An APU, or auxiliary power unit, is a small jet engine built into the tail of an aircraft that generates electricity and compressed air independently of the main engines. It’s the reason your cabin lights, air conditioning, and overhead screens are already running when you board a plane sitting at the gate with its engines off. That small circular opening you may have noticed at the very back of the fuselage is the APU’s exhaust port.
What the APU Actually Does
The APU serves two core functions: it produces electrical power and it produces compressed air (called bleed air in aviation). Electrical power runs the cockpit instruments, cabin lighting, galley equipment, and entertainment systems. Bleed air feeds the air conditioning packs that keep the cabin at a comfortable temperature and pressurize it during certain operations.
The third and arguably most critical job is starting the main engines. Aircraft engines are massive turbines that can’t spin up on their own. Bleed air from the APU is routed to an air turbine starter motor connected to each engine, spinning the engine’s core fast enough for fuel to ignite and combustion to take over. This air-driven approach produces the necessary torque from a much smaller, lighter unit than an electric or hydraulic starter would require. Once the main engines are running and generating their own electrical and pneumatic power, the APU is typically shut down for the flight.
How It Works
An APU is essentially a compact gas turbine engine with its own fuel supply, compressor, combustion chamber, and turbine. It connects to a gearbox that drives an electrical generator. A separate load compressor produces the bleed air used for air conditioning and engine starting. Unlike the main engines, the APU doesn’t produce meaningful thrust. All of its energy goes into generating electricity and compressed air.
Starting the APU itself requires nothing more than the aircraft’s onboard batteries. The batteries spin the APU’s turbine to a speed where it can sustain its own combustion, at which point it takes over and begins supplying power to the rest of the aircraft. This self-sufficiency is what makes the APU so valuable: a plane can arrive at a remote gate with no ground power hookup and still operate all its systems.
Where It’s Located
On virtually all commercial airliners, the APU sits inside the tail cone, the very rearmost section of the fuselage. The circular exhaust port at the back of the plane is the easiest way to spot it from the outside. This tail-mounted position keeps engine noise, heat, and exhaust gases away from the cabin and cargo areas. The next time you’re walking across a tarmac to board, look at the back of the aircraft and you’ll likely see (or hear) the APU running.
Ground Operations
The APU’s most familiar role plays out before every flight. While the aircraft is parked at the gate, the APU powers the environmental control system that cools or heats the cabin, runs the lights passengers see when boarding, and keeps essential avionics alive so the flight crew can complete pre-departure checks. Many airports offer external ground power units and preconditioned air hookups as alternatives, and some airports encourage or require airlines to use those ground connections instead of the APU to reduce fuel burn, emissions, and noise on the ramp.
After pushback, pilots use the APU’s bleed air to start one engine, then often the second. Once both engines are stable and producing their own power, the crew shuts the APU down. The reverse happens after landing: the APU is restarted on the taxiway or at the gate so it can take over from the engines, allowing them to be shut down as soon as the aircraft parks.
In-Flight and Emergency Use
Although the APU is mostly associated with ground operations, it can also run in flight. Most modern APUs can be started at altitudes up to 39,000 feet. There are limits on what they can do at high altitude, though. One common APU model, for example, provides both bleed air and electrical power up to 22,500 feet but can only supply electrical power above that altitude. The thinner air at higher altitudes reduces cooling airflow inside the unit, which limits how hard it can work.
This in-flight capability becomes critical during emergencies. If a main engine fails or an electrical system malfunctions, the crew can start the APU to restore power to flight instruments, hydraulic pumps, and other essential systems. For twin-engine planes certified to fly long routes over oceans (under rules known as ETOPS), the APU is considered so important that it must be fully operational before the flight can depart. Federal regulations require the APU’s electrical and pneumatic systems to be working to the unit’s full designed capability for these extended-range operations. The logic is straightforward: when you’re hours from the nearest airport over open water, a backup power source isn’t optional.
What You Notice as a Passenger
You’ve probably experienced the APU without knowing it. That brief moment of silence when the cabin lights flicker and the air conditioning pauses during boarding is often the aircraft switching from gate power to the APU, or vice versa. The low hum you hear in the cabin before the main engines spool up is the APU running in the tail. And the high-pitched whine followed by a deep rumble during engine start is bleed air from the APU spinning the main engine’s turbine to life.
If you’ve ever sat on a plane at a gate during a delay and noticed the cabin getting warm, it may be because the APU was shut down to conserve fuel or because airport rules limited its use. Without either the APU or a ground air connection, the aircraft loses its ability to cool the cabin.