Long before an airliner’s main engines start, another gas turbine may already be running. The Auxiliary Power Unit, or APU, is a compact turbine engine installed primarily to provide electrical and pneumatic power when the main engines are unavailable or when using them would be inefficient. On many commercial airliners it is mounted in the tail cone, although exact location and capability vary by aircraft type. The FAA defines an APU as a gas-turbine unit used to supply auxiliary power, while EASA maintains a dedicated certification specification, CS-APU, for these machines. [1] [2]
The APU is not a miniature third propulsion engine in the normal sense. It does not usually provide meaningful thrust to move the aircraft through the air. Instead, it converts fuel energy into shaft power and hot compressed air. Those outputs can be used to run an electrical generator, supply pneumatic air for air-conditioning packs, and provide the compressed-air source needed to start the main engines. Depending on aircraft design, the APU can also serve as a backup source in flight. [3]
Why an airliner needs an APU
An aircraft parked at the gate still needs electricity. Flight-deck displays, cabin lighting, avionics, fuel pumps, ventilation, galley equipment and many other systems require power before the main engines are started. Airports can provide external ground power, but an airliner also needs an independent onboard source for locations where suitable ground support is unavailable or for phases of operation in which ground equipment has been disconnected. [1]
The APU gives the aircraft that independence. The same unit can often provide both electrical and pneumatic energy, which makes it especially useful during the transition from a parked, externally powered aircraft to a self-sufficient aircraft ready to start its engines and taxi. The precise loads that may be supplied at any moment are defined by the aircraft’s electrical and pneumatic system design and operating procedures. [3]
An APU is a real gas turbine
Although physically smaller than a main turbofan, an APU uses the same broad gas-turbine principle. Air enters a compressor, fuel is added and burned in a combustor, and the hot gas expands through a turbine. The turbine extracts enough energy to drive the compressor and the accessory loads attached to the unit. The exhaust gas then leaves through the APU exhaust, which is commonly visible near the aircraft tail. [2]
The arrangement differs between APU models. Some use a single-spool architecture; others divide functions differently. The generator and pneumatic extraction capacity are sized around the aircraft’s requirements. EASA’s CS-APU certification framework covers mechanical integrity, operating limits, control systems, fire protection, overspeed protection, starting and other characteristics expected of an aircraft gas turbine. [2]
How the APU starts itself
An APU cannot produce power until its compressor and turbine are rotating fast enough for the combustion process to become self-sustaining. It therefore needs a starter. Depending on aircraft design, the initial starting energy can come from the aircraft battery, an external electrical source or another approved electrical supply. The starter accelerates the APU while the control system sequences fuel and ignition. [3]
Modern APUs normally use electronic control systems that monitor speed, temperature and other parameters throughout the start. EASA AMC 20-2B specifically addresses essential APUs equipped with electronic controls and requires failures of those controls, power supplies and interfaces to be assessed as part of both APU and aircraft certification. [3]
The APU generator
One of the APU’s most important jobs is generating electricity. The APU accessory gearbox drives an electrical generator that can feed the aircraft’s AC electrical system through contactors and buses controlled by the aircraft power-management architecture. Once the APU generator is online, the aircraft can operate independently of the airport’s ground-power unit. [1]
The generator does not simply connect directly to every electrical load. Modern airliners divide electrical power into buses and use protection, switching and load-shedding logic to ensure that failures do not unnecessarily remove power from essential systems. The APU generator is one source within that wider electrical network, alongside engine-driven generators, batteries, external power and, on some aircraft, emergency sources. [3]
Why the APU is useful before engine start
Main engines are powerful but inefficient devices to run purely for ground electrical supply. Starting them earlier than necessary would consume fuel, create noise, generate jet blast and increase engine operating time. The APU lets the aircraft remain electrically alive and climatically controlled while the main engines stay shut down until departure procedures require them. [1]
Airports and operators still prefer ground power where practical because an APU burns fuel and produces emissions. The presence of an APU therefore does not mean it runs continuously throughout every turnaround. Airlines may connect 400 Hz ground power and pre-conditioned air at the stand, then start the APU later as part of departure preparation. The exact policy is operator- and airport-specific.
Pneumatic power: compressed air as an energy source
Many conventional airliners use compressed air as a major secondary-power source. APU compressor air can be extracted through a bleed-air system and supplied to the aircraft pneumatic manifold. From there it can feed air-conditioning packs and, when required, the pneumatic starters used to rotate the main engines. [1]
This does not mean the APU sends its exhaust directly into the cabin. Pneumatic bleed is taken from compressed air within the APU before combustion products are exhausted. The air then passes through aircraft ducting and the environmental-control system. Cabin conditioning involves temperature control, cooling, mixing, pressure regulation and filtration processes separate from the APU itself.
