What is an aircraft GPU, and how is it used?

An aircraft ground power unit (GPU) is an external source of electrical power used while an aircraft is parked. It powers lights, avionics, electrically driven pumps, ventilation and other systems without running the engines or APU, and may support engine starting when the aircraft’s starting system can use that electrical supply.

In Aviation & Real-World Flying terminology, a GPU may be a mobile battery or engine-driven cart. Airports also provide fixed electrical ground power at some stands. Once accepted through the aircraft’s external-power contactor, either source feeds the electrical buses much like an onboard generator.

What does an aircraft ground power unit power?

A GPU supplies the aircraft’s electrical systems on the ground while reducing fuel use, noise and APU running time. Depending on the aircraft, that can include:

• Cockpit displays, radios and navigation equipment.
• Cabin, exterior and maintenance lighting.
• Fuel pumps, hydraulic pumps and other electrically driven equipment.
• Ventilation fans, galleys and passenger-service systems.
• Battery charging through the aircraft’s own charging system.
• Engine-control, ignition and starting circuits.

Cabin cooling is a common source of confusion. A GPU can run electrically powered air-conditioning equipment if the aircraft has it, but many aircraft need APU bleed air or a separate pre-conditioned-air unit for effective heating or cooling.

What voltage and frequency does a GPU provide?

The GPU must match the aircraft’s approved voltage, frequency, phase and current requirements. Most transport aircraft use 115/200-volt, 400 Hz, three-phase AC ground power, while some also accept 28-volt DC. Many smaller aircraft use a DC external supply appropriate to their 12- or 24-volt electrical system.

Those figures are not interchangeable, and the connector’s appearance is not enough to establish compatibility. Crews and ground staff use the aircraft placard, operating manual and approved ground equipment. Our explanation of why aircraft use 400 Hz power covers the weight and equipment advantages of that standard.

How is ground power connected and used?

Ground power is connected through a dedicated external-power receptacle and then selected or accepted from the cockpit. The precise sequence is aircraft- and operator-specific, but the normal flow is:

1. Secure the aircraft. Apply the parking brake or chocks and follow the operator’s no-movement safeguards before personnel approach the receptacle.
2. Verify the power source. Confirm that the GPU has the correct output and inspect the cable and connector for contamination, damaged pins or overheating.
3. Connect the cable. Ground staff fully seat the keyed connector and position the cable so it cannot be pulled out or struck by equipment.
4. Make power available. The GPU is enabled according to the equipment procedure. A cockpit indication such as AVAIL confirms that acceptable external power has reached the aircraft.
5. Put power on the buses. The crew selects external power or confirms that the aircraft has accepted it automatically, then checks voltage, frequency, generator and bus indications before adding major loads.
6. Transfer and disconnect. Before departure, the crew verifies that an APU or engine generator has taken the load. External power is removed using the approved sequence, and ground staff disconnect and stow the cable.

The cable should never simply be pulled while it is carrying the aircraft’s load. Incorrect sequencing can arc the connector, damage its pins or leave the aircraft unexpectedly unpowered.

What is the difference between a GPU and an APU?

A GPU is external ground equipment, whereas an auxiliary power unit is installed inside the aircraft and travels with it.

Crews generally choose a GPU when they only need ground electricity and suitable equipment is available. They use the APU when the aircraft needs an independent source, pneumatic air or services the GPU cannot provide. See our guide to how an onboard APU supplies aircraft systems for the other half of that distinction.

Can a GPU start an aircraft engine?

A GPU can support an engine start, but it cannot start every type of engine by itself. The answer depends on whether the starter needs electrical power or compressed air.

On a conventional jet with an air-turbine starter, the GPU powers avionics, fuel pumps, ignition and engine controls but does not provide the pneumatic airflow that turns the engine. That normally comes from the APU, a running engine or a separate air-start unit. Aircraft with electric starters may use an approved GPU for starter current, often with the aircraft battery still connected as required by the checklist.

How do you use a GPU in a flight simulator?

In a flight simulator, requesting the GPU and placing it on the aircraft’s electrical buses are usually separate actions. Higher-fidelity aircraft commonly follow this sequence:

1. Request or connect ground power through the aircraft tablet, ground-services panel or simulator interface.
2. Wait for the cockpit’s external-power available indication.
3. Select EXT PWR or the equivalent control.
4. Confirm that the expected buses and displays are powered.
5. Start the APU or engines and verify that an onboard generator has taken over before removing the GPU.

A mistake we see constantly is assuming that a visible ground cart means the aircraft is powered. The cart may be connected while the external-power contactor remains open. Conversely, some simulators model electrical power without displaying a physical cart.

If the GPU is connected but the cockpit remains dark, check the external-power indication, battery-master configuration and the add-on’s ground-service conditions. If everything dies when the GPU is removed, no APU or engine generator had taken the load. Our A320 electrical-source and bus-transfer guide explains the cockpit logic, while the Fenix A320 cold-and-dark procedure shows ground power in a complete simulator start sequence.