Was [towing unit or motors in landing gear] ever tested or assessed?
Yes.
2014 Main Gear Using the Auxiliary Power Unit (APU) generator to power motors on the main wheels, the EGTS taxiing system allows aircraft to push back without a tug and then taxi without requiring the use of the main aircraft engines.
One wheel on each main gear is equipped with an electric motor to drive the aircraft From EGTS Airbus signs MoU with Honeywell and Safran to develop electric taxiing solution for the A320 Family From Airbus press release Lufthansa Technik has proven with a technology demonstrator that aircraft can be driven on ground by electric motors.
From Lufthansa Technik (with video) 2015
Towing Unit
As of 2015 Taxibot, a semi-robotic towbarless tractor which meets and connects to aircraft, is the only alternative taxiing system certified and currently in use by airlines in the market.
From Wikipedia 2016
Nose Gear Honeywell and Safran have pulled the plug on their jointly conceived electric taxiing system, while rival developer WheelTug aims to enter service with its system in 2018.
"
After thorough market analysis", the US supplier says, the two partners "agreed to stop work on the Electric Green Taxiing System due to dramatically lower oil prices and the current aviation industry's economic environment".
EGTS had been scheduled to enter service this year, with Honeywell and Safran having planned to produce a system for line- and retrofit on A320-family and Boeing 737 jets.
Meanwhile, Gibraltar-headquartered WheelTug plans to enter service with its nose gear-mounted electric drive system in 2018....
initially on 737s and, at a later stage, on A320-family ...
Neither Airbus nor Boeing are supporting the programme From FlightGlobal
aviation.stackexchange.com/users/1289/RedGrittyBrick
Variations on this kind of scheme and designs are currently being worked on.
From what I remember, using electric motors are still pretty expensive, and also you're using them for a very short time frame, such that you're lugging around extra weight that is useful for a tiny portion of time(aka more fuel burn).
Tugs of various sorts have been used at various places as well.
Also consider that various engines need time to warm up before applying full throttle, sometimes just a few seconds, others a few minutes.
aviation.stackexchange.com/users/2785/slookabill
This is an older question, and I have no answer, just experience:
When I flew from Frankfurt (FRA) to Geneve (GVA) in January 2015, we were towed by a tug nearly all the taxiway to the runway.
We turned to the parallel taxiway "N north" to get rid of the tug, then went back on taxiway "N" to the runway.
Here is our way as tracked by my mobile GPS: The pilot told us in advance that we will to this, and that it is some kind of experiment / project / study about if it works.
(I guess more from logistics / operation side) From my data, I also have some time information: 12:12:
End of push-back 12:15:
Leaving terminal 12:21: turn right on "N North" 12:22: get rid of tug, start engines12:27:
Start moving again, and back to "N" 12:29: arriving at runway 12:31:
accelerating If I remember correctly, the engines were started when we were on "N north", so 5-6 minutes before takeoff.
However, we were taxiing to that place 20 without engines.
It would be interesting to know how much fuel could be saved by this procedure (It was a A320 or similar, I don't know), but at least, the aircraft didn't need any extra equipment.
(And yes, it was an ordinary tug, not this electrical stuff or similar.)
aviation.stackexchange.com/users/4170/sweber
Along with fitting electric motors to allow push-back without a tug and more efficient taxiing, there is a plan to use tugs controlled by the aircraft for the taxi.
aviation.stackexchange.com/users/1576/NobodySpecial
One simpler alternative for multi-engine aircraft is to taxi out with only some (or just one) of the engines running, starting the remainder within a few minutes of take-off.
It might not save as much fuel as, say, a tug, but it can be done with existing aircraft and without any extra equipment.
The major drawback of any of these approaches is that running time during taxi contributes to check-out of the fuel, fuel system and other engine systems which all have to perform properly for a safe take off.
Delaying engine startup reduces/limits the opportunity for such checks.
Nevertheless, I think this practice has been done/is being done in some situations.
aviation.stackexchange.com/users/3218/Anthony X
Does it make sense towing airplanes to the head of airstrip by electric means, whether internal or external?  Taxiing from the gate to the runway on jet power seems like a waste of fuel. Is it possible and it would make sense to tow the airplane to the start of take-off run by a motorized towing unit or perhaps with electric motors in the airplane's landing gear? Was this ever tested or assessed?
Does it make sense towing airplanes to the head of airstrip by external (eg electric) means?
aviation.stackexchange.com
<taxiing><electric-engine><towing>
aviation.stackexchange.com/questions/9101
aviation.stackexchange.com/users/3424/Urquiola
A number of ways are being investigated for bringing the plane to the take-off area without burning jet fuel. These include tugs and electric motors. There is also ataxxing system using an Auxiliary Power Unit. That allows the plane to push baxk and taxi without using the main aircraft engines. Specific examples of equipment used for this purposes include the EGTS taxiing system and Taxibot, a semi-robotic tractor without a towbar. This is the only alternative taxiing system that is currently being used by airlines. Various kinds of tugs have also been used and there are plans for aircraft-controlled tugs to be used in the future. Work is currently underway on designs to allow aircraft to be towed electrically. These include aircraft-controlled tugs for taxiing and installation of electric motors. However, the use of electric motors is costly and not necessarily beneficial as extra weight is being carried for a very short time, resulting in increased fuel burn.
Specific examples of equipment used for this purposes include the EGTS taxiing system and Taxibot, a semi-robotic tractor without a towbar. This is the only alternative taxiing system that is currently being used by airlines. Various kinds of tugs have also been used and there are plans for aircraft-controlled tugs to be used in the future. Work is currently underway on designs to allow aircraft to be towed electrically. These include aircraft-controlled tugs for taxiing and installation of electric motors. The use of electric motors is costly and not necessarily beneficial as extra weight is being carried for a very short time, resulting in increased fuel burn.