The Ground Handling Blog

Mototok's blog for Hangar Professionals

Written by Mototok on February 12, 2018 // 5:10 PM

Ground Power Unit facts you need to know before buying


So you own an FBO and you need to update your inventory of ground support equipment. Great! Modern design processes and technology have made ground support equipment, particularly ground power units (GPUs) much smaller and more efficient than those of yesteryear. But before you press any buy buttons, there are some things you need to know about different types of units, how they function, and which GPU is going to be the best unit for your operation.

What is a Ground Power Unit?

To put it succinctly, a ground power unit is any device which delivers the specified power (frequency, current, voltage) to an aircraft through any number of different means. Some units plug into a standard 3-phase electrical system, others are a large battery pack, many units use either a gasoline or diesel engine, and yet even others use a small turbine engine which produces both bleed air for engine starting and electrical power for the aircraft.

GPU fact #1: Get the right power!

It is widely understood that not all electricity is the same, so no need to dig into the specifics. When you are looking at a GPU for your fleet or ground operation, it is really, really important to make sure you buy a unit which is compatible for all of the aircraft.

General aviation and smaller jets and turboprop aircraft operate on a direct current (DC) system, basically the same as your car. They may be either 12-volt or 24-volt systems, which require 14-volt DC or 28-volt DC power generation to maintain the charge on the batteries used for electrical function of the aircraft.

Large business jet and all airliners, on the other hand, run an alternating current (AC) electrical system, generally in the 400 Hz band. This is the type of electricity which runs our houses, buildings, offices, etc. These large aircraft run AC systems because the inherently require much more energy than a DC system can provide. Think back to the last time you were on an airliner: interior lights on the entire flight, electrical outlets on every seat, TVs built into every headrest, not to mention the unbelievable power requirements of a fly-by-wire system like those used on every single Airbus! The energy demand is absolutely staggering so a DC system is just not suitable for the mission.

GPU fact #2: Internal combustion power or electrical rectifier?

GPUs are not universally powered by internal combustion engines; a lot of organizations opt for all-electrical frequency converters which convert standard, 3-phase electrical power to the specified AC or DC rate.

The decision here rests on what your mission looks like. Gas and diesel powered units are much more portable than an electric unit, but then again you have to provide fuel, and upkeep the engine as well as the electrical components. Also, these units are loud and produce considerable noise pollution.

Having worked on a lot of airport aprons and used both varieties extensively, the all-electric ground power units are so much nicer. They are absolutely quiet, only producing a barely perceptible hum. This is also a major safety feature; the engine on a powered unit must be run at a high power setting when the generator is engaged so they are incredibly loud and make verbal communication impossible for anything less than shouting.

GPU fact #3: Multifunction machines

While not altogether common by modern manufacturers, military organizations used hybrid machines for quite some time which were powered by a small turbine engine used to create low pressure, high volume air for jet engines. The secondary function on certain units was the addition of a generator and rectifier to provide electrical power to the aircraft. The most prolific unit is the A/M32A-60 gas turbine generator/compressor set.

No manufacturer produces them presently as new stock, but they are included here for an important reason: these units are readily available to the public as a military surplus item. Since they produce both AC and DC voltage, they are a viable option for many operations with the added bonus that they will run on just about any fuel available, save for high octane fuels. They burn diesel, kerosene, and all jet fuels with ease making them highly versatile units.

GPU fact #4: Specialized ground power units

One type of unit which was previously unfamiliar to us is made by AERO Specialties, and is a skid-mounted diesel unit which may be either placed semi-permanently or it may be hard mounted onto an aircraft tug.

This is a great solution for pushback operations where aircraft are coming in and out of spots all day. It frees up personnel by not necessitating a separate GSE unit in addition to the tug. This reduction of clutter is a boon for ramp safety.

