What Is a Ground Power Unit?

AIRCRAFT GROUND POWER UNITS

A Comprehensive Guide to What They Are, Why They Are Needed,

and the Different Types Used Across Aviation

Introduction

Every time a commercial airliner, military jet, or business aircraft sits on the ground with its engines shut down, it faces a fundamental challenge: without running engines, the aircraft has no power. Modern aircraft are extraordinarily complex electrical and mechanical systems – they require electricity to run avionics, lighting, air conditioning, hydraulics, flight computers, cabin systems, and a host of other critical equipment, even when completely stationary. The solution to this challenge is the Ground Power Unit, universally known in the aviation industry as a GPU.

GPUs, like those sold by Red Box Aviation, are a cornerstone of modern aviation ground operations. They are a category of specialised support equipment found at virtually every commercial airport, military air base, and fixed-base operator (FBO) around the world. Despite their critical role, they are largely invisible to the travelling public, operating silently beneath wings and beside fuselages while passengers board and disembark.

This article provides a thorough examination of aircraft GPUs – what they are, the engineering principles behind them, why they are indispensable, and the wide variety of types deployed across the full spectrum of aircraft in service today.

A Ground Power Unit is a device that provides electrical power to an aircraft while it is on the ground and its main engines or Auxiliary Power Unit (APU) are not running. In its most fundamental form, a GPU converts one energy source – typically diesel fuel, aviation fuel, or electricity from the airport grid – into the specific type and quality of electrical power required by the aircraft.

Aircraft electrical systems operate on very specific power standards that differ significantly from ordinary commercial electricity. Most commercial and military aircraft require 115-volt, 400Hz three-phase alternating current (AC) power. This 400Hz frequency – ten times higher than the 50Hz used in UK households or the 60Hz used in North America – was chosen for aviation applications because higher-frequency electricity allows for smaller, lighter transformers and motors aboard the aircraft, a critical advantage in weight-sensitive aerospace applications.

Some aircraft also require 28-volt direct current (DC) power for specific systems such as battery charging, certain avionics, and emergency lighting. Modern GPUs are often capable of delivering both AC and DC power simultaneously or switching between them as required.

Why Are GPUs Needed?

1. Engine-Off Operations

When an aircraft lands and its engines are shut down at a gate or hardstand, the aircraft’s onboard electrical generators – which are driven directly by the engines – cease to produce power. Without a source of ground power, the aircraft is essentially dead: no lighting, no air conditioning, no avionics, no entertainment systems, and no ability to support passengers or prepare for the next flight.

A GPU allows all of these systems to operate continuously throughout the ground turn, from the moment engines are shut down after arrival to the moment they are started again before departure.

2. Engine Starting

Starting the main engines of a large jet aircraft requires enormous amounts of electrical power to spin the starter motors to sufficient speed. While aircraft carry onboard batteries that can perform engine starts in an emergency, routine engine starting from batteries alone would deplete them rapidly and cause significant wear. A GPU provides the robust, high-current power supply needed for a reliable, consistent engine start, protecting the aircraft’s own batteries for genuine emergencies.

3. Avoiding APU Use

Most modern commercial jet aircraft are fitted with an Auxiliary Power Unit – a small gas turbine engine, typically located in the tail cone, that can generate electricity and compressed air independently of the main engines. While the APU is a capable system, there are compelling reasons to avoid using it on the ground whenever a GPU is available.

• Fuel cost: APUs burn significant quantities of jet fuel, adding directly to airline operating costs.
• Noise pollution: APUs generate considerable noise, a growing concern at airports with residential neighbours and strict noise abatement regulations.
• Emissions: APU exhaust contributes to local air quality pollution within the airport environment, increasingly regulated under environmental legislation.
• Wear and maintenance: APUs have limited service lives measured in hours and cycles; every hour of unnecessary ground running accelerates costly maintenance intervals.
• Sustainability: With aviation facing intense pressure to reduce its environmental footprint, GPU use is a straightforward way to reduce fuel burn and CO2 emissions during ground operations.

For all of these reasons, airport operators, airlines, and regulatory bodies increasingly mandate or strongly incentivise the use of GPUs over APU operation wherever adequate ground power infrastructure exists.

4. Passenger Comfort and Cabin Preparation

During boarding and deplaning – which can take 30 to 60 minutes or more on a widebody aircraft – passengers, crew, and ground staff are present in the cabin. Climate control systems, cabin lighting, in-flight entertainment loading, galley power for catering, and lavatory systems all require electrical power. A GPU ensures passenger comfort and allows cabin crew to prepare the aircraft efficiently without relying on fuel-consuming APU operation.

