Aircraft GPU (Ground Power Unit) Product

Overview

An aircraft Ground Power Unit (GPU) is a self-contained power generator mounted on a towable trailer, providing 90–180 kVA of 400 Hz 3-phase AC power to commercial and military aircraft on the ground. The Prime Mover Engine (turbocharged diesel) drives a AC Generator Assembly through a constant-speed-drive (CSD) unit, maintaining frequency at exactly 400 Hz ±0.5 % regardless of load or engine throttle variation. The Cable Reel & Connector stores 300 feet of aircraft umbilical cable, connected via a Type 61A or 62A MIL-DTL-27618 connector to the aircraft power inlet.

Aircraft depend on 400 Hz ground power during gate operations: servicing, boarding, catering, and maintenance. Running the aircraft auxiliary power unit (APU) for hours is expensive (60–100 gallons/hour fuel burn) and adds unnecessary operating hours to the engine. Towing a GPU to the aircraft and establishing shore power is faster, cheaper, and quieter—essential at noise-sensitive airports like London Heathrow or Tokyo Narita.

How it works

The Diesel Engine starts via electric starter motor, running at variable rpm depending on operator demand. The Electronic Governor continuously adjusts fuel injection to maintain output shaft speed constant at 1500–1800 rpm. The Constant-Speed Drive Unit (constant-speed-drive) is a hydraulic transmission that internally varies displacement, ensuring the generator input shaft rotates at precisely the rpm needed to generate 400 Hz output.

The AC Generator Assembly (synchronous alternator) produces 115/208 V 3-phase AC, sampled by the Automatic Voltage Regulator voltage regulator and the Frequency Lock System phase-lock system. If frequency drifts above 400 Hz, the frequency-lock oscillator signals the engine governor to slow down; if frequency drops below 400 Hz, the governor speeds up. The feedback loop maintains ±0.5 % frequency regulation within 1 second.

The Cable Reel & Connector is motorized; a ground crew member operates a wireless remote control button, extending the cable to the aircraft (typically 100–200 feet at the gate) and retracting it after power-down. The Aircraft Connector mates to the aircraft power inlet with an audible click and electrical interlock.

Electrical safety

The Protection Relay System system monitors three critical parameters:

• Overcurrent (50/51): If output current exceeds 150 % of nameplate (e.g., 27 A for a 90 kVA unit), the overcurrent relay energizes the generator field breaker within <100 ms, shutting down power. This protects aircraft systems from being overloaded.
• Overvoltage (59): If output voltage rises above 132 V AC (115 V nominal + 15 %), indicating Automatic Voltage Regulator failure or excitation runaway, the relay trips the field.
• Underfrequency (81): If frequency falls below 395 Hz, indicating extreme engine loading or imminent stall, an alarm sounds. This is a warning; the aircraft must shed non-essential loads.

Modern GPUs include soft-starters that limit inrush current when first connected to aircraft (which have large capacitive loads). Without current limiting, mating the connector can produce a 5000+ ampere transient, damaging avionics.

Thermal management

The Cooling System dissipates 50–100 kW of continuous heat from the engine and generator. The Engine Radiator is oversized to operate at sea level in summer heat (40 °C ambient). At higher altitudes (Denver, Mexico City), air density is lower, reducing radiator effectiveness; GPU output must be derated (e.g., 150 kVA unit produces only 120 kVA at 5000 ft).

The Coolant Pump is mechanically driven by the engine; if the pump fails or cooling airflow is blocked (by dirt, leaves), coolant temperature rises rapidly, triggering a shutdown alarm at 100 °C.

Operations at the gate

A typical aircraft gate operation:

1. GPU is towed to the gate and positioned beside the aircraft power inlet (usually under the fuselage, 50–100 feet from the cockpit).
2. The engine is started and allowed to stabilize for 30 seconds. The HMI Display Panel confirms 400 Hz ±1 Hz and 115 V ±3 V.
3. A ground crew member presses the wireless Wireless Start System remote button, extending the cable.
4. The cable is manually connected to the aircraft power inlet; the cockpit confirms power-in via an illuminated indicator light.
5. The flight crew pilots can now use air conditioning, galley power, and cabin pressurization without burning APU fuel.
6. When the aircraft is ready to push back (typically 30–90 minutes later), the cable is disconnected.
7. The reel motor retracts the cable, and the engine is shut down.

Trailer towing and logistics

The GPU trailer weighs 15000–20000 lbs loaded. It is towed by an airport tug (typically a "tug-and-tow" configuration allowing 30000 lb combined weight). At busy airports (Atlanta, Dubai), dozens of GPUs are deployed across the ramp, servicing aircraft simultaneously. Large carriers (Delta, Emirates) own their own GPU fleets; smaller carriers lease from ground service contractors.

The Pintle Hook Coupling is an automated coupling allowing a tug to rapidly connect and disconnect GPUs at the gate without manual labor. The Ballast Weights is carefully balanced to maintain stable towing characteristics at highway speeds (up to 65 mph between gates/maintenance facilities).

Maintenance and operating costs

Diesel fuel costs $0.50–1.00 per gallon at airports. A GPU burning 8 gallons/hour at full load incurs $4–8/hour fuel cost. The Diesel Engine requires oil changes every 250–500 hours (typical oil capacity: 10–15 gallons). The Engine Radiator coolant is flushed every 2000 hours. The Umbilical Cable insulation can crack from UV and mechanical abrasion; inspection every 500 hours is recommended.

The Synchronous Alternator and Constant-Speed Drive Unit are sealed units with no field serviceable parts. If either fails, the entire GPU is removed from service and sent to an authorized repair facility (typically Boeing-certified maintenance centers). Repair costs can exceed $50000 for major overhauls.

Modern GPUs log all operational data via the Data Logger. Fleet managers download logs weekly to identify underperforming units before catastrophic failure.

Evolution: Electric GPUs

Emerging electric GPUs are being deployed at airports with adequate grid power (Frankfurt, Amsterdam). A 200 kVA electric GPU plugs into a 480 V 3-phase facility supply, requiring no engine fuel or maintenance. Transition time from engine-start to full power is <10 seconds vs. 30–60 seconds for diesel GPUs. However, electrics are capital-intensive ($250k–500k each) and require 500 kW of facility power, limiting simultaneous deployment on smaller ramps. Hybrid GPU designs (diesel engine + large battery pack + electric motor) promise the best of both: quiet operation during boarding (electric motor only) and full power for high-demand scenarios (engine + motor combined).