The Auxiliary Power Unit (APU) is a small gas turbine engine typically located at the rear of an aircraft’s fuselage. It plays an important role in ensuring the smooth operation and functionality of various systems on board modern commercial aircraft. Essentially a self-contained generator, the APU serves as an independent power source for the aircraft, separate from the main engines, providing supplementary power while the aircraft is on the ground and during some phases of flight.
The primary function of an APU is to generate electrical power and, in some cases, pneumatic power. This capability proves essential during ground operations such as pre-flight checks, boarding, and servicing. The APU is instrumental in maintaining essential systems when the main engines are inactive during layovers or while taxiing on the ground.
How does the Auxiliary Power unit work?
The APU is a jet engine, and operates on the same principles as the main engines. It draws fuel from the aircraft’s fuel system and utilizes a combination of air and fuel to generate power. Unlike the main engines, the APU is not directly connected to the propulsion system and doesn’t drive the aircraft forward. Instead, its sole purpose is to generate electrical power and, in some cases, compressed air for various systems.
The APU typically features a compact design, allowing it to fit snugly within the aircraft’s structure. It is started using an electric starter motor or a pneumatic starter, and once operational, it becomes a self-sustaining power source. Once the main engines are engaged and running, the APU can be shut down, conserving fuel and reducing unnecessary wear and tear.
Why do we use the APU?
- Power Generation: The APU is a self-contained power source that generates electrical power independently of the main engines. Once activated, it produces the necessary electrical energy to run various aircraft systems, including air conditioning.
- Air Compression: In addition to electrical power, some APUs are designed to produce compressed air. This compressed air is crucial for running pneumatic systems on the aircraft, such as the air conditioning packs.
- Air Conditioning Packs: Aircraft typically use air conditioning packs, which are integrated systems responsible for cooling and pressurizing the air before it enters the cabin. These packs utilize both electrical power and compressed air to function effectively.
- Ground Cooling Mode: Many modern aircraft APUs have a specific mode known as the “Ground Cooling Mode.” In this mode, the APU operates at a lower power setting optimized for providing essential services on the ground, including air conditioning. It ensures a controlled climate inside the aircraft while conserving fuel and minimizing emissions. Prior to passenger boarding, the APU can be started to pre-condition the cabin. This involves running the air conditioning systems to achieve a comfortable temperature inside the aircraft before passengers embark, contributing to a more pleasant travel experience.
- Minimizing Engine Use: Instead of relying on the main engines for power on the ground, which can be inefficient and environmentally taxing, the APU allows the aircraft to conserve fuel and reduce emissions while still meeting the power requirements for various systems, including air conditioning.
Alternatives to the APU
Like the main engines, the APU burns jet fuel. As such, limiting APU usage is a common part of airline sustainability strategies. Here are some of the most common alternatives to the APU.
Ground Power Units (GPU)
Ground Power Units are external power sources that supply electrical power to the aircraft while it is on the ground. GPUs are often stationed near the aircraft during boarding, maintenance, or other ground operations. They connect to the aircraft through external power receptacles, obviating the need for the APU. GPUs are commonly used at airports equipped with appropriate infrastructure.
Onboard Batteries:
Some modern aircraft are equipped with advanced onboard batteries that can provide electrical power for ground operations. These batteries are typically charged during flight and can sustain essential systems, including avionics and lighting, without the need for the APU or external power sources (such as the ground power unit).
Pre-conditioned Air (PCA) Units:
PCA units supply conditioned air to the aircraft’s ventilation system while on the ground. These units are separate from the APU and are primarily designed to provide a comfortable cabin environment for passengers and crew. PCA units are particularly useful in warm climates where air conditioning is crucial during ground operations.
Combination Systems:
Certain ground support systems integrate features of both GPUs and air conditioning units. These combination systems provide electrical power and pre-conditioned air simultaneously, eliminating the need for separate ground support equipment.
Hybrid and Electric Ground Support Equipment:
With advancements in electric and hybrid technologies, there is ongoing development of ground support equipment that relies on electric or hybrid power sources. This includes electric tugs for aircraft movement and other electrically powered equipment that reduces reliance on traditional fuel-based systems.
Each alternative has its own set of advantages and limitations. GPUs and external power sources offer cost-effective solutions, especially at airports with established infrastructure, while onboard batteries and advanced systems contribute to reducing emissions and improving sustainability. The choice of alternative depends on factors such as airport facilities, aircraft capabilities, and the operator’s priorities in terms of efficiency and environmental impact.
Cover photo: Gabriel Leigh
![An example full flight summary response from the Flightradar24 API: { "data": [ { "fr24_id": "0987654321", "flight": "SK1415", "callsign": "SAS1415", "operated_as": "SAS", "painted_as": "SAS", "type": "A20N", "reg": "SE-DOY", "orig_icao": "ESSA", "orig_iata": "ARN", "datetime_takeoff": "2023-01-27T05:15:22", "runway_takeoff": "12R", "dest_icao": "EKCH", "dest_iata": "CPH", "dest_icao_actual": "EPWA", "dest_iata_actual": "WAW", "datetime_landed": "2023-01-27T06:15:10", "runway_landed": "27L", "flight_time": 3600, "actual_distance": 1007.74, "circle_distance": 6245, "category": "Passenger", "hex": "4A91F9", "first_seen": "2023-01-27T05:06:22", "last_seen": "2023-01-27T06:18:10", "flight_ended": "true" } ] }](https://www.flightradar24.com/blog/wp-content/uploads/2025/04/Flight-summary-API-cover-300x148.jpg)
![An example full flight summary response from the Flightradar24 API: { "data": [ { "fr24_id": "0987654321", "flight": "SK1415", "callsign": "SAS1415", "operated_as": "SAS", "painted_as": "SAS", "type": "A20N", "reg": "SE-DOY", "orig_icao": "ESSA", "orig_iata": "ARN", "datetime_takeoff": "2023-01-27T05:15:22", "runway_takeoff": "12R", "dest_icao": "EKCH", "dest_iata": "CPH", "dest_icao_actual": "EPWA", "dest_iata_actual": "WAW", "datetime_landed": "2023-01-27T06:15:10", "runway_landed": "27L", "flight_time": 3600, "actual_distance": 1007.74, "circle_distance": 6245, "category": "Passenger", "hex": "4A91F9", "first_seen": "2023-01-27T05:06:22", "last_seen": "2023-01-27T06:18:10", "flight_ended": "true" } ] }](https://www.flightradar24.com/blog/wp-content/uploads/2025/04/Flight-summary-API-cover-768x378.jpg)