Running the air-conditioning packs
On a bleed-air aircraft, the APU can supply the air-conditioning packs while the main engines are shut down. Pack valves regulate the flow into the environmental-control system, where heat exchangers and an air-cycle machine reduce the temperature before conditioned air is distributed through the cabin. This is why passengers can board a cool or heated aircraft before the engines start.
The APU’s pneumatic output is finite. Supplying several packs, starting an engine and generating a high electrical load simultaneously may exceed the permitted operating envelope, so aircraft systems and checklists sequence loads. The exact number of packs allowed during engine start, and whether an APU bleed valve must be opened or closed for particular phases, are aircraft-specific operating details. [2]
How the APU starts a main engine
A large turbofan cannot start from rest simply by introducing fuel and ignition. Its compressor must first be rotated to a sufficient speed. On many conventional airliners, high-pressure pneumatic air is directed through a starter air valve to an air-turbine starter attached to the engine accessory gearbox. The starter converts compressed-air energy into shaft rotation and accelerates the engine core.
The APU is a convenient source for that compressed air. During an ordinary ground start, APU bleed air flows through the aircraft pneumatic manifold to the selected engine starter. As the engine accelerates, fuel and ignition are introduced under the engine’s approved starting sequence. Once the engine reaches the point at which it can continue accelerating without starter assistance, the starter system disengages.
Cross-bleed starting
The APU is not the only possible pneumatic source on many aircraft. After one main engine is running, bleed air from that engine may be used to start another engine. This is known as a cross-bleed start. It can be useful when the APU is unavailable or in particular operational circumstances.
Cross-bleed starts often require a higher thrust setting on the operating engine to provide sufficient pneumatic pressure, which has implications for jet blast and ground safety. The exact procedure is aircraft-specific and is performed only according to the approved flightcrew documentation. The existence of cross-bleed capability does not remove the operational convenience of the APU.
Why the APU remains running during some departures
Although the APU may be shut down after the main engines have started, operators can keep it running for defined phases when required. One reason may be to provide additional electrical redundancy. Another may be to supply pneumatic loads so the main engines do not have to provide as much bleed air during a performance-critical phase such as take-off.
Whether that produces a performance benefit depends on aircraft design because using the APU also consumes fuel and may impose its own limits. Some aircraft procedures call for APU operation for particular system configurations, dispatch conditions or low-visibility operations. These are type- and operator-specific decisions rather than a universal airline rule. [3]
Can an APU run in flight?
Yes, many transport-aircraft APUs are approved for operation in flight, but capability can depend on altitude and the function being demanded. Electrical-generation limits may differ from pneumatic-bleed limits because providing compressed air places different demands on the APU compressor. The Aircraft Flight Manual defines the approved altitude envelope for the installed unit.
In-flight availability can make the APU an important backup source. If an engine-driven generator is lost, an APU generator may restore part or all of the normal electrical architecture. If pneumatic capability is available at the relevant altitude, it may also support environmental-control or other systems. EASA AMC 20-2B uses the term “essential APU” for installations where continued APU function can be important to safe aircraft operation following other failures. [3]
APU altitude limits
A gas turbine becomes more difficult to start as air density and pressure decrease with altitude. Compressor mass flow also changes, affecting both combustion stability and pneumatic output. For this reason, aircraft manuals often specify one maximum altitude for APU operation, another for starting the APU, and potentially lower limits for use of APU bleed air.
Those numbers vary significantly among aircraft families and APU models, so they should never be generalised. The certification requirement is that the APU demonstrate safe operation within its declared operating envelope. EASA CS-APU covers declared ratings, operating limitations and environmental conditions as part of certification. [2]
APU fuel supply
The APU normally burns the same approved aviation turbine fuel as the main engines. Fuel is delivered from the aircraft fuel system through an APU fuel-control path. The electronic or hydromechanical control system meters the fuel needed for starting, acceleration, steady operation and load changes.
Because the APU can run while the main engines are shut down, the aircraft must have a way to provide the required fuel pressure in that condition. Depending on design, this may involve an electric fuel pump, a dedicated feed arrangement or suction capability. Exact tank selection and pump logic are aircraft-specific and should be taken from approved system manuals rather than assumed.
