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Have you ever sat at the gate while boarding a plane and heard a low, persistent hum coming from the back of the aircraft? While many passengers assume it is the main engines idling, it is actually the Auxiliary Power Unit (APU). This small gas turbine engine, usually tucked away in the tail cone, is the unsung hero of the ramp, providing the independence and safety margins required for modern commercial flight.
Table of Contents
- What is an Auxiliary Power Unit?
- Core Functions: Ground and Flight Operations
- The Cost of Independence: Fuel and Emissions
- Leading Manufacturers and Innovation
- Summary of Key Takeaways
- Sources
What is an Auxiliary Power Unit?
The APU is a self-contained, small-scale jet engine that functions primarily as a generator [1]. Unlike the massive turbofans under the wings, the APU does not provide thrust. Instead, its job is to burn aviation fuel to spin a shaft connected to an electrical generator and a load compressor.
While the history of human flight saw early aircraft relying on hand-cranks or external battery carts, the introduction of the gas-turbine APU—pioneered on the Boeing 727 in 1963 [2]—revolutionized how planes operate at remote airports.
No, the APU does not provide any thrust for the aircraft. Its sole purpose is to burn fuel to generate electricity and pneumatic pressure for the plane’s internal systems.
The APU is typically a small gas turbine engine tucked away in the tail cone of the aircraft. You can often see its small exhaust hole at the very tip of the plane’s tail.
Core Functions: Ground and Flight Operations
Modern aircraft are “energy hungry” machines. Even before the wheels leave the tarmac, they require massive amounts of electrical and pneumatic (air) pressure.
1. Ground Power and Climate Control
When the main engines are off, the APU provides the 115V AC power needed for cockpit avionics, cabin lighting, and galley equipment. More importantly, it powers the Environmental Control System (ECS). On a 95-degree day in Phoenix, the APU is what allows the air conditioning to keep the cabin habitable while passengers board.
2. Starting the Main Engines
Jet engines are too large to be started by a simple electric motor like a car. They require a massive “kick” of high-pressure air to spin the internal turbines to a speed where fuel can be introduced. The APU provides this “bleed air” to the air starters on the main engines [1].
3. In-Flight Emergency Redundancy
If an engine fails during flight, the APU acts as a vital backup. It can be started mid-air to provide electrical power to flight instruments and, in some cases, the pneumatic air required to attempt an engine restart [2]. For twin-engine jets flying long-overwater routes (ETOPS), a functioning APU is often a legal requirement for dispatch to ensure a layer of safety if one engine dies [1].
Jet engines are too large for electric starters; they require high-pressure ‘bleed air’ to spin the turbines. The APU provides this necessary pneumatic kick to get the main engines up to speed.
Yes, the APU serves as a vital emergency backup during flight. If an engine fails, the APU can be started mid-air to provide electrical power to cockpit instruments and assist in restarting the main engine.
For twin-engine jets flying long distances over oceans, a functioning APU provides a critical layer of safety redundancy, ensuring power is available if one of the primary engines fails.
The Cost of Independence: Fuel and Emissions
While the APU makes a plane self-sufficient, that independence comes at a price. A narrow-body aircraft like a Boeing 737 or Airbus A320 burns approximately 120 kg (40 US gallons) of fuel per hour just to run the APU [1]. Wide-body jets can burn more than double that amount.
Because of the fuel burn, noise pollution, and carbon emissions, many modern airports now encourage or mandate the use of:
Ground Power Units (GPU): External electrical cables plugged into the plane.
Pre-Conditioned Air (PCA): Large yellow hoses that pump cooled air directly from the airport’s central plant into the cabin.
Using these external sources allows pilots to shut down the APU, saving the airline money and reducing the airport’s environmental footprint. This move toward electrification is a precursor to the future of electric planes, where traditional gas-turbine APUs may eventually be replaced by high-capacity battery banks or hydrogen fuel cells.
| Feature | APU Operation | Ground Power (GPU/PCA) |
|---|---|---|
| Power Source | Internal Turbine | Airport Grid |
| Fuel Consumption | 120-250+ kg/hr | Zero (Aircraft) |
| Noise Level | High (Hum) | Low/Silent |
| Emissions | Direct CO2/Particulates | Minimal (at source) |
A narrow-body aircraft like a Boeing 737 burns approximately 120 kg or 40 US gallons of fuel per hour. Larger wide-body jets can consume more than double that amount.
To reduce noise and emissions, many airports provide Ground Power Units (GPU) for electricity and Pre-Conditioned Air (PCA) hoses for climate control, allowing pilots to shut down the APU entirely.
Leading Manufacturers and Innovation
The APU market is highly specialized, dominated by three major players:
Honeywell Aerospace: The world’s largest producer, having built over 100,000 units since 1948 [3]. Their latest models, like the HGT1700, focus on reducing fuel burn by 10% and emissions by 25%.
Pratt & Whitney: Currently partnering with the PBS Group to develop next-generation APUs with the “highest power density on the market” for both military and civil aircraft [4].
Safran Power Units: A leader in regional and military rotary-wing power solutions.
The market is dominated by Honeywell Aerospace, Pratt & Whitney (in partnership with PBS Group), and Safran Power Units. Honeywell is currently the world’s largest producer.
Current innovations focus on reducing fuel burn and emissions by up to 25%. In the future, traditional gas-turbine APUs may be replaced by high-capacity battery banks or hydrogen fuel cells.
Summary of Key Takeaways
- Primary Purpose: The APU is a small turbine in the tail that provides electricity and air pressure when the main engines are off.
- Self-Sufficiency: It allows planes to operate at airports that lack ground power carts or air conditioning units.
- Critical Safety: It serves as a backup power source in the event of a dual-engine failure or other mid-flight emergencies.
- Operational Cost: Running an APU is expensive, consuming 40 to 100 gallons of fuel per hour depending on the aircraft size.
Action Plan for Travelers and Enthusiasts
- Listen for the “Hum”: Next time you board, listen for the high-pitched whistle at the back of the plane—that is the APU working to keep your seat cool.
- Look for the Exhaust: When walking through the terminal, look at the very tip of an airplane’s tail. The small hole you see is the APU exhaust pipe.
- Support Ground Electrification: Airlines that use GPUs and PCA hoses instead of APUs at the gate are significantly more eco-friendly.
The APU may be small compared to the engines that propel the aircraft, but without it, the modern experience of comfortable, reliable, and safe commercial flight would be impossible.
| Category | Key Takeaway | ||
|---|---|---|---|
| Primary Role | Provides electricity and pneumatic air without main engines. | Critical Safety | Backup power for flight instruments and engine restarts. |
| Ground Ops | Enables air conditioning and engine starting at the gate. | ||
| Efficiency | High operational cost; replaced by ground units when available. |
You can usually hear a low, persistent hum or a high-pitched whistle coming from the back of the aircraft. This sound indicates the APU is providing the electricity and air conditioning for the cabin.
Yes, because the APU burns aviation fuel and creates noise pollution. Airlines that utilize airport-provided electrical cables and air hoses instead of the APU are considered more environmentally friendly.