WPR19LA205

The pilots reported that they were approaching the airport ramp area for landing from the east with a wind of 10 knots from the north. About 25 to 30 feet above the ramp at an airspeed of about 25 knots, the left seat pilot reduced left pedal pressure to turn the helicopter toward the right into the wind, then subsequently increased left pedal pressure to stop the turn; however, the helicopter continued right and began to spin. The pilot lowered the collective, and the right seat pilot took the flight controls in an attempt to slow the spin. The helicopter impacted the ground hard, spreading the landing skids and resulting in the tail rotor contacting the ramp. Postaccident examination of the airframe and engine did not reveal any anomalies that would have precluded normal operation. The helicopter’s altitude above the ramp, its low forward speed, and the right turn were conducive to the onset of a loss of tail rotor effectiveness (LTE). Given the lack of mechanical anomalies, it is likely that the loss of control was due to LTE.

On July 27, 2019, about 2215 Pacific daylight time, a McDonnell Douglas 369E helicopter, N438RC, was substantially damaged when it was involved in an accident near Eugene, Oregon. The two commercial pilots were seriously injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight. The pilots reported that they were approaching the airport ramp area for landing from the east with a wind of 10 knots from the north. About 25 to 30 feet above the ramp at an airspeed of about 25 knots, the left seat pilot reduced left pedal pressure to turn the helicopter toward the right into the wind, then subsequently increased left pedal pressure to stop the turn; however, the helicopter continued right and began to spin. The pilot lowered the collective, and the right seat pilot took the flight controls in an attempt to slow the spin. The helicopter impacted the ground hard, spreading the landing skids and resulting in tail rotor contact with the ramp. Postaccident examination of the airframe and engine did not reveal any anomalies that would have precluded normal operation. The helicopter was mostly intact. Control continuity was established throughout the helicopter with no anomalies noted. Examination of the powertrain revealed a fracture of the main driveshaft flexible joint, and a fracture of the tail rotor driveshaft. All fracture features displayed signatures consistent with overstress. Federal Aviation Administration Advisory Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters, states, "Loss of tail rotor effectiveness (LTE) is a critical, low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control." In addition, the AC lists the conditions under which LTE may occur: "Any maneuver which requires the pilot to operate in a high-power, low airspeed environment with a left crosswind or tailwind…" and "There is greater susceptibility for LTE in right turns. This is especially true during flight at low airspeed since the pilot may not be able to stop the rotation."