CEN20LA167 · MD Helicopter 369 E

The pilot of the police helicopter reported that, while making a right orbit over a scene during a night flight, he felt a “strong vibration” in the controls, and the helicopter rotated rapidly to the right. The pilot recalled no unusual sounds, warning horns, or caution or warning lights before the event. According to the pilot, the helicopter was “spinning like [the] tail was not functioning,” and he responded by performing the emergency procedure for “loss of tail rotor.” He lowered the collective and pushed the cyclic forward “to gain forward airspeed and airflow over the vertical stabilizer.” Automatic dependent surveillance data showed that the helicopter began a tight right turn as its groundspeed accelerated from 10 to 30 knots. The groundspeed remained at 30 knots for about 5 seconds before slowing to 20 knots. The right turn then continued and tightened, and the helicopter flew straight for the final 5 seconds of flight. The helicopter descended rapidly until it impacted an unoccupied building and terrain, which destroyed the helicopter. Postaccident examination of the helicopter revealed no evidence of preimpact failures of the tail rotor control or drive systems. Further, there was no evidence found of a preimpact failure of the helicopter structure, main rotor system, cyclic and collective flight controls, or the engine. Based on the available data, the reason for the vibration described by the pilot could not be determined. A video taken by a ground witness showed the helicopter in a rotating descent before impact. Evaluation of the video revealed that the helicopter’s yaw rate increased from 146° to 178° per second while the helicopter was visible and that the helicopter’s yaw was to the right, which was opposite the rotation of the main rotor blades. However, the video did not record the onset of the rotation. A performance study considered whether a loss of tail rotor effectiveness (LTE) or a vortex ring state had occurred during the accident sequence. If the pilot’s statement that he accelerated to try to gain control of the spinning helicopter corresponds with the increase in speed from 10 to 30 knots, the yaw would have begun before that time and preceded the final tightening right turn. During the 30 seconds before the increase in speed, the helicopter was on a track where it would have encountered the reported winds as a right quartering headwind of low magnitude; this wind was not conducive to main rotor disc interference LTE, weathercock stability LTE, or tail rotor vortex ring state LTE. Further, the study determined that a vortex ring state was not consistent with the helicopter’s apparent level flightpath at the likely onset of the spin, and a vortex ring state does not usually result in an uncontrolled spin. Regarding loss of translational lift LTE, the performance study determined two factors that might have contributed to the uncommanded right yaw event: the increased anti-torque requirement when the helicopter was below the onset of translational lift and the right rolling moment induced by the introduction of translational lift when the helicopter was accelerating from low speed. However, the helicopter was not equipped with a flight recorder that could have provided additional data about when the yaw event began, the helicopter’s attitude and power, and the pilot’s inputs; therefore, the investigation could not determine the reason for the uncommanded right yaw.

Verbatim NTSB analysis and probable cause from the NTSB dataset