Ground Power Unit (GPU), alternatively referred to as Aircraft Ground Power, is an external power source for aircraft on ground. It is a mobile or stationary device that can be connected to the electrical system of an aircraft to provide either 120V AC or 28V DC power. The unit supplies power for starting the auxiliary power unit, lighting, avionics and air conditioning while remaining independent of aircraft internal fuel. Ground power units convert power, drawn from an external generator or the airport grid, to the correct voltage and frequency, assuring reliable ground-side electrical support.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a GPU in an airplane?
The Ground Power Unit (GPU) provides electrical power to aircraft while they are on the ground, and it is a mobile or stationary device. The GPU provides the necessary electrical power to operate the on-board systems, and ground power units may be found in other configurations.
The Ground Power Unit (GPU), sometimes referred to as Aircraft Ground Power, is an external self-contained device that supplies electrical energy to an aircraft when its main engines are not running. Serving as a temporary outlet for supplying power, it delivers either 115 Volt Alternating Current at 400 Hz or 28 Volt Direct Current. By doing so, it fulfills the electricity needed to run avionics, enabling operation of lighting, instruments, the air conditioning system and providing power to flight controls and navigation - without draining fuel or relying on the on-board auxiliary power unit. Modern GPU families include fixed units permanently installed at gates or in aircraft hangars, along with mobile GPUs, which are towable or wheeled units that can provide power anywhere on the airfield and are suitable for maintenance procedures and heavy-engine starts.
What is a ground power unit used for?
A Ground Power Unit (GPU) is equipment used at airports to supply electrical power to an aircraft while it is on the ground. When an aircraft is on the ground it requires electricity to run lights, avionics, and air conditioning without using the aircraft's engines or auxiliary power unit (APU). Ground power units are therefore used to give power to various aircraft systems like cockpit instruments, cabin lighting, passenger boarding, servicing, cleaning crews, maintenance checks and pre-flight inspections. It helps operate avionics and enables novice pilots to practice instrument procedures in a low-pressure setting. GPUs additionally help simplify pushback operations.
Ground power units provide clean, regulated power for engine starting and can be used for starting the auxiliary power unit or turbine starting. DC ground power unit is used for turboprop airplanes and general aircraft, delivering 28.5 V DC output, whereas large commercial aircraft use 115 V 400 Hz alternating current supplied by solid-state AC frequency converters, termed Fixed Electrical Ground Power. Hybrid units combine diesel generators and battery power. Battery carts offer 28.5 V DC output through high discharge batteries, contributing to environmental friendliness and quiet operation. By eliminating the need to run onboard APUs or main engines during ground operations, ground power units reduce fuel consumption, emissions, and noise pollution.
What is the difference between an APU and a GPU?
The difference between an APU and a GPU is that an APU is an engine inside the tail of the aircraft, a compact jet engine that functions as an autonomous unit and uses the aircraft's jet fuel. It generates electricity to power onboard systems including the air conditioning, lights, and avionics, and it supplies bleed air to start main engines. The APU is used on the ground and in flight, providing power after the main engines are shut down and during taxi-in.
A GPU, by contrast, is external equipment that delivers electrical power only, supplying 115 volts of alternating current at 400 Hz without needing to run the APU. GPUs provide higher power outputs compared to auxiliary power units and are suitable for heavy-duty operations like engine starts and maintenance procedures. Using a GPU instead of the APU for ground power extends the APU's lifespan and reduces maintenance costs, because the GPU eliminates the need to run the APU while the aircraft is on the ground. When a suitable GPU is available, the APU can be switched off after the main engines have been started during taxi-out, reducing fuel consumption and wear. At smaller airports the correct GPU is not available, so the APU is used throughout the whole turnaround, but wherever possible the GPU is preferred to reduce emissions, noise, and operating expense.
How is a ground power unit connected to an aircraft?
A cable connects the GPU to the aircraft. When an aircraft is on the ground and requires external power, the ground crew connects the GPU to the aircraft through a cable. The GPU connects with the aircraft through specially designed power supply outlets, and the ground crew aligns the GPU's connector with the aircraft's power receptacle, guaranteeing the pins match up correctly. These receptacles are found near an aircraft's nose or wing.
GPU can be connected to the electrical system of aircraft while on ground to provide 120V AC or 28V DC power. Smaller aircraft operate with 28 Volt Direct Current. Larger aircraft have 2 or 4 standardized 90 kVA connectors, and aircraft use 400Hz AC current at high amperage. The power required by the aircraft depends on its size.
What are the main components of a ground power unit?
The main components of a ground power unit are listed below.
- Power Source/Generator: It is typically a diesel or gasoline engine for mobile units, or an electric motor for fixed/solid-state units, which powers the alternator to generate electricity.
- Alternator/Generator Assembly: It converts mechanical energy from the engine into electrical energy, producing either DC or 400 Hz AC power.
- Control Panel and Monitoring Systems: Allows operators to start/stop the unit and monitor output voltage, frequency, and current to ensure compatibility with aircraft systems.
- Distribution & Control System: Manages the power output and contains safety features like circuit breakers to protect the aircraft's electrical components.
- Output Cables & Connectors: Heavy-duty, standardized cables used to connect the GPU directly to the aircraft's electrical receptacle.
- Cooling System : Radiators and fans that maintain engine operating temperatures.
- Fuel Tank: Fuel storage for the internal combustion engine.
- Chassis/Trailer: It is often mobile, mounted on wheels or a trailer for towing around the airport.
What are the types of ground power unit?
Ground Power Unit types are categorized by mobility type and by power source. By mobility type, GPUs are fixed, mobile, or portable. Fixed electrical ground power units are permanently installed at gates or in hangars and are best for regular, high-frequency operations. They eliminate exhaust emissions and minimize noise, making them best suited for enclosed spaces. By power source, GPUs include electric GPUs, battery-powered eGPUs, and diesel-powered units. Battery-powered eGPUs are battery-powered, compact, and easy to handle; they eliminate exhaust emissions and minimize noise, making them best for enclosed spaces. Diesel-powered GPUs are internal combustion units that require fuel and engine maintenance, produce emissions and noise, and are being phased out due to environmental and operational drawbacks. Ground power units can also be categorized by power output type. Combination units can deliver both 400 Hz AC and 28V DC power, offering versatility for mixed fleets.
