On July 5, 1983, a Hiller UH-12E (registration N4028K) was involved in an aviation accident near Brooten, MN. Investigators recorded the probable cause as: The thrust bearing, part number 25200-3, in the tail rotor drive was installed backwards, which allowed the beveled drive gear to move forward and become disengaged from the tail rotor drive gear. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter lifted off from a truck flatbed and began spinning. The pilot landed beside the truck, but main rotor blades struck it. Investigation found a tail rotor thrust bearing installed backwards, causing gear disengagement.
Incident
A helicopter had just lifted off from the flatbed of a truck when it started to spin. The pilot responded by lowering the collective and reducing power, then landed the helicopter beside the truck. During touchdown, the main rotor blades struck the truck.
Investigation
An investigation into the incident revealed that the thrust bearing, part number 25200-3, in the tail rotor drive had been installed backwards. This incorrect installation allowed the beveled drive gear to move forward and become disengaged from the tail rotor drive gear, leading to a loss of tail rotor effectiveness and the resulting spin.
Probable cause
The thrust bearing, part number 25200-3, in the tail rotor drive was installed backwards, which allowed the beveled drive gear to move forward and become disengaged from the tail rotor drive gear.