What Is Ground Power Units?

I spent fifteen years on the ramp at DFW before moving into ground support equipment sales. Back then we called GPUs "the carts" and nobody thought much about them. You hooked up the cable, hit the switch, waited for the green light. Done. The pilots could run their systems without burning jet fuel on the ground.

A ground power unit does exactly what the name says. It gives an aircraft electrical power while the engines are off. Simple concept. Not so simple execution.

Here's something most people outside the industry don't realize. Aircraft don't use the same electricity as your house. Your wall outlet delivers 120 volts at 60 Hz. That frequency-60 cycles per second-works fine for refrigerators and air conditioners. But it makes electric motors heavy.

Weight kills you in aviation. Every pound you save on motor weight is a pound you can fill with paying passengers or cargo. So aircraft engineers went with 400 Hz back in the 1950s. At that frequency, motors and transformers shrink dramatically. A 400 Hz motor might weigh half what a 60 Hz motor weighs for the same power output.

The tradeoff is transmission losses. Higher frequency means more energy bleeds away over distance. But inside an aircraft, the distances are short. The weight savings more than compensate.

This creates headaches on the ground. You can't just run an extension cord from the terminal building. The airport grid runs at 50 or 60 Hz like everywhere else. Something has to convert that utility power to 400 Hz.

That something is the GPU.

The simple version: a GPU takes whatever power source it has-diesel engine, utility electricity, battery pack-and produces 115 volts AC at 400 Hz. Or 28 volts DC for smaller aircraft. The output goes through a fat cable to a receptacle near the aircraft nose.

The complicated version involves frequency converters, voltage regulators, protection circuits, and about a thousand things that can go wrong at 2 AM when a flight is delayed and everyone is screaming.

I've seen solid-state units from ITW GSE run flawlessly for eight years with nothing but filter changes. I've also seen brand new diesel units catch fire because someone forgot to check the coolant. Equipment is only as good as the people maintaining it.

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Most airports run a mix of both.

Fixed units sit at the gate. They pull power from the airport electrical grid and convert it. The cable reaches the aircraft through a pop-up pit in the concrete or a hose reel mounted nearby. No diesel exhaust. No fuel costs. No scheduling headaches. But you're committed. If that unit breaks, you're scrambling for a mobile backup while passengers watch through the window.

Mobile units ride on wheeled carts. Ground crews tow them around with tractors. Old school operations ran everything this way. The flexibility is real. Aircraft parks at a remote stand with no fixed power? Roll a mobile unit over there. Cargo operation on the far side of the field? Same deal.

The downsides are also real. You're burning diesel to tow these things back and forth. More traffic on the ramp means more opportunities for accidents. And someone has to coordinate which unit goes where. That sounds trivial until you have 200 flights a day and three units down for maintenance.

Bridge-mounted units hang under the jet bridge. They move with the bridge, so the cable is always close to the aircraft receptacle. Clever solution. But mechanics hate them because you can't service the unit while a flight is at the gate. Try explaining to your supervisor that you need to take a bridge out of service during the afternoon push.

Ten years ago, battery-powered GPUs were a curiosity. Lead-acid packs were too heavy and didn't last. You'd get maybe three or four aircraft turns before needing a recharge.

Lithium iron phosphate changed that.

The newer battery units can handle ten or twelve narrow-body turns on a single charge. They weigh less than half what lead-acid equivalents weighed. And they're quiet. I mean genuinely quiet. No diesel rumble. No exhaust. Ground crews noticed immediately.

Japan Airlines started running lithium battery GPUs at Matsuyama Airport last year. Their stated reasons were emissions reduction and noise. But I suspect the real driver was maintenance costs. Diesel engines need oil changes, filter replacements, injector cleanings. Battery packs need almost nothing.

AERO Specialties sells a unit with lithium iron phosphate cells that contain no cobalt. That matters because cobalt supply chains are a mess-ethically and logistically. The batteries also transfer to other ground equipment when they can't handle the high-demand GPU duty anymore. Second-life use. Smart.

Start Pac has been at this since 1997. Jim Wurth, one of the founders, was an Eastern Airlines pilot who flew helicopters after retirement. Story goes he got stuck somewhere remote when his MD500 battery wouldn't crank in cold weather. He engineered his own starting unit. His wife Judith saw the business opportunity. Twenty-five years later they've shipped over 50,000 units.

Their portable lithium units weigh ten pounds. Ten. You can throw one in the baggage compartment as insurance. Pilots flying into unfamiliar airports love them.

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A Boeing 737 or Airbus A320 needs one 90 kVA connection. A Boeing 777 needs two. The receptacle on a narrow-body sits about two meters off the ground. Wide-bodies put them closer to three meters, so you need a ladder.

Undersizing is dangerous. Overpaying for capacity you don't need is just wasteful.

I've watched operators buy the biggest unit available because they figured it would cover everything. Then they complained about fuel consumption or charging times. You want the right tool, not the biggest tool.

Zurich Airport published data a few years back comparing emissions from aircraft auxiliary power units versus ground power. The APU produces roughly ten times more CO2 per hour. Ten times.

The European Union battery regulation kicking in requires manufacturers to declare production carbon footprints. That's going to push procurement decisions. Airlines and airports tracking scope 3 emissions need this data.

Diesel GPUs aren't going away tomorrow. But the direction is clear. Every major manufacturer is expanding their electric and battery product lines. ITW GSE, TLD, AERO Specialties-all of them.

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Figure out your actual needs first. How many gates? What aircraft types? How many turns per day? What's your electrical infrastructure look like?

Fixed solid-state units make sense for high-traffic gates with good utility power. Battery units work well for remote stands or operations where you can't run electrical infrastructure easily. Diesel still has a place for backup power or locations with unreliable grids.

Don't buy based on the spec sheet alone. Talk to operators running the same equipment. Ask about failure modes and parts availability. A unit that performs beautifully in the manufacturer's test facility might struggle in Phoenix summer heat or Minneapolis winter cold.

And budget for training. I've seen expensive equipment damaged because nobody taught the ramp agents how to properly connect the cable. The interlock LED exists for a reason.