Introduction
Every aircraft, from a two-seat training helicopter to a wide-body airliner, needs electrical power long before its own engines are turning. Avionics need to boot up, lights need to come on, hydraulic and fuel systems need to be checked, and — critically — the engines themselves usually need an outside electrical push to start. Supplying that power on the ground is the job of Ground Power Units, universally known in the industry simply as the GPU.
It’s an unglamorous piece of equipment. It doesn’t fly, it rarely gets photographed, and passengers boarding a jet bridge have almost certainly never noticed the humming cart parked beneath the nose of their aircraft. Yet the GPU is one of the quiet enablers of modern aviation: it protects aircraft batteries, cuts fuel burn and emissions, reduces engine wear, and keeps maintenance crews working efficiently around the clock. This article traces how ground power evolved from hand-cranked magnetos and battery carts to today’s solid-state, and increasingly electric, systems — and looks at how the needs of a light aircraft or helicopter differ sharply from those of a commercial airliner.
Life Before the GPU
In the earliest decades of powered flight, “ground power” barely existed as a concept, because aircraft barely had electrical systems to power. Early aero engines were started by hand-swinging the propeller or, on larger aircraft, by inertia starters cranked manually or by a small external motor. Electrics, where fitted at all, ran off small onboard batteries charged by an engine-driven generator once running.
As aircraft grew more complex through the 1930s and into the Second World War, this approach became untenable. Bombers and transports carried increasingly sophisticated radios, lighting, and instrumentation, all of which drew down the battery even before engine start — and a battery flat from overnight parking or repeated pre-flight checks could leave a valuable aircraft grounded. Ground crews responded with rudimentary external battery carts and, on military airfields, dedicated “starter trucks” fitted with generators that could be wheeled up to an aircraft and connected via cable to crank the engines over and supply house power simultaneously. These were the direct ancestors of modern aircraft GPUs, though they were often improvised: converted vehicle alternators, truck batteries wired in series, or generators salvaged from other equipment.
Without such support, operators managed in ways that put real strain on aircraft and aircrew:
- Battery-only starts, which risked deep-discharging the aircraft’s own battery, shortening its life and sometimes leaving too little charge for a safe engine start.
- Hand-starting on light piston aircraft, still common today on some small types, which is physically demanding and carries real safety risk from the propeller.
- Running the engines longer than necessary simply to keep systems powered during loading, maintenance checks, or crew changes — burning fuel and adding wear for no operational benefit.
- On aircraft that had one, relying on the onboard Auxiliary Power Unit (APU) — a small onboard turbine generator — for extended periods, which itself burns fuel, is noisy, and has an efficiency rate of only around 8–14%, producing gaseous emissions and noise that affect both local air quality and the people working around the aircraft.
None of these were good long-term solutions, and as aircraft — particularly airliners — became larger and more electrically dependent, purpose-built ground power equipment became essential rather than optional.
Why GPUs Matter
The core purpose of a GPU has remained constant even as the technology has changed: to supply reliable external electrical power to an aircraft on the ground, so its own batteries and engines don’t have to do the work. That single function delivers several distinct benefits.
Protecting the aircraft’s battery and electrical system. Aircraft batteries are expensive, safety-critical components, and repeated deep discharges shorten their working life. Using a GPU for pre-flight checks, maintenance, and engine start keeps the onboard battery topped up and reserved for genuine emergencies, such as an in-flight electrical failure.
Enabling reliable engine starts. Many aircraft, particularly larger turboprops, business jets, and airliners, need a strong, stable current to turn a starter motor or start generator. A GPU can deliver a higher and more consistent starting current than an aircraft battery alone, especially in cold weather when battery performance drops and engine oil is thicker.
Reducing fuel burn, emissions, and noise. This is the single biggest driver behind GPU adoption at commercial airports. Running a jet’s onboard APU, or worse, one or more main engines, just to power cabin systems during boarding or maintenance wastes fuel and adds unnecessary noise and pollution around the gate. Because aircraft ground emissions can account for as much as 75–80% of a busy airport’s total emissions, switching that load onto grid-fed ground power makes a measurable difference to both airline fuel bills and local air quality.
Supporting maintenance and turnaround operations. Ground engineers frequently need full electrical power — for avionics testing, lighting, hydraulic pump operation, or running environmental control systems — without starting an engine at all. A GPU lets that work continue safely and efficiently, day or night, hangar or ramp.
Cutting engine wear. Every unnecessary engine start and stop adds cycles to components with finite service lives. Using external power for anything that doesn’t require the engines running reduces maintenance costs over the life of the airframe.
How GPUs Have Evolved
From improvised carts to purpose-built diesel generators
By the mid-20th century, dedicated ground power carts — a diesel or petrol engine driving a generator, mounted on a wheeled chassis — had become standard equipment at military bases and civil airports alike. These units were rugged and simple: an engine, an alternator, some voltage regulation, and a heavy cable with an aircraft-specific plug. Diesel-engine-driven GPUs of this basic design are still manufactured and widely used today, prized for being self-contained and usable anywhere on the airfield without depending on mains power.