Load changes are a real turbine-control problem
When a large electrical load is connected or an air-conditioning pack opens, the APU suddenly has to supply more power. Without control action, turbine speed could decrease. The APU control system detects operating changes and adjusts fuel flow and other control variables to maintain the required speed and operating margins. [3]
Likewise, removing a large load can create a tendency for speed to rise. Overspeed protection is therefore a key certification requirement. An APU is expected to remain controlled through the load transients it will encounter in service rather than behaving like a simple fixed-throttle turbine. [2]
APU fire detection and extinguishing
The APU operates in an enclosed part of the aircraft with fuel, hot surfaces and rotating machinery, so fire protection is fundamental. Transport aircraft normally provide APU-compartment fire detection. Where required by the aircraft design and certification basis, an extinguishing system can discharge agent into the compartment.
APU fire procedures can differ significantly between ground and flight operation. Some aircraft have automatic ground shutdown or extinguishing logic because no pilot may be monitoring the flight deck during a servicing event. The exact automatic functions should not be assumed across aircraft types. EASA’s APU and transport-aircraft certification frameworks address fire-zone protection, controls and failure effects. [2]
Why the APU has its own air intake and exhaust
A gas turbine needs a substantial airflow even though the unit is small compared with a main engine. The aircraft therefore incorporates a dedicated APU inlet door or inlet arrangement to supply combustion and compressor air. The exhaust is routed safely overboard through a tailpipe designed to keep hot gases clear of critical structure and ground personnel.
Inlet and exhaust design has to work across ground and flight conditions. Ram pressure, crosswinds, icing, rain ingestion and aircraft attitude can all affect the flow field. The APU installation is therefore certified as a complete aircraft installation rather than simply bolting a generic turbine inside the tail. [3]
Noise and emissions
Anyone standing near the rear of an airliner at the gate may hear the APU’s high-pitched exhaust noise. The unit also burns fuel and produces exhaust emissions. For that reason, airports may restrict unnecessary APU operation, particularly where fixed electrical ground power and pre-conditioned air are available.
Reducing APU use can save fuel and reduce local emissions, but operational safety and system requirements remain the priority. An airline cannot simply forbid APU use if the aircraft requires it for an approved dispatch condition, engine start or system backup. Local environmental procedures therefore work alongside, not instead of, the aircraft’s technical requirements.
The APU is not the same as ground power
External ground power supplies electricity from airport equipment. A ground air-conditioning unit can separately supply conditioned air. An air-start cart can provide compressed air for engine starting. The APU can often replace all or several of these pieces of equipment by producing electrical and pneumatic power onboard.
That flexibility is why the APU remains valuable even at highly equipped airports. Ground services reduce the need to run it, but they do not make the onboard unit redundant. The aircraft needs an independent source for transition, remote operations and approved backup functions.
The APU on more-electric aircraft
Not every modern airliner uses pneumatic systems in the same way. The Boeing 787, for example, moved many functions traditionally powered by engine bleed air to electrical power. Its APU architecture therefore operates within a different aircraft-level energy system from a conventional bleed-air transport. This is one reason APU capability should always be described in relation to a particular aircraft rather than as a universal set of outputs.
The high-level principle remains the same: the APU is an independent onboard turbine that supplies auxiliary energy. Whether that energy is predominantly electrical, pneumatic or a combination depends on the aircraft’s system architecture.
What happens when the APU is shut down
After the main engines are providing their own electrical and pneumatic power, the APU may no longer be required. Shutdown is normally controlled rather than instantaneous. The system removes loads, terminates the fuel command in the approved sequence and allows the unit to decelerate. Some designs include a cool-down period to reduce thermal stress after high-load operation.
Electronic control continues monitoring during shutdown. If the APU has been shut down because of a fire, overspeed or other protective condition, the sequence can differ from a routine shutdown. Exact timings and switch selections belong in the aircraft operating manual.
A compact power station inside the aircraft
The most useful way to think about the APU is as a small onboard power station. It takes fuel and air, runs a gas turbine at controlled speed and turns that energy into electricity and, on many aircraft, compressed air. Those outputs keep the aircraft alive at the gate, run environmental-control systems, provide the energy to start large turbofans and can add an independent backup source in flight. [1]
Its importance is easy to underestimate because it usually sits out of sight in the tail and produces no visible propulsion. Yet an airliner without an available APU may require additional ground equipment, different starting procedures or dispatch restrictions. The APU’s contribution is not thrust but independence: it allows the aircraft to create the electrical and pneumatic energy needed to prepare itself for flight and, where approved, support itself after other power sources are lost.
Verified Sources / References
- Federal Aviation Administration — Auxiliary Power Unit information and approvals. FAA overview of APUs and their auxiliary-power role.
- EASA — CS-APU Auxiliary Power Units, Amendment 1. European certification specifications covering APU design and operation.
- EASA AMC 20-2B — Certification of Essential APUs Equipped with Electronic Controls. Guidance on APU electronic control, redundancy, aircraft interfaces and essential-APU installations.
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