A small treble rig (127ft mast) was on its first well with a top drive unit installed. The rig had been operating with the top drive for 3-4 weeks. While tripping out of the hole, a stand was stood back in the mast and the traveling block and TDU lowered to the floor and the pipe elevators were made up to the pipe in the slips. Upon taking up the load to pull the stand, a noise was heard above the floor. The traveling block and TDU then started to free-fall. The driller attempted to halt the fall of the blocks with the brake but this had no effect. The blocks and TDU fell until the elevators struck the rotary table and stopped. Although no-one was injured in this event some damage was sustained to the Top Drive and the rig was shut down for 8 days while an investigation, equipment inspections and repairs were carried out.