Aircraft Ground Power: 400 Hz vs 28 VDC, Fixed vs Mobile, and Pre-Conditioned Air

A search such as "GPU for DXB Terminal 3" or "ECAC-compliant battery GPU for DXB" looks like a simple equipment request, but it sits on top of a real engineering decision: which electrical supply does the aircraft on this stand actually need, and is a fixed or mobile unit the right answer. Getting ground power wrong means either an aircraft that cannot accept the supply, or a parked jet running its auxiliary power unit (APU) on the apron — burning fuel, generating noise, and producing emissions the airport is increasingly accountable for. This brief explains the two power standards, the fixed-versus-mobile choice, and where pre-conditioned air fits, for a Gulf procurement audience.

Why ground power exists

When an aircraft is parked at a gate or remote stand, it still needs electrical power for cabin lighting, avionics, galley, environmental systems and pre-flight checks. It can generate that power itself by running the APU — a small turbine engine in the tail — but the APU burns jet fuel, is noisy, and emits exhaust at the stand. The purpose of fixed or mobile ground power is to supply the aircraft from the ground so the APU can be shut down, cutting fuel burn, noise and apron emissions. In Gulf heat the same logic extends to conditioned air: rather than run the APU to drive the air-conditioning packs, a pre-conditioned air unit supplies cooled, conditioned air directly to the cabin.

400 Hz vs 28 VDC — two different supplies

Commercial transport aircraft do not all want the same electrical supply. Two standards dominate, and they are not interchangeable.

• 400 Hz, 115/200 V AC three-phase is the standard primary electrical supply for the great majority of commercial jets — the A320 family, A330/A350, 737, 777, 787 and so on. Aviation settled on 400 Hz rather than the 50/60 Hz used in buildings because higher-frequency transformers and motors can be much smaller and lighter for the same power, which matters on an aircraft. A ground power unit feeding these aircraft must deliver clean, regulated 400 Hz at the right voltage, within the tolerances the aircraft's electrical system accepts.
• 28 V DC is a low-voltage direct-current supply used mainly by smaller aircraft, general aviation, helicopters and some business jets, and for specific functions such as engine starting on certain types. A 28 VDC ground power unit is a different machine from a 400 Hz unit, and many GSE makers build both or build combined units.

The practical point for procurement: the supply must match the aircraft on the stand. A widebody gate needs 400 Hz at high amperage; a general-aviation or rotary-wing apron may need 28 VDC; a mixed-use stand may need both. Output power for large aircraft is significant — widebody 400 Hz units are typically rated in the order of 90 kVA or more, and a stand may need more than one point or a higher-rated unit for an A380-class aircraft.

Diesel, electric and battery ground power

GPUs are also distinguished by how they generate or source the power:

• Diesel-engine GPUs (mobile) carry their own diesel generator. They are flexible and need no fixed infrastructure, but they burn fuel and emit at the stand — increasingly the thing airports are trying to eliminate.
• Solid-state / electric GPUs are static converters that take mains power (50/60 Hz from the grid) and convert it to clean 400 Hz. Fixed units mounted on the boarding bridge or stand are the lowest-emission, lowest-noise option where grid capacity exists.
• Battery-electric mobile GPUs are the growing middle ground: a mobile unit with no diesel engine, drawing from an onboard battery that is recharged from the grid. These appear in searches as "battery GPU" and are attractive where a stand has no fixed converter but the airport wants to eliminate diesel. They must be sized so the battery can supply a full turnaround without exhausting.

"ECAC-compliant" in a GPU query usually refers to the broader airport-equipment and apron-safety expectations rather than a single GPU performance standard; the substantive electrical compliance comes from the standards below.

The governing standards

• ISO 6858 specifies general requirements for aircraft ground support electrical supplies.
• SAE ARP and AS documents (notably the SAE AS standards covering 400 Hz ground power quality and characteristics) define the power-quality envelope — voltage, frequency stability, harmonic distortion — that the aircraft electrical system expects.
• EN 12312 series (aircraft ground support equipment) covers the safety and design requirements for the GSE itself, including electrical GPUs.
• The aircraft manufacturers' airport-planning documents state the external power the type accepts.

A GPU that delivers out-of-tolerance power can trip the aircraft's protection or, worse, damage equipment, so power quality — not just nameplate kVA — is a real evaluation criterion.

Pre-conditioned air (PCA) — the heat half of the problem

In the Gulf, ground power alone does not let the APU shut down, because the cabin still needs cooling. Pre-conditioned air units supply cooled, dehumidified, conditioned air to the parked aircraft through a duct to the aircraft's PCA connection, so the air-conditioning load is taken off the APU.

PCA sizing is the critical Gulf variable. A PCA unit specified for a temperate climate will be badly undersized at 45–50 °C ambient, leaving the cabin hot and pushing crews to start the APU anyway — defeating the entire investment. Specifications for Gulf airports should state the design ambient (worst-case summer) and the largest aircraft on the stand, and require the unit to hold cabin temperature at that combination. Like GPUs, PCA comes in mobile (often diesel-driven) and fixed (electric, bridge- or stand-mounted) forms, and the same emissions logic favours fixed electric units where grid capacity allows.

GCC-specific considerations

• Ambient heat dominates PCA sizing and stresses GPU power electronics and cooling — specify tested high-temperature performance.
• Dust and sand clog filtration and attack connectors; IP-rated enclosures and serviceable filters are needed.
• Grid capacity determines whether fixed electric GPU/PCA is feasible or whether battery/mobile units bridge the gap at remote stands.
• Emissions and APU-reduction commitments at Gulf hubs increasingly make fixed electric 400 Hz + adequately sized PCA the default for new gates, with battery-electric mobile units for stands lacking fixed infrastructure.
• In-region service for power electronics and refrigeration plant matters; these are maintenance-intensive assets.

What this means for procurement

Start from the aircraft and the stand: which supply (400 Hz, 28 VDC, or both), at what power rating, for the largest type served; whether fixed electric is feasible given grid capacity, or whether battery/diesel mobile units are needed; and the PCA capacity required at local worst-case ambient for that aircraft. Only then does the supplier list matter. Established suppliers across 400 Hz, 28 VDC, PCA and combined units include ITW GSE (which now consolidates the former Hobart and AXA Power ground-power brands), Cavotec (whose airport division now trades within Dabico Airport Solutions), Guinault, JBT, Powervamp, Red Box and Twist, among others; many bridge vendors integrate fixed GPU and PCA into the boarding-bridge package. The badge is the last decision, after the electrical envelope and the heat case are settled.