Powerful stuff

posted on 27th June 2018

Aircraft, even when on the ground, are power-hungry – and, given the proliferation of passenger comforts, air conditioning and on-board electrical systems – not to mention the sheer increase in plane size – they are more so than ever before. Chris Lewis reports  

Power can be supplied on the ramp in many ways. Plug-in mains power is increasingly being used, and can often be convenient and cost-effective, as well as being very quiet, although there is, arguably, some loss of flexibility compared with portable GPUs (ground power units).

Fixed ground power exists in the form of permanent installations that convert locally-available mains power to 400Hz power or whatever power is required – either carried out centrally or at the point of use by frequency converters. (Aircraft generally cannot use the available mains power, which is typically supplied at 50Hz or 60Hz frequency.)

The main alternatives to in-ground power units are mobile GPUs and they remain a common sight at airports all around the world. They can still be the most cost-effective solution at less busy airports where the high cost of fixed power cannot be justified or where a variety of aircraft sizes or types have to be handled, requiring different power voltages.

GPUs come in all shapes and sizes and can provide DC and/or AC to keep an aircraft powered on the ground while its engines are switched off. They are also used to start the aircraft’s engine and the aircraft auxiliary power unit (APU).

Aircraft systems can be run on the APU, of course, but unnecessary use of the APU is increasingly being frowned upon, both by airline accountants and, more and more, local residents – many airports now ban unnecessary use of APUs for noise and other emission mitigation reasons.

GPUs are much cheaper to run that APUs, both in terms of the fuel burned and maintenance costs. Essentially, a GPU is a diesel generator while an APU forms part of the aircraft system and requires maintenance at set intervals, so minimising their use is good economics. APUs also burn aircraft fuel, which is generally more expensive than diesel.

Cutting APU use to a minimum could save something like US$5,000 per gate per day, while one GSE manufacturer described the noise difference as “equal to the difference between elevator music and Woodstock”.

US-based Unitron, a major supplier of GSE, says that all-electric GPUs (not to be confused with rotary diesel generator or motor generator types) are becoming smaller, more reliable and lighter, thanks to “a new hybrid of 1980s space-age high frequency power technology known as Power Factor Correction (PFC)”, observes chairman Ray Beutel. This is now beginning to be applied to the input rectifier stages of newly developed higher power equipment, he adds.

While an earlier version of this high frequency technology, called Pulse Width Modulation (PWM), has become standard for the output stages of some types of GPUs, the last significant improvement to the input rectifier stages was as far back as the 1950s when phase shifting multi-pulse technology was introduced for use in higher powered equipment to decrease the high input current distortion levels being fed back onto utility lines by the traditional six-pulse rectifier circuits found in many of these kinds of electrical systems.

Some of these offending systems included higher power communication, navigational and radar equipment, as well as all-electric GPUs, also referred to as solid state frequency converters (SSFCs) or transformer rectifier units (TRUs), depending upon whether they supplied an alternating or direct current output voltage (VAC or VDC respectively). These outdated 12, 18, and 24 pulse rectifier stages, capable of reducing high input distortion of 30% caused by a six-pulse rectifier, were able to attenuate input current distortion down to levels of 10%, 8%, and 5% as the pulse count increased. Unfortunately there was a penalty for this early technological advance in terms of equipment size, reliability and cost.

But today, as more GPUs incorporate PFC input rectifier technology, resulting in current distortion levels of typically 5% or less (sometimes as low as 3%), the price of these more advanced configurations continues to come down both in terms of initial price and operating cost. The size, weight, and reliability of these newer PWM input circuits has also greatly improved. Unitron adds that it offers one of the largest selections of GPUs, SSFCs, and TRUs available in the market, all using this latest PFC technology.

Unitron LP began offering its all-PFC Rectifier solid state power conversion product line in 2009; a few other GPU manufacturers offer this latest technology but thus far only in AC GPUs and not 28VDC or 270VDC output units. The company notes that its 28VDC hybrid technology, which combines both the high frequency input PFC and an intermediary PWM stage, results in a 28VDC GPU that is only half the weight of any other GPU of this type. This reduced weight ratio is of special benefit for mobile power carts and is also the only mobile 28VDC power cart that complies with the NEC 18 inch rule for hazardous area requirement imposed for use in aircraft maintenance hangars.

