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If I understand correctly, there are two way to connect a tug: with or without a bar; and the tug can push back or pull forward the airliner.

  • When using a bar, how is the tug connected to the airliner? Is there a standardized fixation? Is there any articulation on both sides of the bar?
  • When using a towbarless tug, the tug lift the nose wheel. How is it done? Is there limitation on nose-wheel dimensions?
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You're correct that there are two ways to connect the tug with an aircraft.

  • In case a tow bar is used, it is fixed laterally at the nose landing gear, while allowing slight movement for height adjustment. Usually, the tow bar may pivot freely laterally and vertically at the end that attaches to the tug rather than the aircraft end, which is fixed. The following image shows the tow bar attachment point in Airbus A380.

A380 nose wheel

Image from alamy.com.

  • The attachment points differ from aircraft to aircraft, so the tow bar basically operates as an adapter between the aircraft and the tug.

Tow bars

"Pins for pushback". Licensed under Public Domain via Commons.

When using a towbarless tug, the nose wheel is simply scooped off the ground using hydraulic (for large aircraft) or electric power. The tug has a movable arm, which is opened and the tug is moved and placed so that the nose wheel fits inside the fixed arms of the tug. The movable arm is then closed. The movable arm and the opposite side has sloping edges, over which the wheels slide and the nose is lifted off as the arm closes due to hydraulic power.

This video shows how its done in case of Airbus A380.

Video from Youtube: How to lift up the world's largest commercial aircraft the Airbus A380. by user Kasavubu

The nose wheel dimensions and aircraft weight do matter and tugs are certified for different ramp weights.

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As mentioned there are two ways used for pushback: towbar and towbarless (TBL) push trucks.

There is a third way that is not as common: the Power Push Unit (PPU).

enter image description here
Source: airliners.net

Unlike the standard pushback process that uses a pushback truck and a towbar attached to the nose wheel, the PPU is a remote controlled truck directly attached to one of the main landing gear bogies. It pushes the aircraft backwards and the pilot steers the nose wheel based on the instructions of the PPU operator.

Based on one company, the way it works is via "pressure rollers [that] grip the tyres. Rotational friction forces drive the aircraft's wheels."

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    $\begingroup$ The braless pish truck, every pilot's wet dream! Love it $\endgroup$
    – DeltaLima ♦
    Commented Nov 5, 2015 at 20:07
  • $\begingroup$ You should add pictures and references/links to improve your answer. $\endgroup$
    – Manu H
    Commented Nov 6, 2015 at 9:39
  • $\begingroup$ Still i dont know how to add photos $\endgroup$
    – Hmb
    Commented Nov 6, 2015 at 11:09
  • $\begingroup$ paste in links, @Hmb, and someone will post them for you. Or, click the little "Image" icon (6th from the left) and it will walk you through the process. $\endgroup$
    – FreeMan
    Commented Nov 6, 2015 at 15:10
  • $\begingroup$ Video: youtu.be/1KyT02nqaus $\endgroup$
    – Řídící
    Commented Nov 6, 2015 at 21:05

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