An APU starts a jet engine by feeding high-pressure compressed air into a small turbine attached to the engine’s core, spinning it fast enough for fuel to ignite and the engine to sustain itself. The whole process takes roughly two minutes from the moment a pilot presses the engine start switch, and the APU’s role ends once the engine reaches about 35% of its maximum core speed.
What an APU Actually Is
The auxiliary power unit is a small gas turbine engine, usually tucked inside the tail cone of the aircraft. It has its own combustion chamber, its own fuel supply drawn from the aircraft’s main tanks, and its own compressor and turbine stages. When running, it does two things: it drives an electric generator that powers the aircraft’s electrical systems on the ground, and it spins a second compressor that produces a steady supply of high-pressure air. That compressed air is the key ingredient for starting the main engines.
The APU itself starts from battery power. The aircraft’s batteries (needing at least 22 volts DC) spin a small electric starter motor inside the APU. Once the APU’s rotor hits about 5% speed, its fuel valve opens and igniters fire. By around 50%, the APU’s own combustion is producing enough power that the electric starter disconnects. By 95% speed, ignition shuts off and the APU is running self-sustained, ready to supply air and electricity to the rest of the aircraft.
How Compressed Air Reaches the Engine
Aircraft use a network of ducts, valves, and pressure regulators called the bleed air system. When the APU is running, compressed air is “bled” from its compressor section and routed through these ducts to whichever engine the crew wants to start. The pilot opens the APU bleed valve, and pressurized air flows forward through the fuselage to the engine pylon.
At the engine, this air enters a component called the air turbine starter. Think of it as a small, high-speed turbine bolted to the engine’s core shaft. The pressurized air from the APU hits the starter’s turbine blades, converting pneumatic energy into mechanical torque. That torque spins the engine’s high-pressure compressor shaft (known as N2), forcing air through the engine’s own compressor stages even though no combustion is happening inside the engine yet.
The Engine Start Sequence
Once the pilot selects engine start, the sequence follows a predictable pattern governed by the engine’s rotation speed, measured as a percentage of maximum RPM.
First, the air turbine starter alone accelerates the engine’s core from a standstill. The compressor blades begin pulling ambient air into the engine. At around 20% of maximum speed (the exact threshold varies by engine type), two things happen nearly simultaneously: the igniters begin firing inside the combustion chamber, and fuel valves open to spray jet fuel into the combustor. The igniters have typically been activated slightly before fuel arrives so the combustion chamber accumulates enough heat for reliable light-off.
Once the fuel ignites, the engine enters a shared-power phase. The APU’s air is still spinning the starter, but now the engine’s own turbine is also producing power from combustion. These two forces work together to accelerate the core. By about 35% N2 speed, the engine generates enough power from its own combustion to sustain acceleration without any outside help. At or near 50% N2, the air turbine starter automatically disengages (or the pilot disconnects it manually, depending on the aircraft type), and ignition is deactivated shortly after. The engine continues accelerating on its own to idle speed.
From the pilot’s perspective, the process involves pressing a start button, monitoring a few gauges to confirm temperatures and speeds are rising normally, and then moving the fuel lever or engine master switch to introduce fuel at the right moment. On modern fly-by-wire aircraft like the Airbus A320 family, much of this is automated once the start switch is selected.
Why Air Instead of Electricity
Jet engines are massive, and their compressor shafts are heavy. Spinning one from a standstill to 20% or more of its maximum speed requires an enormous amount of torque. An electric motor powerful enough to do this directly would be extremely heavy and draw far more current than onboard batteries could supply. Compressed air solves this elegantly: the APU generates continuous high-pressure airflow, and the air turbine starter converts that into the brute rotational force needed.
This is also why ground crews sometimes connect an external air supply (called an air start unit or ASU) to the aircraft. If the APU is broken, a truck-mounted compressor can push air through the same duct system to start the first engine, bypassing the APU entirely.
Cross-Bleed Starts: Using One Engine to Start Another
On a twin-engine aircraft, the APU typically only needs to start one engine. Once that engine is running, its own compressor produces bleed air that can be routed through a cross-bleed valve to the other engine’s air turbine starter. This is called a cross-bleed start.
The procedure requires the crew to open the cross-bleed valve connecting both engine bleed systems while closing the bleed valve on the receiving engine to prevent reverse airflow. The running engine needs to produce at least 25 to 30 PSI of bleed pressure. If pressure drops during the start (which is common, since the starting engine is consuming a lot of air), the crew advances the throttle on the running engine to compensate. Once both engines are running, the cross-bleed valve returns to its normal automatic position.
Cross-bleed starts are routine on multi-engine aircraft and serve as the standard backup when the APU is unavailable.
Newer Aircraft Are Going Electric
The Boeing 787 Dreamliner broke from the traditional pneumatic approach. Boeing’s engineers noticed that conventional air turbine starters sat idle for the vast majority of each flight, doing nothing after the engine start was complete. The 787 replaced them with dual-purpose electric motor-generators attached to the engine’s intermediate-pressure compressor stages. During startup, these units act as powerful electric motors to spin the engine. Once the engine is running, they switch roles and function as generators, each producing 250 kVA of electrical power.
This design allowed Boeing to eliminate the entire bleed air system from the 787’s engines. Without bleed air ducting, the engines run more efficiently because no compressed air is being siphoned off for cabin pressurization, de-icing, or engine starts. The tradeoff is a much larger electrical system: the 787 generates roughly ten times more electrical power than a conventional airliner of similar size, using that electricity to handle functions that bleed air traditionally covered.
The 787’s APU still exists, but its primary job shifted to generating electricity rather than providing pneumatic air. It powers the electric starter-generators during engine start on the ground, keeping the fundamental relationship intact: the APU provides the energy, just in electrical form rather than compressed air.