A Line Mechanic reports that during an aircraft towing procedure; the towbar pin sheared and the B767 continued forward; dragging the Tug until the tow lug fractured. The aircraft went over the top of the Tug; damaging the Radio Altimeter Antenna; Galley Drain Mast and Fuselage. Mechanic did not disconnect Steering torsion links.

Narrative

I was assigned to move a B767 aircraft from deicing pad to hanger. I drove the Tug with towbar to the aircraft with Mechanic 'X;' Mechanic 'Y' drove the stairs truck. Mechanic 'X' unhooked the towbar from the back of tug; pushed it to the B767 and I latched the bar to the aircraft; I did not have a bypass pin or substitute. Decided to tow aircraft with hydraulics 'off.' Mechanic 'Y' went into the cockpit. I pushed aircraft off deice pad to Taxiway 'Z;' a 90-degree turn; went forward; turned 90-degrees from Taxiway 'Z' to Taxiway 'J;' no issues. Halfway thru final turn from Taxiway 'J' to Taxiway'J-1;' the towbar pin sheared. I heard a loud noise and sudden unauthorized change of direction of towing tractor (tug). The B767 aircraft continued forward; dragging the tug until the tow lug fractured. The B767 went over the top of the tug; causing damage to radio altimeter antenna; galley drain mast and damaging fuselage. This was confusing; because I made two 90-degree turns without incident; but during the 30 degrees turn into the final turn; the (towbar) pin sheared. In my past experience (18 years Line pushbacks at this air carrier); if there was a hydraulic bypass problem; the pin has sheared almost immediately; a few degrees into the first turn. Some aircraft did not have a hydraulic bypass for towing.For many years; tow bars were labeled 'hangar only' and stayed there. Using 'hanger only' towbars gave you some confidence that they were not Line tow bars; that were damaged and dumped there. Our Maintenance Procedures Manual says (bypass) pin must be installed during all towing situations.Immediately notified Tower Maintenance Supervisor by personal cell phone.I suggest bypass pins be placed on towbars; as in the past.