APU, ground power and PCA: how an aircraft gets power on the ground

The engines are off, the passengers are getting out, but in the cockpit all the displays are still lit, the cabin stays pleasantly cool and the galley ovens are running. The energy has to come from somewhere. A parked airliner has two options: it supplies itself through its auxiliary power unit (APU), or it is plugged into the airport – with ground power (GPU) and preconditioned air (PCA). This guide explains how the three systems work, why airports would rather see cables than kerosene and how to tell from the fence what is running right now.

The short answer

An aircraft on the ground needs three things: electricity for avionics, lighting, galley and inflight entertainment, conditioned air for the cabin and – shortly before departure – compressed air or enough electrical power to start the engines. The APU in the tail can supply all of it. The airport can take over electricity and conditioned air, so the APU is only needed shortly before the engines are started. How much of this is possible at a stand depends on the equipment of the position and on the rules of the airport in question.

The APU: the power plant in the tail

The auxiliary power unit is at its core a small gas turbine. On almost all jets it sits in the tail cone, behind the pressure bulkhead – that is why on many airliners you see a small exhaust opening at the very back, sometimes in the center of the tip, sometimes offset to the side. The APU draws fuel from the normal tanks and produces two things from it:

• Shaft power, which drives a generator through a gearbox. That feeds the electrical system, on large airliners typically at 115 volts AC and 400 hertz.
• Compressed air (bleed air), either from its own load compressor or tapped from the turbine’s compressor. This air feeds the air-conditioning units (packs) and the air starters of the main engines.

The APU is started electrically, in the simplest case with the onboard batteries. That makes it the key to an aircraft’s independence: with charged batteries and a working APU, a crew can power up all systems and start the engines even at a remote stand without any ground equipment.

A bit of history

Auxiliary units have existed almost as long as powered flight: as early as World War II, large bombers used small piston engines to drive generators, and early German jet engines were started with their own small two-stroke engine, the Riedel starter. The Boeing 727 of the 1960s is considered the first jet airliner with an auxiliary gas turbine. It was meant to manage even at small airports without much ground equipment – exactly the argument that justifies the APU to this day.

Who builds APUs?

The market is essentially shared by two suppliers: Honeywell as the clear market leader and Pratt & Whitney Canada, a subsidiary of Pratt & Whitney. Pratt & Whitney Canada supplies, for example, the APUs for the Airbus A380 and the Boeing 787. In the A320 family there was long a choice between a Honeywell unit and Pratt & Whitney’s APS3200; on the A320neo Airbus dropped the Pratt & Whitney option, and the APU there comes from Honeywell. For spotters the manufacturer is usually not visible, but sometimes audible: different APU models sound noticeably different.

The APU in flight

Normally the crew shuts the APU down as soon as the engines are running and their generators have taken over the electrical system – some airlines leave it on until after takeoff so as not to load the engines with bleed air for air conditioning at liftoff. In cruise the APU is then a backup: if an engine generator fails, it can supply power. For twinjet long-haul flights under ETOPS rules, which run far from alternate airports, a reliably startable APU therefore plays an important role.

The best-known example of its use in an emergency is US Airways Flight 1549 in January 2009: after the bird strike and the loss of both engines, the crew started the APU so that the aircraft stayed supplied with electricity until the ditching in the Hudson.

You shouldn’t confuse the APU with the ram air turbine (RAT). That is a small wind turbine that deploys from the fuselage or wing in an emergency and is driven by the airstream. The RAT has neither an engine nor fuel and supplies only the bare essentials for flight controls and important instruments – on the ground it plays no role.

Ground power: the GPU

The ground power unit supplies electricity from outside. A cable is plugged into an external receptacle on the aircraft, usually on the underside of the forward fuselage. The connectors are internationally standardized, so the same equipment fits almost all airliners. Widebodies often have several connections because a single cable isn’t enough for their needs.

You see three designs on the apron:

Design	How it works	Where you see it	Typical advantage
Mobile diesel GPU	combustion engine drives a generator, on a trailer or vehicle	remote stands, maintenance areas, smaller airports	flexible, needs no grid connection
Fixed converter at the stand	converts grid power to 400 hertz, cable comes from a pit or a mast	apron positions of modern airports	quiet, no exhaust at the aircraft
GPU at the jet bridge	converter hangs under the bridge, cable is plugged in directly	terminal positions with a bridge	short distances, quickly connected
Mobile electric GPU	battery- or grid-powered unit on wheels	increasingly at airports with climate targets	flexible without diesel exhaust

Why 400 hertz?

Household power in Europe is 50 hertz. Aircraft use eight times that, because transformers, generators and electric motors can be built smaller and lighter for the same output at higher frequency. In an aircraft every kilogram counts, and on the ground that has consequences: the airport can’t simply offer an outlet but needs frequency converters that bring the grid to the onboard format. Smaller aircraft, such as turboprops and business jets, additionally or primarily work with 28 volts DC – that is why many GPUs have both outputs.