5. Maintenance and Avionics Work

Whenever engineers and technicians need to carry out maintenance, avionics work, software updates, or system testing on the ground, aircraft electrical systems must be powered up. A GPU provides a stable, clean, and controllable power source for this work, far more suitable than running the main engines or APU simply to power a diagnostic computer.

Types of Ground Power Unit

Ground power units are not a single, uniform product. The range of aircraft types in service – from a single-engine light aircraft to a 600-seat double-deck widebody airliner, from a fast jet fighter to a heavy transport helicopter – demands a wide variety of GPU types, sizes, and configurations. GPUs can be broadly categorised in several ways: by power source, by mobility, and by the type of aircraft they serve.

A. Diesel-Engine GPUs (Motor GPUs)

The most common type of GPU in use at airports worldwide, diesel-engine GPUs contain an internal combustion engine – almost invariably a diesel – driving an electrical generator. They are self-contained units requiring no external infrastructure and are therefore extremely flexible in deployment.

• How they work: A diesel engine drives an alternator or generator set calibrated to produce 115V/400Hz AC output. The engine speed is typically governed electronically to maintain precise frequency regardless of electrical load.
• Advantages: Fully mobile and self-powered; no need for airport electrical infrastructure; can be driven to any location on an airfield; rugged and reliable in adverse weather conditions; immediately available.
• Disadvantages: They consume diesel fuel and produce exhaust emissions; generate noise; require regular maintenance of the engine; contribute to the very emissions and noise problems that GPUs are supposed to mitigate compared to APU use.
• Typical applications: General aviation airports without fixed electrical infrastructure; remote stands and hardstandings; military operating bases; emergency and backup power.

B. Static Frequency Converters (SFCs) – Fixed Ground Power

At modern, fully-equipped commercial airports, the most sophisticated and environmentally preferred solution is the Static Frequency Converter – a permanently installed unit that draws power from the airport’s main electrical grid and converts it to the 400Hz, 115V output required by the aircraft.

• How they work: Airport utility power (typically 50Hz in Europe or 60Hz in the US) is fed through solid-state power electronics that convert it to the precise 400Hz, 115V, three-phase AC standard. Modern SFCs use advanced power electronics – insulated-gate bipolar transistors (IGBTs) and pulse-width modulation (PWM) – to produce extremely clean, stable output with very low harmonic distortion.
• Advantages: Zero local emissions; very low noise; highly stable and clean power output ideal for sensitive avionics; low operating costs once installed; minimal maintenance compared to diesel units; integrates seamlessly with airport sustainability targets.
• Disadvantages: Expensive infrastructure investment; only available at fixed gate positions; not deployable to remote stands without significant cable runs.
• Typical applications: Jet bridges and fixed gate stands at major international airports; maintenance hangars; airline engineering bases.

Many SFC installations are integrated into the jet bridge (also called the jetway or passenger boarding bridge) itself, with the GPU connection point located on the bridge structure where a ground handler can connect it directly to the aircraft nose or wing area with minimal cable run.

C. Solid-State / Battery-Electric GPUs

An emerging and rapidly growing category, battery-electric GPUs replace the diesel engine with a large lithium-ion battery pack, using the same solid-state frequency conversion electronics as fixed SFCs to produce 400Hz AC output from stored DC energy.

• How they work: Large lithium-ion or lithium iron phosphate (LFP) battery packs, typically with capacities of 20-100 kWh, store electrical energy that can be deployed on demand. An onboard inverter and frequency converter produces the 400Hz output. The unit recharges from a standard 400V three-phase supply when not in use.
• Advantages: Zero local emissions and near-zero noise in operation; fully mobile like a diesel unit but without the fuel costs and exhaust; low running costs; rapidly improving battery technology is extending range and capacity.
• Disadvantages: Higher capital cost than diesel units; limited operational duration before recharging is required; recharging takes time; performance may degrade in extreme cold; battery replacement adds long-term cost.
• Typical applications: Large progressive airports with sustainability commitments; airlines seeking to reduce ground emissions; stands without fixed SFC installations; urban and noise-sensitive airports.

Major GPU manufacturers including ITW GSE, TLD, and Hitzinger have released electric GPU products, and many airlines – particularly in Europe – are actively transitioning their ground fleets toward electric equipment as part of scope 3 emissions reduction strategies.

D. Hydraulic GPUs

Separate from electrical GPUs, some aircraft also require ground-supplied hydraulic power for maintenance operations – testing hydraulic systems, operating control surfaces, cycling landing gear, and so forth. Hydraulic GPUs generate and supply hydraulic fluid at the correct pressure (typically 3,000-5,000 psi depending on aircraft type) through external connections.

