The pilot and tactical flight officer (TFO) onboard the law-enforcement helicopter were performing right turns around a ground altercation over an ocean peninsula at night when the helicopter began to spin rapidly to the right. The pilot applied corrective control inputs but was unable to arrest the rotation, and the helicopter descended into the water. The pilot sustained minor injuries and the TFO was fatally injured.Examination of the airframe and engine did not reveal any anomalies that would have precluded normal operation.Although the pilot reported that the helicopter was traveling at a speed of about 50 knots before the spins began, review of flight track data and onboard imaging revealed that, after factoring relative wind, the helicopter had essentially transitioned to a hover shortly before the event started and was flying almost perpendicular to the direction of travel for about 30 seconds before entering the rotation. This discrepancy was likely because the pilot was fixated on the scene below as it became obscured by buildings, and he was concerned about the safety of ground patrol officers who had just arrived.The nature of law enforcement flights can result in pilots needing to perform tight- radius, uncoordinated turns in a high-power and low-airspeed regime. Such conditions create an environment where unanticipated right yaw may occur, with a greater susceptibility for a loss of tail rotor effectiveness (LTE) in right turns. Flight operations at low altitude and low airspeed in which the pilot loses situational awareness from the dynamic conditions affecting control of the helicopter are particularly susceptible to this phenomenon. Additionally, the helicopter was equipped with a ducted fan anti-torque system, rather than a conventional tail rotor, which was more susceptible to encountering unanticipated right yaw at higher speeds.Page 2 of 17 WPR22FA101While the right yaw is usually correctable, the response must be appropriate and rapid, otherwise the condition may quickly increase to a point where recovery is not possible. Additionally, for the accident helicopter model, if aft cyclic was applied during the early recovery phase, the yaw rate can rapidly increase. An effective recovery is also dependent on the pilot's ability to use external visual references to coordinate corrective control inputs. Due to the night conditions and the helicopter’s proximity to open water, the pilot likely did not have a horizon or accurate external visual reference at the time the helicopter encountered the unanticipated right yaw.Although the pilot stated that he immediately applied forward cyclic and full left foot pedal in accordance with the approved recovery technique, the helicopter’s imaging system camera pitched up rapidly at the onset of the spin, indicating that the helicopter likely was in an immediate nose-down attitude. Under these circumstances, with the ground immediately filling the windshield, it is possible that the pilot initially instinctively pulled aft on the cyclic, thereby exacerbating the early stages of the spin. Once the spin had progressed, recovery would have been difficult.Both crew members had recently undergone water egress training, and the pilot was able to use it effectively to exit the helicopter after it sunk following the accident. Evidence suggests that the TFO survived the impact essentially uninjured and began the process of self-extracting. He was positioned on the lower right side of the helicopter, which was on the seabed, and would have needed to crawl through the cabin to climb out of another door or window. He had begun the process of extracting himself, but eventually drowned and was found partially out of the left door window.The pilot had a significant amount of flight experience in the helicopter, much of it at night, and had recently received training in the tail rotor-equipped version about two weeks before the accident. That training included a section on LTE, but training records indicated that the last time he had received unanticipated right yaw training specific to the accident helicopter type was about seven years before the accident. The recovery techniques for the two situations are similar, however, and the phenomenon and its recovery are well understood, especially for a pilot with his experience.The helicopter was equipped with a yaw stability augmentation system designed to reduce pilot workload by continuously adjusting the vertical control surface on the tailcone to correct out-of-trim flight. Postaccident examination revealed that the actuator for the control surface was at its full deflection, likely because of the system attempting to correct the extreme yaw encountered during the spinning descent. Detailed examination of the augmentation system did not reveal any anomalies, and although an electrical inductor within the actuator appeared to have burnt out, its damage signatures appeared to be fresh and were possibly a result of investigative testing after the unit had been damaged by corrosion following saltwater immersion. The augmentation system’s control authority was negligible at the speeds the helicopter was traveling before the spin began; therefore, an uncommanded control surface Page 3 of 17 WPR22FA101hard-over would not have contributed to the spin entry or inhibited the pilot’s ability to recover from it.The pilot started the day earlier than usual at 0400, having spent the preceding few days on leave out of state. He reported for duty after taking a connecting commercial flight to get home, almost 12 hours after waking up. The accident then occurred about 3.5 hours later, with his duty day due to finish 23 hours after he woke up. Although he took a nap on the earlier flights, the short nature of the flights meant that his sleep would have been interrupted and insufficient to have overcome the accrued sleep debt. The police department did not have policies for crew rest requirements before reporting for duty, and it is likely that the pilot was beginning to show signs of fatigue during the flight.