What ground power can’t do

Electricity alone doesn’t get the big engines of classic jets going. Their starters are air turbines that need compressed air. The usual chain is therefore: GPU or battery starts the APU, the APU supplies bleed air, the bleed air spins up the engines. Without an APU you need a separate air start unit (ASU) on the ground, a compressor cart that feeds in compressed air through a hose.

PCA: conditioned air from the airport

Electricity is only half the battle. A full cabin heats up quickly at the stand in summer and cools down in winter. On board, the air-conditioning units handle that, working with the APU’s bleed air. If the APU is to stay off, the air has to come from outside. That is pre-conditioned air (PCA): an air-conditioning unit on the ground cools or heats air and blows it through a thick, flexible hose into a connection on the belly of the fuselage. From there the aircraft’s air-conditioning system distributes it into the cabin as usual.

PCA also comes mobile on wheels and permanently installed, especially often directly at the jet bridge. You can easily recognize the hose: it is much thicker than a ground power cable and hangs from the bridge or from a ground pit to the fuselage. How such terminal and remote positions are set up is described in the article Gates, jet bridges and remote stands.

APU or ground equipment: the direct comparison

Criterion	APU	GPU + PCA from the airport
Energy source	kerosene from the onboard tanks	mostly the airport’s power grid, partly diesel
Noise at the stand	clearly audible, high whine	quiet, especially with fixed installations
Exhaust on the apron	yes, right at the aircraft	none on site with fixed installations
Wear	counts APU operating hours, maintenance costs for the airline	spares the APU
Independence	full autonomy, even at remote positions	only where equipment is available
Engine start	supplies compressed air for the starter	electricity alone is not enough on classic jets

The table explains why airports and airlines are moving in one direction. Ground equipment reduces noise and air pollutants directly at the building, where ground staff work, and saves the airline fuel and maintenance costs on the APU. Many airports therefore limit in their conditions of use how long an APU may run before departure and after arrival if the position is equipped with ground power and PCA. The exact time windows and exceptions – for example in extreme heat, with technical problems or for types with special needs – differ from airport to airport and are set out in the respective operating rules and aeronautical information publications.

The routine at the gate: who takes over when

During the turnaround, the energy supply changes several times. A typical sequence at a well-equipped terminal position looks like this:

How pushback itself works is explained in the article Pushback, and which vehicles are used is covered in the post on aircraft tugs. If you want to get to know the people behind the cables and hoses, read the overview Working on the apron.

Reading it at the fence: what is running right now?

For spotters, the energy supply is more than technical trivia. It tells you what phase an aircraft is in – especially where you look at terminal positions, for example from the Hamburg observation deck or the Düsseldorf observation deck.

A few practical tips from this knowledge:

• Estimate departure: The switch from ground power to APU is a reliable sign that departure is near. If you see it, you have time to aim the camera at pushback and taxiway.
• Allow for heat shimmer at the tail: APU exhaust is hot. If you photograph an aircraft from the rear side with a long focal length, the air behind the tail can soften the image. A step to the side often helps more than any editing.
• Record sound: The spool-up of an APU is a popular subject for audio and video recordings. How to capture engine and APU sound cleanly is in the guide Recording engine sound. Don’t underestimate the noise level on the apron – more on this in the article Aircraft noise and your hearing and in the post Measuring aircraft noise.
• Ask on apron tours: On an apron tour you get close to terminal positions and can see bridge GPUs and PCA units up close.

Typical situations on the apron

Not every position is equipped the same way, and you can tell from the operation.

Remote stands without a bridge: Here passengers go to the aircraft by bus. Ground power there comes either from a fixed installation in the ground or from a mobile cart. If neither is available, the APU stays on for the entire stay. At remote positions of big hubs such as Munich or Frankfurt, it is worth looking at which solution is in use.

Defective APU: Under certain conditions an APU may be temporarily out of service without the aircraft having to stay on the ground. Then the aircraft needs ground power, conditioned air and a start cart at every airport. If you see a compressor cart with a thick hose at the fuselage while the engines are being started, that is often the reason.

Winter mornings: After a cold night at the stand, the cabin needs a lot of heat. Besides PCA and APU, de-icing is then often part of the program too – a good day for detail photos of the apron.

Hot days: At high temperatures, ground air-conditioning units reach their limits, especially on large, full aircraft. Some airports allow longer APU running times in such cases. You can hear it: on hot afternoons there is clearly more whining on the apron.

Maintenance area: In hangars and at maintenance positions, aircraft often run for hours on ground power while technicians work on systems. How the major checks proceed is described in the article Aircraft maintenance.

Conclusion

A parked aircraft is never really off. Either it supplies itself through its APU in the tail, or the airport takes over with ground power at 115 volts and 400 hertz and with preconditioned air through the PCA hose. Because ground equipment is quieter and cleaner, airports increasingly rely on it and limit APU use. The auxiliary power unit still remains indispensable: as an independent energy source at simple positions, as a supplier of compressed air for engine start and as a backup in flight.