Total power loss involving a Bell 206B, VH-NDL, 92 km north-east of Finke (ALA), Northern Territory, on 12 November 2013

On 12 November 2013, a Bell 206B helicopter registered VH-NDL, departed a camp site located 51 NM SE Alice Springs, Northern Territory on a charter flight with the pilot and three passengers on board, in visual meteorological conditions. About an hour into the flight and 2 NM from the landing area the pilot commenced a slow descent from 2,000 feet above the ground (AGL). When lowering the collective, the pilot heard an intermittent grinding noise above the cockpit. The pilot checked the gauges, with nothing unusual noted. The noise continued to develop, and the pilot elected to land. As the helicopter descended through 400 feet (AGL), a clunking noise was heard, and power was lost to the main rotor. The pilot initiated an auto rotation and briefed the passengers for an emergency landing. During the touched down at about 0705, the main rotor blade severed the tail boom. The pilot secured the helicopter, waited for the main rotor to slow and assisted the passengers to exit the helicopter. The helicopter sustained substantial damage; the occupants received nil injuries.

ATSB examination of main drive shaft found that the forward outer coupling had failed, in overload, into five segments. The surfaces of the segments had evidence of discolouration due to over temperature and most of the surfaces had turned into red oxide (which forms in air at high temperatures, estimated to be over 500 °C). There was no detected grease that is needed for lubrication to reduce friction (heat) between and the rotating parts. Without the grease, the gear teeth on the forward inner spherical coupling softened, deformed, fractured and became jammed, resulting in the forward outer coupling shattering into the five segments. The four Temp-Plate indicators (which indicate when there has been excess temperature) were not present on the forward outer coupling exterior. There were no detected remnants of the forward rubber boot that is part of the seal assembly for containment of the grease.

Aviation Short Investigations Bulletin - Issue 32