Why don't large commercial airplanes have electric motors in the wheels?

Question

The cost of jet fuel during taxiing can be a pain. Concorde in particular had very large fuel expenditure for taxiing.

So why not put electric motors in at least 2 wheels to drive the plane that way? They would not have to be the most powerful motors in the world---only strong enough to get the plane up to a few miles per hour, or normal taxi speed.

EDIT: I don't think this is a duplicate of this other question. If anything, the savings cited in the answer of that question seems to imply that my question, asking why it isn't implemented, is actually a pretty good question.

Well, you might as well guess where the electricity to run those motors would come from; sooner or later, the jet engines themselves. — SMS von der Tann, Jan 18 '16 at 23:09
@SMSvonderTann Yes but unless I'm totally mixed up, power generation from jet engines is much more efficient than small (taxiing) amounts of thrust from jet engines. — DrZ214, Jan 18 '16 at 23:18
How is this question a duplicate? The question of What are the savings of...? is very different than the question of "Why isn't that technology used? — J W, Jan 18 '16 at 23:49
related, perhaps duplicate: Does it make sense towing airplanes to the head of airstrip by external means? — reirab, Jan 19 '16 at 05:46
To me this is not a duplicate: the other asks if it would be convinient, this asks why currently it is not the case. — Federico, Jan 19 '16 at 14:02

Kromen · Answer 1 · 2016-01-20T09:08:19.947

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In fact, this is currently on test.

This test is conducted by Airbus, Safran and Honeywell for the Airbus 320 familly on this aircraft.

An engine is installed on the wheels of the main landing gear

As you might know it take long time and lot of money to certified a system in aeronautics, this is why it's not in use yet.

I other hand, you need to run engine in idle for about 5 minutes in order to warm up the engine and stabilise systems (hydraulics, gyroscopes, etc)

You can find more here : http://www.greentaxiing.com/

Edit 1 : edit to add @Ben elements

edited Jan 20 '16 at 09:08

answered Jan 19 '16 at 09:43

Kromen

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This doesn't answer the question, "Why...". Has the technology been unavailable? Has the electric power source been weak? Have the existing design concepts been developed only so recently that they are not yet available? – J W Jan 19 '16 at 12:50
@JonathanWalters "As you might know it take long time to certified a system in aeronautics, it's this is not in use yet.". You can understand "use" by available. – Kromen Jan 19 '16 at 15:30
That doesn't look like an electric motor to me. Is that...an internal combustion engine? – DrZ214 Jan 19 '16 at 15:33
@DrZ214 I assure you this is a electrical engine :D – Kromen Jan 20 '16 at 08:58
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Terminology in English is combustion --> engine (usually), electric --> motor (always.) Apart from that, a good answer. – Level River St Feb 15 '16 at 00:21

score 6 · Answer 2 · answered Jan 19 '16 at 12:14

As Kromen points out, such a system is in development. There are just two main impediments:

1) The not-insignificant costs of design, certification, installation and operation.

2) Jet engines can't just be turned on as the aircraft lines up on the runway. The startup sequence itself can take a few minutes, and then they need to be running at close to idle for up to about 5 minutes to allow all of the aircraft systems (hydraulics, gyroscopes, etc) to stabilise.

These aren't insurmountable obstacles, and the system should have great benefits at busy airports with long taxi times. But this helps explain why it hasn't exactly been a top priority over the years.

Probably increasing emissions / environmental controls have increased the focus here, beyond simple commercial fuel cost considerations. — WOPR, Dec 18 '16 at 23:11

Why don't large commercial airplanes have electric motors in the wheels?

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