The latest military aircraft are now all electric with no hydraulic equipment aboard and are serviced and maintained using 270VDC GPUs; moreover, some of the newest commercial aircraft such the Airbus A380 and the Boeing B787 are likewise all-electric aircraft using 270VDC as their primary voltage, although so far both have on-board rectifier systems to facilitate existing 400HZ GPU supplies worldwide.

Another factor to have influenced GPU design has been the rise of the big Middle Eastern airlines, which have looked to units than can operate at 60°C or 65°C rather than 55°C. Unitron points out: “Past solutions used to offset these high-temperature locations have been to simply overrate the GPUs rather than change their traditional temperature range. But with the Middle East being such a centre for airport expansion, combined with the apparent funds to support this activity, the demand is finally high enough to actually make a change to the GPUs, as well as other GSE, rather than working around the issue. Unitron’s next generation of GPUs will be designed to meet or exceed the Middle East ‘hot spot’ environment.”

Drawbacks

GSE supplier JBT AeroTech’s Scott Gwilliam, business manager, jetway airport service equipment, points out that – though in many ways desirable – there are downsides to increased GPU use. “One disadvantage is that they have increased the activity on the ramp around the aircraft, crowding which can result in delays and lead to safety issues. Not only that, but some locations are becoming accustomed to having these resources available and are not prepared when they are not available.”

Many types of equipment are available from various manufacturers, but one size does not fit all. Factors such as the local climate, the nature of the carrier and aircraft type all affect the sort of equipment that might be required. Just to quote a few examples, power units can be mobile electric, mobile diesel, fixed inverter, central inverter or central rotating and all have their benefits and disadvantages. Air-conditioning units (ACUs) are even more complex than power units, primarily because their performance is so strongly influenced by environmental conditions.

The changing requirements of newer aircraft have also had an effect on the supply of ground support equipment, especially the A380, B787 and A350. Gwilliam continues: “Size alone was the biggest issue with respect to supporting the A380. Imagine three boarding bridges, three to four catering trucks, four power cables, four air-conditioning hoses along with everything else required to service that aircraft while on the ground.”

Though smaller, the B787 has required some unique power and air supply requirements. Solid state converters have to survive 1,100 Amp inrush current when the aircraft connects to ground power and not all legacy equipment is capable of handling this current. The B787-8 is equipped with a single 203mm (8 inch) diameter connection point for air conditioning. Most of the existing air-conditioning equipment could only produce the necessary pressure and flow in ‘Jumbo’ mode, thus requiring modification of the equipment to deliver the higher flow and pressure through a single port.

The power requirements for new aircraft should also be taken into account. Most modern aircraft require equipment that can provide around 25% more power than the older generation models. “This can cause problems for the source of power at the building because incoming power cable and breakers may now be undersized.”

Gwilliam adds: “When I get on a modern aircraft now, I expect a personal in-seat monitor, laptop power adapter and USB power ports that can power my Kindle and recharge my sound-cancelling head-phones. In addition, every new version of an aircraft stretches the plan by 6.1m (20 feet), but doesn’t change the duct diameter inside the aircraft distributing air to the cabin, electronics and cargo areas. The end result is that newer aircraft are requiring increasing volumes of air and lower temperature air – air colder than has ever been supplied for cooling the aircraft.”

Another alternative – hybrid traction

Hybrid (diesel electric) traction for ground power units (GPUs) is now a practical proposition, argues US-based ground support equipment manufacturer and distributor Aero Specialties. The company markets a wide range of products, but its top-selling GPU is the JetGo diesel-hybrid GPU that the company manufactures itself.

GPUs have to fulfil two basic functions. They need to provide continuous power for aircraft on the ground, sometimes for long periods. But they also need to give short bursts of higher power for engine starting – and it is this that governs the size of the GPU engine and generator, even though that level of power may only be needed for short periods.

