as long as we seem to have a bunch of technical experts here, can I ask about these fancy tugs that I've seen at some airports? They have a sort of tuning fork shape if you look at them from above, and they surround the aircraft nose wheel with their arms and lift the whole front end up! Is there some advantage to this -- less stress or something? They must be quite expensive. At AMS for example where I saw one, the pushback doesn't use any other ground guys watching the wings either -- the tug operator just grabs the wheel and pushes. Maybe they have certainty that there won't be other vehicles out there.
Also, the UX gates at IAD on the other hand, go completely simple and cheap on the tugs -- it's like they're little more than electric-powered (it seems) style pallet pulling motors. I guess if it gets the job done!
Goldhofer tugs - they have been German specialist manufacturers for hydraulic heavy-lift road trailers for years....see here:
http://www.goldhofer.com/english/pro...gschlepper.php
And to answer your query on how the tugs work, here:
http://www.goldhofer.com/english/pro...infuehrung.php
(From what I can observe now from the photograph, it lifts the wheel up a bit, then pulls the plane along, just like it would do normally, securing the nose gear's wheels/tires, (using forces normally encountered by the nosegear, and at the same places), rather than attaching itself to the nosegear pylon/support (whatever it is called))
Then look at the other products they have, which shows you their history in the field of heavy cargo transport....
Most of the large planes at FRA are being moved around with these tugs.