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Electric Taxi – A New Brand New Era in Green Aviation Practice

.Ask anyone in the street about pollution and noise, and most folk will immediately talk about the road transport industry, or, if like me, they live near a major airport, then they would probably refer to the airlines.

Over the last fifty years, air travel has opened up a whole new dimension to travellers. Whether travelling on business, or taking the family away, air travel enables people to reach some of the remotest parts of our planet.

During the early and mid parts of the 20th century, air travel was expensive, and only those travellers with access to a large amount of disposable wealth could afford to fly. 

This was in part caused by the relative lack of supporting infrastructure, but the size of aircraft was also a limiting factor.

The biggest direct operating cost for any airline is that of fuel, and the current smaller aeroplanes were unable to offer the economies of scale necessary to place flying within the reach of the average man. 

To put this into perspective, in the early 1960s, the workhorse of the sky was the Boeing B707, which had a seating capacity of about 140. 

On the 22nd January 1970 Pan Am introduced the very first Boeing 747-100 into service. This aeroplane changed the face of aviation forever.  With its massive seating capacity, of more than double that of the 707, the costs for air travel fell dramatically, and even the poorest backpacker could save enough money to make a transatlantic or transpacific flight.

Over the years, developments of the 747 have continued, and as an example, a British Airways 747-400 will carry 345 passengers over vast distances.

But there are always other factors.  The 1973 oil crisis made fuel costs escalate rapidly, and a number of airlines went out of business. Those that survived recognised the need for newer far more fuel efficient aircraft.

Aircraft manufacturers rose to the challenge, and many new aeroplane were developed, constructed from much lighter materials, including polymers and carbon fibre materials. 

Engine manufacturers have developed cleaner, quieter and far more fuel efficient engines, and new software driven control systems enable aircraft to fly far higher, out of the worst of the weather, and at altitudes where engines are even more frugal.

Sadly, this is still not enough.  The global energy crisis continues, and international concern with  climate change is driving fuel costs upwards.

Airlines are looking to save costs wherever they can.  Most airlines will defer operating the Auxiliary Power Unit (APU) until shortly before boarding, and some airlines have established a policy that requires aircraft to be taxied with one engine shut down.

The economics of this are sound, and saving may be made.

According to Airbus Industrie an Airbus A320 fitted with CFM56 engines will burn 250kg of fuel conducting a twenty minute average taxi time. A single engine taxi of the same duration will burn a reduced amount of 190kg.

Using IATA fuel data, jet fuel (Jet A-1) costs £0.3613 per kilo so a single engine taxi will cost the operator £68.65.  Two engines £72.26. This is doubled effectively, as the aircraft also has to taxi in after landing, which again, will take an average of twenty minutes.

Throughout 2014 fuel prices fell by an average of 42.8%, so it is reasonable to assume that they could rise again by the same amount, giving taxi costs of between £98.03 and ££103.19. 

A very simple costing taking into account British Airways fleet of 105 Airbuses, assumes that each aircraft flies 5 sectors a day (5×2 taxies = 10 x 20 minutes x 105) that’s a massive 350 hours of taxiing. 

350 hours x 60 = 21,000 minutes @ 12.5kg/min = 262,500 kg = 262.50 tonnes!

Now the figures look very different. In the above example, fuel currently costs £361.25 per tonne.  

£94,828 to just taxi around the airfield. Remember this is just a single days operation for one short haul fleet. 

Operators will be very keen to both minimise taxi times, and to reduce costs as much as possible during taxiing.

Airbus have been working on a new self propelled taxying system for the Airbus A320 series, known as eTaxi.

This system utilises a powerful air cooled electric motor that drives the main landing gear wheels via a self contained gearbox.

Powered is provided by the APU generator. The eTaxi motor has sufficient power and torque to enable the aircraft to be reversed off the parking stand, and then taxied to the holding point for the departure runway. At this point, the engines may be started.

Naturally, current procedures and checklists would have to be amended and modified to reflect the use of eTaxi to ensure continuation of current ground movement safety.

The eTaxi system offers many benefits.  Airbus’s own studies have shown that even greater fuel savings may be made than by using single engine taxying. 

Using the AP/eTaxi and a single engine for taxying equates to a fuel burn of 140kg, and full electric taxying only 40kg for the same 20 minute taxy.  

 Using the same fleet data as before, the savings are considerable. 

350 hours x 60 = 21,000 minutes @ 2kg/min = kg = 42.00 tonnes!

With fuel in our example currently costing £361.25 per tonne, 42 tonnes costs £15,172.50, a massive daily saving of £79,655.50!

Naturally,  there is a weight penalty for the eTaxi equipment, consisting of motor, gearbox, wiring harness and software and control equipment, but Airbus Industrie quotes this as being about an extra 400kg, and over a 500nm sector, this would require an additional fuel burn of 16kg.

Overall the use of eTaxi with both engines shut down, and including a 5 minute engine warm up and a 3 minute engine cool down, will offer a trip fuel saving of about 3% on a typical A320 sector of 700nm. 

So, the airline accountants will be happy with the considerable direct financial savings.  However, there are many other associated benefits by using an eTaxi. 

During taxying operations, aircraft frequently have to stop, accelerate, turn and hold in position.  This places wear on the brakes, and incurs fuel penalties every time that the thrust levers are opened to recommence taxying.  

As eTaxi is a direct drive system, the normal wheel brakes become redundant, the braking being delivered through the gearbox itself.  

 Environmentally, eTaxi makes a lot of sense.  The use of clean electricity for ground movements will significantly reduce the amount of NOx (Nitrogen Oxides such as Nitric Oxide and Nitrogen Dioxide) and CO (Carbon Monoxide) found in the local atmosphere.  Noise levels will also be significantly reduced. 

An additional benefit is a reduced exposure to the risk of the engine ingesting foreign objects, and extending the time between mandated engine inspections and checks.  

Bearing in mind that the biggest cost for an airline is fuel. Last year British Airways spent £3.5 Billion pounds on fuel. Most large national carriers will be spending about the same.  The figures are almost too large to contemplate. 

It would appear then, that any additional costs in retrofitting such devices to an existing fleet will pay for itself many times over, and any airline that specifies new deliveries without this option are potentially wasting millions.

Facts from Airbus Industrie publication FAST 51

Fuel costs from IATA Fuel cost analysis 2015

BA fleet data from http://www.ba.com

BA Fuel costs data from http://www.iag.com

Mark Charlwood©2015. Mark Charlwood is the owner of the intellectual property rights to this work. Unauthorised use is not permitted. If you want to use this article please contact me for permission. Thank you.