The move to solid-state frequency conversion
Commercial airliners standardized on 115-volt, 400-hertz alternating current for their main electrical systems, chosen because higher-frequency AC allows lighter transformers and motors — an important weight saving in an aircraft. Early ground units generated this frequency mechanically, using a motor-generator set tuned to spin at the right speed. From the 1980s onward, power electronics matured enough that solid-state frequency converters became practical: these take standard 50 or 60 Hz utility power and electronically convert it to a clean 400 Hz output, with far greater efficiency, reliability, and precision than a spinning motor-generator set. Solid-state frequency converters and diesel-powered configurations both remain in production today, often built to deliver 400 Hz AC alongside a 28-volt DC output from a single unit.
Fixed and mobile installations
As airports grew, GPUs diversified by how they’re deployed as much as by how they generate power:
- Mobile or towable GPUs are wheeled units, often diesel-powered, that can be moved anywhere on the ramp — essential for aircraft parked on remote stands, at smaller airfields, or for military and charter operations without fixed gate infrastructure.
- Fixed Electrical Ground Power (FEGP), permanently installed at airport gates, feeds power through cables built into the jet bridge or a ground pit, delivering 400 Hz AC directly to the aircraft via a standardized specialised 6-pin plug. This lets both the engines and the tail-mounted APU be switched off as soon as the aircraft is on stand, delivering substantial fuel savings and environmental benefit — a major reason the industry has shifted toward fixed ground power as airlines look to cut costs while meeting tightening environmental regulation.
- Fixed ground power is very often paired with Pre-Conditioned Air (PCA) units, which supply heated or cooled air to the cabin through a separate duct, removing the need to run the APU for climate control as well as electrics.
Matching power type to aircraft: light aircraft and helicopters versus airliners
Not all aircraft need — or can accept — the same kind of ground power, which is why GPUs have split into distinct families.
Light aircraft, helicopters, turboprops, and military types typically run on 24–28-volt DC electrical systems, closely related to their onboard batteries. A 28V DC GPU is best suited to general aviation, business aircraft, and military applications with DC requirements, and is commonly used with legacy helicopters, turboprops, business aircraft, regional aircraft, and many military aircraft applications, supplying power for avionics, maintenance, battery support, or engine start assistance. These units range from small, hand-portable battery packs used for a quick avionics check or engine start, up to larger wheeled or engine-driven carts capable of sustained high current for repeated cold starts. Battery-powered 28V DC GPUs, manufactured by companies such as Red Box Aviation, are lightweight, popular for preflight checks, avionics updates, and field maintenance where access to shore power or fuel-driven units is limited, making them well suited to helicopter operations at remote sites, air ambulance bases, or small grass airfields with no fixed infrastructure.
Commercial airliners and larger business jets run their main systems on 115V 400Hz AC, so they require either a dedicated AC frequency converter or a combination unit. 400 Hz AC units are designed for commercial aircraft, while 28V DC GPUs serve smaller aircraft, business jets, and military applications, with the choice depending on aircraft compatibility. Many modern operators, particularly those handling mixed fleets, use combination GPUs that supply both outputs simultaneously — delivering 45 kVA of AC power alongside a separate 28.5V DC 800A supply from a single unit — so that one piece of ground equipment can service everything from a light twin to a regional jet without swapping carts.
Toward electrification
The most significant recent shift in GPU technology mirrors what’s happening across ground support equipment more broadly: a move away from diesel toward mains-fed and battery-electric systems. Airport authorities are expanding pre-conditioned air infrastructure and accelerating electric PCA deployment to reduce aircraft APU runtime and curb tarmac emissions, driven in large part by tightening air-quality regulation. Major hub airports such as London Heathrow and San Francisco have rolled out electric ground support equipment — including electric ground power and air-start units — as part of broader net-zero commitments, and the wider market for electric ground power and preconditioned air systems is growing steadily, driven by rising demand for energy-efficient ground support equipment and stricter environmental regulation. Where a generation ago a diesel cart idling at every gate was simply how things were done, today’s larger airports increasingly default to fixed, mains-fed electric power and treat diesel GPUs as backup or remote-stand equipment only.
The GPU Today: A Layered Ecosystem
Rather than one device replacing another, the industry has ended up with a layered set of tools matched to different needs:
- Battery packs and hand-portable units for quick starts and avionics checks on light aircraft and helicopters, valued for their portability and independence from external power.
- Towable diesel or electric carts for ramp flexibility at any aircraft type, especially where fixed infrastructure isn’t available.
- Solid-state frequency converters, mobile or fixed, for precise, efficient 400 Hz AC power to jets and airliners.
- Fixed Electrical Ground Power and Pre-Conditioned Air, built into airport gates, for sustained, low-emission power and climate control during turnarounds — now the preferred solution at most major hubs specifically because it lets crews shut down both engines and APU on arrival.
- Combination AC/DC units for operators servicing mixed fleets from one piece of equipment.
Conclusion
The Ground Power Unit’s story is really the story of aviation’s growing dependence on electrical systems, and the industry’s steady effort to meet that demand more safely, cheaply, and cleanly. What began as improvised battery carts and hand-cranked starters has become a sophisticated, purpose-engineered category of equipment: rugged DC packs for a helicopter on a remote pad, solid-state frequency converters for an airliner on a busy stand, and increasingly, mains-fed electric systems built directly into the gate itself. The common thread across nearly a century of development is simple — keep the aircraft’s own batteries and engines for flying, and let ground power handle everything else.