GPU fact #5: Common mistakes when buying a ground power unit

GPUs are very expensive items both in terms of acquisition and upkeep. It might be very tempting to find a used or surplus item which is on the lower end in terms of initial purchase price, but that can easily be offset by using an inefficient machine, choosing a powered unit when electrical would be ample, etc.

GPUs are complicated machines and will require routine maintenance and repair by a competent repair authority, which may mean sending it off. Any time a large item like a GPU is sent off for repair via truck, it incurs a substantial added cost to the organization sometimes to the tune of several thousand dollars for a round trip. If the machine bought second-hand ends up being cantankerous, your organization could easily end up in the red for more than the difference in just purchasing a new unit with warranty.

Another common mistake when purchasing a GPU is either over-purchasing or under-purchasing. Neither one is inherently hazardous or dangerous, but they both may prove to be costly mistakes. Over-purchasing would be buying a full-size, dual-setting unit suitable for large turbine aircraft when you only really need a DC unit for starting small- and medium-sized turbines and pistons. At least in this scenario you can still utilize the unit, albeit a little underutilized. If your organization under-purchases a GPU, you may end up not failing to use it at all and having to deal with sunk costs.

GPU fact #6: key players in the GPU market

AERO Specialties

AERO Specialties has been a stalwart in the GPU market, as well as most other types of ground support equipment, for nearly three decades. They offer a very comprehensive full line of ground power options ranging from the lightest battery start packs to diesel units rated for the full line of ground power solutions.

Essex Electro Engineers, Inc.

Essex Electro has been in the business since 1964, but you probably have never heard of them. Since much of their business constitutes contracts with the military, their brand recognition is lack to say the least. But if you were to look across the broad expanses of military aprons over the globe, you will see dozens upon dozens of their nondescript, olive green GPUs powering countless aircraft.

Start Pac®

Start Pac® is another manufacturer of ground power units and start packs, which is not likely a household name to anyone outside of the industry. They have long produced battery start packs for all sorts of heavy equipment including diesel locomotives. Anyone who makes a starter pack for trains probably knows a thing or two about starting engines.

GPU fact #7: ground power units can be combined with other machines… sometimes.

This is really a two-fold statement; some GPUs are combined with another machine altogether, while others are merely an add-on to existing equipment.

Most (probably all) modern jets are equipped with a very small turbine engine referred to as the Auxiliary Power Unit (APU). It is installed to provide bleed air through a series of ducting in order to turn the air turbine starters on the main engines. Most APUs are also equipped with a generator, allowing it to produce electrical power for the aircraft. The aforementioned A/M32A-60 carts were really nothing more than an APU placed on a trailer with a protective cabinet around it.

AERO Specialties produces a standalone power unit which is built to be able to mount onto an aircraft tug, dramatically simplifying the logistics of pushback operations. The ability to have power and towing on one, single platform would make the operation so much simpler, and would allow the aircraft to have full electrical power during towing operations instead of just battery power. This is an often overlooked safety function for two reasons:

  1. You have a finite amount of brake applications on battery power alone, and
  2. Radios draw a lot of power and their effective range is reduced when on battery power alone.

Conclusion to ground power units

Ground support equipment is the lifeblood of your ground operations, whether it be a corporate jet, an FBO, or an MRO. Successful ground operations must have quality GSE on hand, and perhaps none more often utilized than the GPU. GPUs are built to last for decades and will do just that with proper care. Beware should you think about used and surplus equipment that it may have already been used for decades and may be at the end of its service life.

There are a lot of factors involved in selecting the correct GPU. Make sure that it is the correct voltage and frequency, and decide early in the planning stage what configuration works best for your operation. It may be a diesel or gas powered unit, where mobility is paramount. Or, an MRO hangar will likely employ a rectifier for their needs, producing quiet, efficient power. Whatever you do, do your homework first because it will pay great dividends in the end.

The right GPU and other ground support equipment is only one of many important features of safe and successful ground handling. Get even more tips and news by subscribing to our blog!

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