• Typical applications: Heavy maintenance checks; aircraft that cannot generate their own hydraulic pressure without running engines or APUs; pre-delivery testing; troubleshooting.

E. Pneumatic (Air Start) Units

While not strictly a ‘power’ unit in the electrical sense, pneumatic ground support units – also known as Air Start Units (ASUs) – are closely related to GPUs in function and are often deployed alongside them. These units supply high-pressure, high-volume compressed air to an aircraft’s pneumatic system for the purpose of starting jet engines.

• How they work: A diesel-driven compressor produces a high volume of compressed air at the pressure and temperature required by the aircraft’s air turbine starters or bleed air systems. The air is supplied through a flexible high-pressure hose to a standard ground connection on the aircraft.
• Why they are needed: While many aircraft can start their engines using bleed air from the APU, some scenarios require an external air source – APU unserviceable, APU not fitted, insufficient APU output for rapid start, or airline policy during APU-off operations.
• Typical applications: Military aircraft without APUs; airline operations where APU use is restricted; emergency situations; high-altitude airports where APU performance may be degraded.

GPUs by Aircraft Category

Commercial Narrowbody Jets (e.g. Boeing 737, Airbus A320 Family)

The single-aisle narrowbody jet is the workhorse of global aviation, operated in the tens of thousands by airlines worldwide. These aircraft operate at high frequency with very fast ground turns – often as little as 25-35 minutes at busy airports – placing premium importance on the speed and reliability of ground power connection.

Narrowbodies typically require a single 90 kVA ground power connection at 115V/400Hz, three-phase AC. Connection is made via a standard ISO 461 plug to a receptacle usually located in the nose or forward fuselage area. Most modern narrowbodies also have a second, smaller GPU receptacle for ground servicing. At well-equipped airports, fixed SFC connections integrated into jet bridges are the norm; at less-equipped facilities or remote stands, a diesel or electric mobile GPU is used.

Commercial Widebody Jets (e.g. Boeing 777, 787, Airbus A330, A350, A380)

Widebody aircraft are larger, more complex, and more power-hungry than narrowbodies. Many widebodies require two GPU connections simultaneously – one per side of the aircraft – to power all systems adequately, particularly when full cabin preparation, full avionics, and catering operations are running concurrently.

The Airbus A380, with its two passenger decks and enormous cabin, is perhaps the most demanding aircraft in service in terms of ground power requirements. It typically requires two 90 kVA GPU connections simultaneously, and some operators use 180 kVA single units capable of meeting the full load from a single connection point.

The Boeing 787 Dreamliner presents a different challenge: its ‘More Electric Architecture’ replaces many traditional pneumatic and hydraulic systems with electrical ones, meaning total electrical power demand is significantly higher than equivalent-size predecessors. The 787 can draw up to 1.45 megawatts of electrical power from the ground when all systems are running.

Business Jets and Corporate Aviation

Business jets range enormously in size and complexity, from small light jets like the Cessna Citation CJ series to large ultra-long-range jets like the Gulfstream G700 or Bombardier Global 7500. Ground power requirements vary accordingly.

Smaller business jets may operate from simple 28V DC GPU trolleys – entirely adequate for powering avionics and modest cabin systems. Mid-size and large cabin jets increasingly require the full 115V/400Hz AC standard. At fixed-base operators (FBOs) catering to business aviation, compact diesel or electric GPU units are standard equipment, often towed to the aircraft on request.

Regional and Turboprop Aircraft

Regional jets such as the Embraer E-series, Bombardier CRJ, and ATR turboprops typically operate from smaller regional airports and domestic hubs, many of which have limited or no fixed SFC infrastructure. These aircraft generally require 28V DC or 115V/400Hz AC ground power, with total demand significantly lower than large commercial jets.

Mobile diesel GPU units in the 30-45 kVA range are common at regional airports. The shorter turnaround times of regional operations make rapid GPU connection particularly important.

Military Fast Jets (e.g. Eurofighter Typhoon, F-35, Tornado)

Military aircraft, and particularly fast jets, present unique GPU requirements. Combat aircraft have no passenger cabin requirements but carry extremely complex and sensitive avionics, radar systems, electronic warfare suites, weapons systems, and flight computers – all of which must be powered up for pre-flight checks and post-flight downloads.

Fast jets often operate from hardstandings and dispersal areas without any fixed power infrastructure, making fully mobile diesel GPUs essential. Military GPUs must be robust, reliable in the field, capable of rapid deployment, and frequently able to operate in extreme climatic conditions ranging from Arctic temperatures to tropical heat.