Derek Rose, responsible for international sales and marketing at Aero Specialties, explains that in traditional GPUs this has resulted in an engine and generator that is “oversized for 80% of the time, to provide enough power for peak starting amperage”, with all the penalties in terms of size, fuel consumption and lack of manoeuvrability that that implies. In the JetGo, the extra power needed for starting is provided by the on-board batteries, which means that the diesel engine requires only enough power to provide the aircraft with continuous electric power and hence can be much smaller and lighter.

There are also advantages in terms of manoeuvrability, he points out – the JetGo is small and light enough to be manhandled into position, rather than having to be pushed by a tug. Moreover, it does not get kicked offline by the circuit protection systems found on many newer aircraft as its batteries absorb any power surges. The batteries also provide a back-up in the event of the JetGo being inadvertently allowed to run out of fuel.

Moreover, “The JetGo can save $8,000 a year in fuel costs alone,” Rose insists. “That’s based on a two-hour daily running time, which is quite conservative, and on US diesel prices, which are lower than in most parts of the world.” And while new, fuel-saving technology usually comes at a price, the JetGo is said to be $5-8,000 cheaper than comparable conventional GPUs.

Hardly surprisingly then, says Rose, that “JetGo is our most popular product” – in fact, by a large margin. The JetGo outsells ‘straight’ GPUs by about five to one these days.

However, the JetGo only provides DC power – no one has yet come up with a viable hybrid GPU for AC. There would be issues in providing a suitable engine and generator and there are also problems to be overcome with storing AC current in batteries.

The market for GPUs and GSE generally has been picking up, says Rose. “In the past few years, our clientèle have been holding on to their money, but now there is a large ageing fleet, and also all the environmental regulations have made it imperative that they replace their fleets.” This is especially true of one of Aero Specialties’ major markets, California, and indeed the JetGo’s small, compact size gives it great ‘green’ credentials in terms of emissions, he says.

The JetGo is also finding many customers outside the US, in Canada, the Caribbean, Australia and, increasingly, Europe. The coming US – EU trade pact could open up the market still further, as EU countries are forced to cut tariffs on imported products and remove technical barriers to trade.

The airport’s viewpoint

Standardisation of GPUs is often a wise policy, points out Copenhagen Airport (Københavns Lufthavne) asset manager Karsten Rahbek Jørgensen. The airport operates AXA 400Hz units (the manufacturer is based 100km up the road in Odense, Denmark) and provides between one and four GPUs per stand, depending on the type of aircraft that normally use it – big beasts like the A380 need four GPUs, smaller aircraft can manage with one.

Like many airports, Copenhagen has strict rules about when aircraft can use on-board APUs – normally, they can be turned on only five minutes before starting and must be turned off again within five minutes of arrival – mainly to comply with strict local noise and emission regulations.

Now, the airport is taking delivery of AXA 2400 GPUs, which will be its new standard model. A dozen are already in operation and the airport is pleased with their performance; they promise less maintenance. That said, the existing AXA fleet gives few problems, says Jørgensen.

ASUs – still vital for some

Air Start Units (ASUs) – sometimes known as air carts – remain an essential piece of airport equipment. Typically, they are vehicles with a built-in gas turbine engine that supplies enough air to start gas turbine engine compressor spools, blowing in the air through hoses attached to the aircraft.

ASUs can be mounted on independent trailers for towing by tugs or, in some cases, mounted on a standard light truck chassis. Usually, the ASU needs only to start one of the aircraft’s engines; once that is running, the others can be started by cross-bleeding.

Although most aircraft can start their engines with their APU, an ASU would be needed should the APU fail for any reason and is seen as an essential aid to reliability by most airports. Reliability is paramount for ASUs, as they are most likely to be needed when aircraft are loaded and ready to go.

Fortunately, perhaps, ASUs are generally fairly ‘universal’ machines, able to start most types of aircraft. Tug Technologies, for instance, advertises that its TMD-250 ASU will handle the engine starting needs of all widebodied aircraft engines requiring 250ppm of continuous airflow apart from the B777. However, sometimes the biggest problem at many airports is actually finding the ASU in a hurry, then getting it to where it is needed.