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To save costs, budget airlines could potentially eliminate the single-engine taxi that is standard.

Companies are looking into things like electric motors on the wheels of airliners for taxi, which could be powered by the APU, using much less fuel than even a single-engine taxi.

But why not just tow all of them using existing towing equipment?


The related post 'Does it make sense towing airplanes to the head of airstrip by electric means?' asks for electric technology (internal or external), this one asks about using existing technology.

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  • $\begingroup$ Does one such system require FAA Authorisation and is it more costly? in terms of electricity for / capital for solar panels and night time batteries for Charging towing vehicles if they are electric or net fuel use per tow in the case of combustion engine vehicles? $\endgroup$ – securitydude5 Feb 22 '18 at 12:19
  • $\begingroup$ why the world isn't as perfect as it could be? because change takes time and money $\endgroup$ – user3528438 Feb 22 '18 at 12:38
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    $\begingroup$ Voting to leave open. What's on that other thread is mostly discussing "cool innovative new ideas" rather than just using the tugs that exist now. One post there talks about a tug being used, but doesn't address the "why aren't they used more" aspect of the question. I don't think this question is answered there, and I think it of sufficiently distinct flavor to leave open as-is. $\endgroup$ – Ralph J Feb 22 '18 at 15:34
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    $\begingroup$ @RalphJ All of the answer etc. and it was still closed... why I dislike SE these days $\endgroup$ – Cloud Feb 22 '18 at 16:03
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    $\begingroup$ That would mean tugs dragging aircraft slowly round the taxiways, increasing delays. Then the tug would have to be released from the aircraft while it holds just short of the runway, Then the tug would have to clear the runway, introducing delays, and make its way back to the ramp to tow another aircraft, introducing congestion and delays. $\endgroup$ – Martin James Feb 22 '18 at 22:40
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Single-engine taxi is used for taxiing out (to the runway) and taxiing in (after landing) when the aircraft/operator allows it.

It is usually done when the taxi time is big, which means at big and busy airports. Use Google Earth to see how complicated big airports are in case you're not familiar with the airport layouts. But for the purposes of your question, let's consider a simple airport.

enter image description here
(wikimedia.org)

As shown, there is a single runway and one major parallel taxiway. Typically airplanes land and takeoff in the same direction.

First off too many tow trucks is too much traffic, too much money, and an increased chance of accidents on the ground.

Let's say in the above picture the plane is towed to where it says (11) for takeoff, the tow truck will detach, then what? It will have to occupy the runway and exit (delaying departures and arrivals).

Same thing for arrivals, tow trucks will need to wait by the exits, and with the time it takes to attach, the exit will be occupied. If not the exit but farther down, then you're already almost at the gate.

So the why not is basically: more traffic that doubles when the tow truck detaches, expenses (trucks, drivers, maintenance), reduced runway capacity, bigger chance for ground accidents, and a very complicated coordination task.

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    $\begingroup$ your answer is excellent - I agree fully. I might add that the ATC problems with managing all of the tow vehicles with separate radio communication requirements would make the proposal totally unworkable. $\endgroup$ – 757toga Feb 22 '18 at 14:30
  • $\begingroup$ Once a plane is under control of a tow truck, does ATC need to keep with both the pilot(s) and the truck driver(s) or just the truck driver(s)? $\endgroup$ – ivanivan Feb 22 '18 at 22:11
  • $\begingroup$ @757toga Not speaking about the fact that the tow drivers would have to be all reasonably qualified to manage all of ATC stuff, pilot communication etc. It's not rocket science, but not a trivial thing. $\endgroup$ – yo' Feb 25 '18 at 21:57
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Towing is much slower than taxiing.

When taxiing, you have on the order of 25 MW available, and taxi speeds seem to be 55 km/h.
Towing vehicles have on the order of 500 kW, and towing speed about 5 km/h.

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  • $\begingroup$ “have 25 MW available” – how do you arrive at this number? Even with the full 430 kN a Trent XWB can offer (I daresay it won't during taxiing, but let's leave that out), you can at 55 km/h extract at most 6.57 MW for kinetic purposes. $\endgroup$ – leftaroundabout Feb 22 '18 at 16:13
  • $\begingroup$ "on the order of " indicates I'm not quoting exact numbers, merely indicating an order of magnitude. The important point is that the aircraft's engines are two orders of magnitude more powerful than a tow tractor. $\endgroup$ – Hobbes Feb 22 '18 at 16:21
  • $\begingroup$ Well I daresay it's only one order of magnitude realistically speaking, but suppose the point stands nevertheless. But I don't think this is really the reason why towing isn't used – it would certainly also be possible to design towing vehicles with a few megawatts of power, but probably not safe or economic to use them on airports. $\endgroup$ – leftaroundabout Feb 22 '18 at 16:22
  • $\begingroup$ @leftaroundabout: Unless you're towing with jets your tow vehicle will have to weigh too much to be remotely reasonably controlled as it's limited by tractive effort rather than motive power. $\endgroup$ – Joshua Feb 22 '18 at 21:08
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  1. Its slow to tow. Even with a towbarless (which can tow faster) it is not ideal.
  2. Danger of miscommunication/slow communications.
  3. Most airliners cannot be towed with 2 engines or more running as the idle thrust is high. Therefore at least one may need to be started just before takeoff. This is not good for engines going from cold to take-off thrust in a few minutes. Also a no-start or fault on start will require a tow back to the gate.
  4. A taxi to runway in a large airport would take half an hour to an hour. A push-pack..? 10-15 minutes at the most? Towing to the take-off points will require a big increase in the number of pushback equipment.

Having said that there have been cases where aircraft have been towed into position on or near the entry point of the runway. One case was United's first non-stop from New York (or was it Newark) to Hong Kong back in the late 90s. Aircraft was towed and I believed lined up with the runway to ensure the flight started off with max fuel.

Another scenario would be if the entry into the runway was not at the very start and the turning pan was either not existent or not suitable. To get every inch of runway the aircraft may be towed onto and backed-up to the start of the runway.

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    $\begingroup$ "A taxi to runway in a large airport would take half an hour to an hour." Can you give an example or two of airports where this might be the case in reasonable traffic conditions? I'm quite certain it would not be the case at Amsterdam Schiphol, at least, and that likely qualifies as a pretty large airport by most reasonable peoples' standards. $\endgroup$ – a CVn Sep 10 '18 at 14:07
  • $\begingroup$ @aCVn: Doesn't taxiing to/from one particular runway at Schiphol already take that long, even with aircraft taxiing under their own power? $\endgroup$ – Sean Jan 11 at 4:14

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