What happened
A Boeing 727 was chocked and awaiting a tug when a mechanic/ramp employee approached the aircraft with a tug and tow bar. The employee stopped the tug, disconnected the tow bar from the vehicle, and connected it to the aircraft's nose gear.
Upon re-boarding the tug to move closer for final attachment, the vehicle failed to stop. The tug continued forward and struck the tow bar, causing the bar to slide off the face of the tug. The movement of the tug continued until contact was made with the underside of the aircraft's nose, resulting in the employee being crushed between the tug and the airplane.
The investigation
Investigators examined the footwear of the tug driver, noting he was wearing a built-up shoe on his right foot. Scuff marks and scraping were observed on the toe of the boot, although the origin of these marks was not verified.
Subsequent testing determined that when the specific shoe was placed under the tug's brake pedal, the braking action provided by the vehicle was insufficient to stop it.