Many military aircraft – including the F-35 Lightning II – use a 270V DC power architecture rather than the traditional 115V/400Hz AC, reflecting the shift toward ‘more-electric’ aircraft design. Specialist military GPUs capable of producing 270V DC at high current are required for these aircraft.

Aircraft such as the Eurofighter Typhoon have very specific GPU requirements defined in their ground handling documentation, and use dedicated military-specification connectors and GPU units supplied to operating air forces.

Military Transport Aircraft (e.g. C-130 Hercules, A400M Atlas, C-17)

Large military transport aircraft share many GPU characteristics with commercial widebody jets – high total electrical load, requirements for sustained ground power during loading and mission preparation, and the need to power troop facilities and cargo systems.

The C-130 Hercules, operated by dozens of air forces worldwide and one of the most widely deployed military aircraft in history, uses 115V/400Hz AC ground power and is routinely supported by military diesel GPU units, often on trailers for rapid deployment from forward operating locations.

The Airbus A400M Atlas, a modern turboprop military transport, has more sophisticated electrical requirements than the C-130 and is typically serviced by modern 90 kVA AC GPUs.

Helicopters

Helicopters present their own GPU requirements, primarily for avionics power-up, maintenance, and pre-flight systems checks. Military helicopters – such as the AgustaWestland AW101 Merlin or Boeing AH-64 Apache – may carry complex avionics and mission systems requiring sustained ground power.

Helicopter GPUs are typically smaller and more portable than those used for fixed-wing aircraft, with compact trolley-mounted units in the 10-30 kVA range being most common. Many helicopters also require 28V DC rather than, or in addition to, 400Hz AC power.

General Aviation (Light Piston and Small Turbine Aircraft)

Light aircraft – Cessna, Piper, Diamond, and similar – have very modest ground power needs. Simple 12V or 24V DC jump packs or battery carts are typically sufficient for engine starting and avionics checks. Purpose-built aviation GPU units are less common in this segment; automotive-derived battery boosters adapted for aviation use are widely deployed.

GPU Type Comparison Summary

The following table provides a condensed comparison of the principal GPU types discussed in this article:

Environmental and Regulatory Trends

The role of GPUs in aviation sustainability has grown enormously in importance over the past decade. Several key trends are reshaping the GPU landscape:

Fixed Electrical Infrastructure Investment

Major hub airports worldwide are investing significantly in expanding fixed SFC coverage, aiming to eliminate diesel GPU use from main terminal operations entirely. Airports including Heathrow, Frankfurt, Amsterdam Schiphol, and Singapore Changi have made substantial commitments to 400Hz fixed power coverage at all main stands.

Electric GPU Fleet Transition

Airlines and ground handling companies have begun transitioning their mobile GPU fleets from diesel to electric. easyJet, Lufthansa, Swissport, and others have announced or implemented programmes to deploy battery-electric GPUs as part of broader ground fleet electrification strategies.

ICAO and IATA Policy Direction

The International Civil Aviation Organization (ICAO) and International Air Transport Association (IATA) have both identified ground operations as a priority area for emissions reduction, with ground power management – maximising use of fixed electrical power and minimising APU and diesel GPU run time – forming a component of broader airport carbon accreditation and sustainability programmes.

Airport Carbon Accreditation

The Airport Carbon Accreditation (ACA) programme, recognised by ICAO and participated in by over 400 airports worldwide, includes ground power electrification as a key metric. Airports are incentivised to demonstrate reductions in ground-level emissions from support equipment including GPUs.

Conclusion

Ground Power Units are far more than a mundane piece of airport equipment. They are a critical enabling technology that sits at the intersection of aircraft engineering, airport operations, airline economics, and environmental sustainability. Without GPUs, modern aviation as we know it – with its rapid turnarounds, complex aircraft systems, and growing pressure to minimise emissions on the ground – would simply not function.

From the compact 24V DC battery trolley supporting a light aircraft at a grass strip, to the 180 kVA static frequency converter embedded in a jet bridge at a major international hub, to the ruggedised 270V DC military GPU operating from a forward air base, GPUs demonstrate remarkable variety in design and application while serving a single, essential purpose: keeping aircraft powered, operational, and ready to fly.

As aviation accelerates its transition toward electrification – both in the air with hybrid and electric aircraft, and on the ground with electric ground support equipment – the GPU will continue to evolve. Battery technology, power electronics, and smart grid integration will drive a new generation of GPU solutions, progressively eliminating the noise, emissions, and fuel costs associated with today’s diesel units. The silent, zero-emission static frequency converter or battery GPU of tomorrow will be just as indispensable as the diesel unit of today – but kinder to both the environment and the communities that live around the world’s airports.