Scenarios:
- Bridges power supply gaps during gate changes or pushback
Maintenance Protocols and Reliability
Routine Maintenance
- Scheduled Inspections: Oil, fuel, air, and electrical systems checked regularly
- Component Replacement: Bearings, starter motors, sensors, and filters changed per manufacturer schedule
- Performance Testing: Electrical and pneumatic output, emission monitoring
- Documentation: All actions logged for regulatory compliance
Reliability
- Modern APUs have mean time between failures (MTBF) of 5,000–10,000+ hours
- Equipped with redundant controls, fire suppression, and fail-safe shutdowns
- Maintenance intervals and procedures governed by FAA, EASA, and OEM guidelines
Environmental and Efficiency Considerations
Emissions and Noise
- APUs emit CO₂, NOₓ, hydrocarbons, and particulates
- Typical noise output: 85–95 dB(A) at close range
- Subject to ICAO Annex 16 and local airport restrictions
Efficiency and Sustainability
- Modern APUs utilize low-emission combustors and digital controls
- Ground Power Units (GPUs) and Pre-Conditioned Air (PCA) at gates reduce APU runtime
- Airlines adopt policies limiting APU use to cut costs and emissions
Common Aviation and Industry Applications
Commercial Airliners:
Standard on jets like Boeing 737/787, Airbus A320/A350, enabling full operational autonomy worldwide.
Business Jets:
Support for private and remote operations with limited ground services.
Military Aircraft:
Field operations, redundancy, and ground system power, some drive hydraulic pumps.
Helicopters:
Medium/large models use APUs for ground power and environmental control.
Other Sectors:
Military vehicles, marine vessels, spacecraft (e.g., Space Shuttle), refrigerated transport, and ground support equipment.
Typical Use Cases: APU in Action
- Turnaround Operations: Maintains systems and comfort during boarding/disembarkation, powers engine start.
- Remote Airfields: Ensures autonomy where no ground power exists.
- In-Flight Backup: Restores critical systems after main generator failure (certified APUs).
- Military Deployment: Enables rapid readiness and maintenance in the field.
- Ground Maintenance: Powers system checks without main engine operation.
Technical Parameters and Specifications
| Parameter | Typical Value (Commercial Jet) | Description |
|---|---|---|
| Electrical Output | 40–120 kVA, 115V AC, 400 Hz | Power for all electrical systems |
| Bleed Air Output | 250–500 lbs/min at 30–45 psi | For ECS, engine starting, anti-icing |
| Fuel Consumption | 100–400 liters/hour (26–106 US gal/hr) | Depends on system load and ambient conditions |
| Startup Time | 60–120 seconds | From startup to operational readiness |
| Operating Altitude | Up to 30,000 ft (if certified) | In-flight operation capability |
| Weight | 150–350 kg (330–770 lbs) | Varies by model and aircraft type |
| Location | Tail cone/rear fuselage (typical) | For noise, safety, and access |
APU vs. Ground Support Equipment
| Function | APU | Ground Support Equipment |
|---|---|---|
| Independence | Fully autonomous | Needs airport infrastructure |
| Power Source | Onboard jet fuel | External electricity or diesel |
| Use Case | Remote locations, redundancy | Major airports, emission reduction |
| Operational Cost | Higher (fuel, maintenance) | Lower (grid power) |
| Environmental Impact | Higher (emissions, noise) | Lower (if electric/grid powered) |
| Flexibility | Immediate availability | Dependent on ground resources |
Maintenance Best Practices
- Adherence to Schedules: Follow OEM and regulatory procedures strictly
- Comprehensive Records: Log all maintenance and replacements
- Predictive Monitoring: Use vibration, oil analysis for early failure detection
- Certified Technicians: Only trained aviation personnel may perform APU work
- Post-Maintenance Checks: Full operational tests after major interventions
Environmental and Regulatory Developments
- Noise Abatement: Airports may restrict APU use at gates, enforcing connections to GPUs and PCA within minutes of arrival
- Emission Controls: New APU models certified to strict standards, airlines limit runtime for sustainability
- Operational Policy: Crew training and SOPs increasingly emphasize APU minimization to reduce environmental footprint
Summary Table: APU Usage and Capabilities
| Application Area | Function Provided | Example Scenario |
|---|---|---|
| Electrical Systems | 115V AC/28V DC for avionics, etc. | Night-time preflight at a remote airfield |
| Engine Start | Bleed air for starter turbines | Main engine start without ground air cart |
| Cabin Climate Control | Bleed air to ECS | Boarding in extreme weather |
| Emergency Power | Backup electricity and air | Generator failure in flight |
| Maintenance | System testing | Hangar checks without main engines |
| Military/Industrial | Power for systems, mobility | Armored vehicle silent watch, refrigerated truck |
Glossary: Key Terms Related to APUs
| Term | Definition |
|---|---|
| APU | Auxiliary Power Unit, a small gas turbine supplying electrical and pneumatic power independently. |
| Bleed Air | Compressed air from a turbine’s compressor, used for ECS, engine start, and anti-icing. |
| ECS | Environmental Control System, manages cabin temperature, humidity, pressurization. |
| GPU | Ground Power Unit, external device supplying electrical power to the aircraft on the ground. |
| PCA | Pre-Conditioned Air, external system providing cabin heating or cooling at the gate. |
| ETOPS/EDTO | Extended-range Twin-engine Operations/Extended Diversion Time Operations, rules for long-range flights. |
Related Resources:
Frequently Asked Questions
- What is the main purpose of an APU in aircraft?
- The main purpose of an APU is to provide electrical power and pneumatic (bleed) air for onboard systems when the main engines are not running. This allows aircraft systems, such as avionics, cabin lighting, air conditioning, and engine starters, to function independently during ground operations and, in some cases, in-flight.
- Where is the APU typically located on an aircraft?
- The APU is most commonly installed in the tail cone or rear fuselage of commercial aircraft. This location minimizes noise and vibration in passenger areas, isolates the unit for safety, and provides easy maintenance access.
- Can the APU be operated during flight?
- Some APUs are certified for in-flight operation, especially in twin-engine aircraft requiring redundancy for extended-range operations (ETOPS/EDTO). In-flight use is primarily for backup power and pneumatic supply following a main system failure.
- How does the APU start and shut down?
- The APU is started using onboard batteries or external power, spinning the turbine via a starter motor until combustion stabilizes. Shutdown involves a cooldown cycle before fuel and ignition are cut off, managed automatically by the control system.
- What are the environmental impacts of APU operation?
- APUs emit carbon dioxide (CO₂), nitrogen oxides (NOₓ), and noise, contributing to ramp emissions and airport noise pollution. Regulations restrict APU use at many airports, and newer APUs are designed to be more efficient and cleaner.
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