Tex Air Helcopters Eurocopter AS350B2 accident

The helicopter was in level flight at 1,000 feet msl, when the pilot felt a thump and a yaw. He moved the anti-torque pedals; however, there was no "tail rotor response." A pilot from another helicopter confirmed that the tail rotor was still turning. The pilot reported that he reviewed the emergency procedure for tail rotor failure and flew the helicopter to a nearby airport. The pilot stated that he maintained about 70 knots indicated airspeed and pressed the "HYD" test button for 5 seconds, then returned it to the normal position, as called for in the procedure. The pilot also reported that at this time he turned off the warning horn which alerts the pilot of low main rotor rpm or loss of hydraulic pressure. He completed three shallow approaches to burn off fuel and to see how the helicopter would respond to control movements. While turning left to downwind, during the fourth go-around, the pilot was having trouble controlling the left yaw when he realized the helicopter's hydraulic system had failed. The pilot initiated an autorotation downwind and instructed the passenger to "pull the [fuel flow control] lever off." When the helicopter touched down, "it slid with a yaw left and flipped over coming to rest on its left side." Examination of the cockpit revealed that the console hydraulic test switch (located next to the horn switch) was pressed in (hydraulics off). According to the manufacturer, the hydraulic test switch cuts off hydraulic power; however, the pilot would not lose control assist until the accumulators were depleted. The horn switch was not pressed in (warning horn deactivated, for the horn to function, the switch has to be pressed in). Post accident testing of the hydraulic system revealed no anomalies. Examination of the tail rotor system revealed that the spider bearing inner race was cut through the rotating housing. Examination of the tail rotor spider revealed that the bearing seal exhibited heat damage, and the bearing would not rotate due to small pieces of cage material loose in between the inner and outer races. No evidence of lubrication was present on the race or balls.

Verbatim NTSB analysis and probable cause from the NTSB dataset