APU stands for auxiliary power unit, a small gas-turbine engine mounted in the tail cone of the aircraft, usually between the horizontal and vertical stabilisers on the right side. It provides electrical power and compressed bleed air and thus allows the aircraft to start and power itself independently of ground services.
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Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is APU in aviation?
APU stands for auxiliary power unit, which is a device on a vehicle that provides energy for functions other than propulsion. An auxiliary power unit is commonly found on large aircraft and is a small engine.
In aviation the Auxiliary Power Unit is a small gas-turbine engine carried on the aircraft so that the machine can operate autonomously on the ground and in the air. Built as a single- or two-shaft high-pressure-ratio core, the APU functions simultaneously as a jet engine, a generator and a pneumatic source: its compressor supplies bleed air for cabin conditioning and for starting the main engines, while an attached generator furnishes 28 V DC or 90 kVA of electrical power to the whole aircraft. Because the APU is independent of the main engines and starts using only the aircraft battery, it is one of the most pivotal systems for safety and operational efficiency and its failure grounds a flight completely.
What is the purpose of an APU in an aircraft?
The purpose of an Auxiliary Power Unit is to serve as an independent power source for the aircraft. On the ground it supplies electrical power to avionics, lighting, and system backup, and provides air conditioning and cabin pressurization without reliance on ground power unit or external air-conditioning unit. APU typically supplies electricity to cockpit avionics, weather radars, flight data recorders, and galley electrics, and drives a generator that can provide power through single or three-phase systems. APU creates enough power to operate vital systems while the aircraft is parked at the gate, and pre-conditions the cabin before passenger boarding. APU enables aircraft to operate autonomously and to start main engines independently of ground services by supplying bleed air to start the main engines. The load compressor provides pneumatic power for engine start. APU acts as a useful backup to some services provided by the main engine and can be used as an emergency electrical power source while airborne to maintain flight controls and navigation in case of main generator failure.
Where is the APU located in an aircraft?
The APU is normally located in the tail cone of the aircraft and is often mounted in the aft fuselage. APU is usually identified by the exhaust pipe at the aircraft tail. APU air intake is generally on the upper left portion of the aft fuselage near the base of the vertical stabilizer. APU air inlet door is located between the horizontal and vertical stabilisers on the right side of the airplane. Some aircraft have a door on the top of the fuselage that opens when APU is running. APU exhaust gases exit on the right side of the aft fuselage. One APU is sufficient for all electrical and pneumatic demands. The Auxiliary Power Unit’s location allows direct link to necessary infrastructures like electrical systems and air conditioning bundles. This isolation assures any possible events with the APU do not endanger main power plants.
How does an airplane APU work?
The APU works in the same way as a conventional jet engine. It draws air, compresses it, mixes it with fuel, ignites it, drives the turbine, and produces a hot compressed exhaust gas. APU uses axial flow power from the turbine to create centrifugal power from the compressor. It utilizes a combination of air and fuel to generate power. APU uses a two-stage centrifugal flow compressor and a reverse flow annular combustion chamber. Fuel, the same kind as used in main engines, is introduced into the APU once it is spinning fast enough. It uses a fuel control unit and a fuel solenoid valve. APU ignites the air-fuel mixture. The hot compressed exhaust gas is directed to another turbine. APU uses a three-stage axial flow turbine. The APU's turbine drives a generator and a compressor. The compressor provides bleed air which is used for engine start and air conditioning. The extra electrical generator creates enough power to operate cockpit avionics, galley electrics, and onboard lighting. The generator provides electrical power, supplying 28 V DC.
When the APU is started with the engines off, electrical power from a ground connection is used. Battery or ground power cart spins up the APU by its electric starter motor. The APU start switch is briefly moved to the START position to initiate the automated start procedure. The startup process begins as soon as the air inlet door is fully open. The fuel solenoid valve opens when the APU is started.
What are the components of an auxiliary power unit?
The components of an auxiliary power unit are listed below.
- Power Section (Gas Turbine): It is the core engine comprising a compressor (single or multi-stage) for combustion air, a combustion chamber, and a turbine to drive the compressor and gearbox.
- Accessory Gearbox: It transfers power from the main shaft to drive components such as the fuel pump, oil pumps, cooling fan, and starter/generator.
- Load Compressor (Bleed Air System): It is a dedicated compressor, or a tapping from the main compressor, that provides pneumatic power for air conditioning, cabin pressurization, and main engine starting.
- Electrical Generator/Alternator: It is driven by the gearbox to generate AC electrical power for aircraft systems.
- Starter Motor: It is a motor, often a starter/generator combination, designed to initiate the APU's rotation during startup.
