The Boeing 727 was chocked awaiting a tug. A mechanic/ramp employee brought a tug and tow bar to the aircraft. After approaching the aircraft, he stopped the tug, disconnected the tow bar from the tug and connected it to the nose gear of the aircraft. He re-boarded the tug to bring it closer to the tow bar for attachment. Rather than stopping, the tug continued forward and contacted the bar, which slid off the face of the tug. The tug continued forward and contact was made with the underside of the aircraft's nose, crushing the employee between the tug and the airplane. The tug driver was wearing a built-up shoe on his right foot. Tests subsequent to the accident revealed that with the shoe placed under the brake pedal of the tug, braking action of the tug was not sufficient to stop the vehicle. Scuff marks and signs of scraping were noted on the toe of the boot, though the source of the scrape/scuff marks was not verified.