An auxiliary power unit, or APU, is a small self-contained engine that provides electricity, compressed air, or hydraulic power to a vehicle without relying on its main engines. You’ll find them most commonly on commercial aircraft, where they keep the lights on and the cabin cool while the plane sits at the gate, but they also serve critical roles in long-haul trucks, military vehicles, and even spacecraft.
How an Aircraft APU Works
Most aircraft APUs are small gas turbine engines, typically tucked into the tail cone of the fuselage. If you’ve ever noticed a small exhaust port at the very back of an airliner, that’s where the APU lives. Despite being compact enough to fit in a closet-sized compartment, it handles two big jobs: generating electrical power and producing compressed air.
The engine itself has three main modules. The power section contains a small compressor and combustion chamber that burn jet fuel to spin a turbine. That turbine drives a load compressor, which forces air into a pressurized stream called bleed air. It also drives an accessory gearbox, which transfers mechanical energy to an electrical generator mounted on the unit. The result is a self-sufficient power plant that can run independently of everything else on the aircraft.
What Bleed Air Actually Does
Bleed air is pressurized air drawn from a compressor and routed through a network of ducts and valves to different parts of the aircraft. The APU’s load compressor is one source of this air. The main engines, once running, are another.
The most important use of APU bleed air is starting the main engines. Jet engines don’t ignite from a spark the way a car engine does. Instead, high-pressure air from the APU is directed into an air turbine starter motor attached to each engine. This spins the engine’s core fast enough for fuel injection and ignition to take over. Without the APU (or an external ground air cart), starting the engines would require another source of pneumatic power.
Bleed air also feeds the aircraft’s air conditioning packs, which filter and cool it through an expansion process before pushing it into the cabin. This is why the cabin stays comfortable at the gate even before the main engines start. On the ground, the APU is often the sole source of both electricity and climate control, eliminating the need for noisy ground power carts and external air conditioning units.
APUs as a Safety Backup in Flight
While the APU primarily runs on the ground, it also serves as an in-flight backup. If a main engine fails or a generator goes offline at 35,000 feet, the APU can start mid-flight to restore electrical power and pneumatic pressure. This redundancy becomes especially important on long overwater routes.
For flights certified under Extended Operations (ETOPS) rules, which allow twin-engine aircraft to fly routes more than 180 minutes from the nearest diversion airport, federal regulations require the APU to be fully operational before the plane can even depart. Both its electrical and pneumatic systems must be working to the unit’s designed capability. The logic is straightforward: over open ocean, hours from any runway, a functioning APU could be the difference between a manageable situation and a serious emergency.
APUs in Long-Haul Trucking
The same basic concept appears in the trucking industry, though the hardware looks very different. A truck APU is typically a small diesel engine mounted on the exterior of the cab, designed to power the sleeper compartment’s heating, air conditioning, and electrical outlets while the driver rests. The alternative is idling the truck’s main engine for hours, which burns far more fuel and produces significantly more pollution.
The U.S. Department of Energy has invested in research on idle reduction technologies, and APUs are one of the leading solutions. Running a small APU for air conditioning produces lower levels of nitrogen oxides, carbon monoxide, particulate matter, and total hydrocarbons compared to idling the full truck engine. California requires particulate filters on APUs installed on 2007 and later model year trucks to push those emissions even lower.
That said, APUs aren’t always the most economical choice for every situation. For trucks that only need bunk heat in cold weather, a simple diesel-fired or plug-in heater is almost always more cost-effective. APUs make the most financial sense when drivers need both heating and cooling across seasons, and when anti-idling regulations make running the main engine illegal during rest stops.
APUs in Spacecraft
The Space Shuttle used a fundamentally different type of APU. Rather than generating electricity or compressed air, each of the Shuttle’s three APUs provided 145 horsepower of shaft power to a hydraulic pump. Those pumps pressurized hydraulic fluid to 3,000 psi, driving 41 hydraulic actuators that controlled flight surfaces, landing gear, brakes, and other critical mechanical systems during ascent and reentry.
These units ran on hydrazine, a toxic but highly reliable rocket propellant that decomposes rapidly when passed over a catalyst, producing hot gas to spin a turbine. The hydrazine system worked but required careful handling and extensive maintenance. NASA later evaluated an electric replacement called the Advanced Hydraulic Power System, which swapped the hydrazine fuel supply and high-speed turbine for a battery-powered electric motor. This electric APU converted 300-volt electrical power into the same 3,000 psi hydraulic output, delivering over 90 kilowatts of stable power with improved efficiency.
Why APUs Matter Across Industries
The common thread across all these applications is independence. An APU lets a vehicle operate essential systems without running its primary propulsion engines. On an airliner, that means passengers board a cool, well-lit cabin while the main engines stay off. On a truck, it means a driver sleeps comfortably without burning 1,500 pounds of fuel per week at idle. On a spacecraft, it meant hydraulic control during the most dangerous phases of flight.
The tradeoff is weight and maintenance. Aircraft APUs add several hundred pounds and require their own inspection schedules, oil changes, and overhaul intervals. Truck APUs add cost and complexity to a rig. But in each case, the fuel savings, emissions reductions, and operational flexibility they provide make them well worth carrying along.