An auxiliary power unit, or APU, is a small gas turbine engine that provides electrical power and compressed air to an aircraft independently of its main engines. You’ll find it tucked into the tail section of most commercial jets, where it runs on jet fuel to keep lights on, air conditioning flowing, and, critically, to start the main engines before a flight. Think of it as a self-contained power plant that lets an airplane operate without being plugged into ground equipment.
The Basic Engine Cycle
An APU works on the same principle as a jet engine, just scaled way down. It runs on what engineers call a Brayton cycle: air gets pulled in, compressed, mixed with fuel, ignited, and the hot expanding gases spin a turbine. That spinning turbine is the source of all the useful work the APU does.
A typical unit, like the APS2300 found on many regional jets, contains three core parts: a single-stage centrifugal compressor that squeezes incoming air, a reverse-flow annular combustor where fuel is burned, and a two-stage axial turbine that extracts energy from the hot exhaust. The turbine shaft connects to a gearbox, and from there the mechanical energy gets split into two outputs: electrical generators and a load compressor. Some APUs use a single shaft for everything, while larger ones (like the Pratt & Whitney PW980 designed for the Airbus A380) use two shafts to handle the higher power demands of a widebody aircraft.
Electrical Power
Generators mounted to the APU’s gearbox convert shaft rotation into electricity. The output varies by aircraft size. A narrowbody jet’s APU typically produces around 90 kVA, enough to run lighting, avionics, galleys, and entertainment systems while the plane sits at the gate. The A380’s PW980 drives two 120 kVA generators. Larger widebody configurations can push even higher, with some setups rated at two 225 kVA generators providing roughly 550 kilowatts of combined capacity.
This electrical output is what keeps a parked airplane “alive” between flights. Without it, airlines would need to connect a ground power unit every time a plane reaches a gate, which isn’t always available or practical, especially at smaller airports.
Bleed Air and Cabin Conditioning
The APU’s second major job is supplying compressed air, called “bleed air,” to the aircraft’s pneumatic systems. A load compressor, driven by the same turbine shaft, forces high-pressure air into the pneumatic ducting that feeds two critical systems: air conditioning and engine starting.
On the ground, with the main engines shut down, the cabin would quickly become uncomfortably hot or cold. Bleed air from the APU routes into the aircraft’s air cycle packs, which are essentially sophisticated cooling machines. These packs take the compressed air, run it through heat exchangers, and expand it to cool it down before injecting it into the cabin. This is why you often feel the air conditioning kick in well before the engines start, and why the APU is especially important during summer turnarounds when cabin temperatures can climb rapidly with passengers boarding.
How the APU Starts a Jet Engine
Starting a jet engine isn’t as simple as turning a key. The engine’s internal turbines need to be spinning at a significant speed before fuel can be introduced and ignited. The APU makes this happen through a specific sequence.
When a pilot sets the engine start switch to the ground position, a starter valve opens, directing bleed air from the APU into a small air turbine motor attached to the engine. That starter motor drives the engine’s accessory gearbox, which spins a drive shaft connected to the high-pressure turbine rotor (called N2). Once N2 reaches about 55% of its operating speed, the starter valve automatically closes, cutting off the bleed air supply. At that point, the engine has enough airflow and combustion going to sustain itself, and it accelerates to idle on its own.
This is why you hear a distinctive whine from the back of the aircraft several minutes before the engines spool up. That’s the APU running, building up the pneumatic pressure needed for the start sequence.
In-Flight Use
APUs aren’t just ground equipment. Most are certified to start and operate at altitude, serving as a backup power source if something goes wrong in flight. The Pratt & Whitney APS5000, for example, can start and run at altitudes up to 43,100 feet while producing 450 kVA of electrical power.
In the most serious emergency scenario, a total loss of engine power, the APU becomes the aircraft’s lifeline. It can supply either bleed air for a pneumatic engine restart or electrical power for an electric starter, depending on the aircraft type. Certification rules require that this restart capability work at high cruise altitudes, with engines recovered before the aircraft descends below 15,000 feet. For certain fuel types with higher volatility, that floor drops to 10,000 feet. Airlines and manufacturers test these scenarios extensively to ensure the APU can reliably come online when everything else has failed.
Fuel and Efficiency
APUs burn the same jet fuel as the main engines, typically Jet A or Jet A-1. Their fuel consumption is modest compared to the main powerplants but not trivial. A narrowbody APU might burn around 100 to 150 kilograms of fuel per hour, which adds up across an airline’s fleet during long ground operations. This is why many airports encourage or require airlines to use ground power connections and preconditioned air hookups at gates, reserving APU use for situations where ground infrastructure isn’t available.
Maintenance and Lifespan
APU maintenance follows a schedule based on operating hours rather than flight hours, since the unit often runs on the ground for extended periods. Typical inspection intervals include general visual checks of drain lines, fuel system components, and the starter motor every 1,000 APU hours. The oil filter element gets replaced at 1,500 hours. More involved checks, like operational testing of the emergency shutdown system, happen at 2,250-hour intervals.
Unlike some engine programs that mandate fixed overhaul intervals, many modern APUs operate under condition-based maintenance programs. Rather than pulling the unit apart at a set number of hours, technicians monitor performance data (exhaust temperature trends, oil consumption, vibration levels) and schedule maintenance when the numbers indicate it’s needed. This approach keeps reliable units in service longer while catching developing problems early.
What’s Changing in APU Technology
The conventional gas turbine APU produces emissions and noise, both increasingly regulated at airports. One of the most promising alternatives is hydrogen fuel cells, which could power non-propulsion systems like lighting, climate control, and cabin pressurization without combustion. The European HYCARUS project has worked on developing a flight-ready fuel cell APU system with a high-pressure hydrogen tank suitable for pressurized passenger aircraft. These systems are still at an early development stage, with commercial applications in even small regional jets considered a long-term prospect rather than something arriving in the next few years.