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Aircraft manufacturers are now planning to use High Torque electric motors on the nose wheel of the aircraft, which is powered from the APU, to taxi the aircraft from the gate to the runway without using its main engines. This is intended to reduce the fuel consumption by the plane while being taxied using the main engines, but also adds additional 150 kgs approximately to the weight of the aircraft.

How much fuel savings are we talking about here? Lets say we are considering an A320?

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    $\begingroup$ There is some information here: greentaxiing.com/benefits.html It's a biased source, so it's not an ideal answer, but it may be a place to start. This specific system, btw, is installed on the main landing gear. It looks like this: upload.wikimedia.org/wikipedia/commons/0/05/… $\endgroup$ – JulianHzg Jul 14 '15 at 12:58
  • $\begingroup$ Sounds messy to me. Extra APU output required and, if it's not a busy day and you get a rapid ATC/takeoff clearance, you would incur delays while holding short or on runway as you struggle to get the main engines started and complete your checklist:( $\endgroup$ – Martin James Jul 14 '15 at 15:45
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    $\begingroup$ There are additional advantages to just fuel savings. Taxiing backwards and pushback without a tug, and, when this option starts to see widespread use, improved safety for ground crews, because the engines could be started much later, during taxiing, well away from the gate/apron. $\endgroup$ – Jörg W Mittag Jul 14 '15 at 17:28
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    $\begingroup$ Electrical taxi propulsion, air-conditioning or bleed air for starting. Pick any one:( $\endgroup$ – Martin James Jul 15 '15 at 16:58
  • $\begingroup$ @MartinJames you don't necessarily need to be rolling while starting the engines. And you can momentarily suspend the air conditioning. Delays tend to be predictable minutes in advance. Though if an engine ever fails to start that could mess up the choreography. Even without this, I've been on flights that shut down some of their engines while parked in an hour-long line on a taxiway. $\endgroup$ – Chris Stratton Aug 14 '18 at 15:45
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Jet engines are not very efficient at low power and speeds, so using an alternative method of propulsion is a good opportunity to save fuel.

An aircraft the size of an A320 will probably burn about 200 kg of fuel during taxi (based on this and this). Assuming fuel costs \$3 per gallon, this is about \$200 per flight.

The cost of the added weight from the electric motor will vary based on flight length, but assuming \$0.05 per pound, the extra cost would be \$16.50. Of course you are saving 200 kg of fuel, so this also helps to offset the weight. The electric motor is powered by the APU which uses much less fuel than the engines.

As KeithS points out, this adds up quickly. Even if United Airlines only equips its 150 A320 series planes with this equipment (which would certainly be a large upfront cost), and those planes make 5 flights each per day, that adds up to \$150,000 in savings every day. This will of course depend on fuel prices, which are actually well under $3/gal at the moment.

A simple way to save some fuel while taxiing is to only use 1 engine (or 2, for four engine planes) to taxi. This doesn't save as much fuel as the electric option, but also has much less upfront cost.

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    $\begingroup$ \$200 may not sound like much, but that's only one plane. There are about 93,000 scheduled flights per day, and a few times that number has exceeded 100,000, so overall this would, if fooot's figure is correct, save the airline industry an average of \$18,6 million daily. In addition, while airlines get bulk discounts and hedge fuel price increases with futures, the spot price for Jet A in the DFW area yesterday averaged \$4.80 a gallon, a 60% increase over fooot's quote, which increases the potential savings proportionally. $\endgroup$ – KeithS Jul 14 '15 at 15:38
  • $\begingroup$ Interesting that the 2nd of your taxi fuel burn links shows 100Kg of fuel burned on the way out, yet only 50Kg burned after landing. I guess the fuel burned in-flight makes the post-landing plane lighter for taxi? $\endgroup$ – FreeMan Jul 14 '15 at 19:44
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    $\begingroup$ @FreeMan Maybe partially, but I think it's more likely to wait in line for takeoff than to wait for a gate. Depends on the airport of course. $\endgroup$ – fooot Jul 14 '15 at 20:12
  • $\begingroup$ That makes sense. $\endgroup$ – FreeMan Jul 14 '15 at 20:13
  • $\begingroup$ What about the overtimes that the APU will do from the gate to the runway? The extra working hours will bring APU much closer to its next maintenance. Or the APU maintenance cost is trivial compared to the fuel saved? $\endgroup$ – Stelios Adamantidis Jul 16 '15 at 9:17

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