- Control Systems (ECU/GCU): These include electronic components like the Electronic Control Unit (ECU) and Generator Control Unit (GCU) that manage starting, monitoring, and automated shutdown sequences (e.g., at 60% speed).
- Lubrication and Fuel System: They provide lubrication to bearings and fuel to the combustion chamber.
The power section is the gas-generator portion of the engine; it contains a small gas turbine engine with a reverse-flow combustion chamber, a single-stage centrifugal compressor and a two-stage power turbine. A tiny inlet flap allows air to reach the first-stage turbine, while air pressure and air-temperature sensors at each stage provide control data. The load compressor is a shaft-mounted compressor that creates centrifugal power, delivering compressed air for pneumatic systems on the aircraft. The compressor stage also provides energy for onboard electrics. The gearbox transmits turbine power through a central shaft that spins fast. Within the gearbox power is transferred to engine accessories like the fuel-control unit, lubrication module and cooling fan. An extra electrical generator is driven by the same shaft via the gearbox; the generator creates enough power to operate cockpit avionics, onboard lighting and galley electrics. A starter motor connected through the gear train performs the starting function of the APU. Nickel Cadmium batteries are used to start the APU when the aircraft is on the ground.
What are aircraft APU specifications?
The aircraft APU specifications are given in the table below.
| Specification | Value |
|---|---|
| AC Voltage | 115 V |
| Frequency | 400 Hz |
| DC Voltage | 28 V |
| Bleed Airflow | 1.8 kg/sec (2.6 lb/sec) |
| Electrical Power | 90 kVA (APS2600), 450 kVA (APS3200 at sea level) |
| Shaft Horsepower | 399.7 kW (536 HP) |
| Operating Altitude | Sea level to 11,887 m (2,000 ft to 41,000 ft) |
| Weight | 136 kg (299 lbs) (dry) |
| Rotor Speed | 49,300 RPM (100%) |
| Overspeed Shutdown | 105% (51,765 RPM) |
| Fuel Consumption | 148 kg/hr (326.2 lb/hr) (178 L/hr) (392.4 gal/hr) |
| Bleed Pressure | 290 kPag (42 PSIG) |
| Certification | 180-minute ETOPS operation |
What are the types of APU in aircraft?
The types of APU in aircraft are listed below.
- Cylindrical APU
- Cylindrical-annular APU
- Annular APU
- Reverse-flow APU
APU types include cylindrical-type, cylindrical-annular type, annular type, and reverse-flow type. Can-type combustion chamber is typical of the type used on turboshaft and APUs. Some APU are piston-driven versions entering the market, while APS2300 APU is an integral bleed gas turbine engine that incorporates a two-stage axial turbine. APUs can provide power through single or three-phase systems. A shaft-mounted load compressor extracts bleed air for cabin conditioning and main engine starting, and concurrently delivers 90 kVA of electrical power.
Which aircraft come with an APU?
All modern jets have their own APUs. Some turboprops have their own APUs as well. The Boeing 727 was the first aircraft equipped with an APU, thus introducing the first commercial APU for a jetliner in 1963, and allowing airlines to avoid reliance on ground power at smaller airports. The turbine was included in all subsequent Boeing aircraft, including the Boeing 737, Boeing 747 families, Boeing 767 whose APU is used for ground power, and Boeing 777 whose APU can be run both on ground and in flight, a capability required by ETOPS regulations. The Boeing 787 Dreamliner is fitted with the APS5000 APU, which delivers only electricity to the aircraft. The Boeing 707 and Douglas DC-8 did not have an APU and their operation relied on ground services or high-pressure air bottles.
The Honeywell 331 is used as an APU in many Boeing and Airbus wide-body jets, and the HGT1700 flies on the Airbus A350. Regional jets and business aircraft carry APUs: the APS2300 is installed in Embraer's fleet of E170/175/190/195 regional jets and in Embraer Lineage 1000 business jets, while a 260 kW unit has already been selected for the Gulfstream V and Bombardier Global Express. The Tupolev Tu-204 will be offered with the AlliedSignal 331-200ER APU. The 36 series is fitted to a wide range of narrow-body airliners, regional jets, and turboprops.
Large American military aircraft were fitted with APUs during World War II. The Supermarine Nighthawk was one of the first military fixed-wing aircraft to use an APU. Larger modern jet fighters are usually equipped with a gas-turbine APU for engine start, whereas the F-16 does not have an APU and instead carries an emergency power unit (EPU). Helicopters like the CH-47D Chinook use an APU. Safran produces APUs for helicopters and business jets.
