CEN14FA193 · Airbus AS350 B3E

The commercial rated pilot planned to depart on a repositioning flight from a medical helipad located on a hospital rooftop with two medical technicians on board. The pilot reported that he completed all of the pretakeoff hydraulic checks and did not note any abnormities with the pedal movement. As the helicopter lifted from the helipad, the pilot expected a slight left turn; however, the helicopter kept turning. The pilot tried to stop the turn without success, and the helicopter then entered a left spin. The pilot reported that the (antitorque) pedals felt jammed or locked in the neutral position. The pilot added that, during the spin, he looked for a light but that he did not recall seeing any (warning) lights. Video footage from a security camera captured the helicopter completing several rotations before it impacted the rooftop and then came to rest adjacent to the helipad.

The helicopter was equipped with a dual (upper and lower) hydraulic system, and the lower system was used to power the single-servo tail rotor servo control and the yaw load compensator. Testing and examination of the lower hydraulic system did not reveal any abnormalities. Data from the helicopter's quick access recorder (QAR) and nonvolatile memory (NVM) from the engine controls were also downloaded; no abnormities were noted.

An examination of the cockpit found the yaw servo hydraulic switch on the collective in the "on" (flight) position, the correct position for the flight. The "ACCU TEST" switch, which controls the accumulator for the tail rotor, was also found in the normal (flight) position. The NVM does not record the positioning of the switches, and analysis of the recorded data provided no indication that the switches were activated during flight.

The investigation tried to determine a reason for the development of the helicopter's spin. Given the pilot's statement that the wind was "relatively calm," which was corroborated by the security camera video footage that showed the wind effect on the nearby smoke and water, a loss of tail rotor effectiveness likely did not occur. Drive continuity of the tail rotor and control continuity from the pedals to the tail rotor were established. No evidence of foreign object debris (FOD), including any witness marks that could be associated with the presence of FOD, was observed in the pedal control system, and there was no evidence indicating that a pedal had jammed.

During takeoff, it is likely that there was an absence of hydraulic boost to the tail rotor pedals, either from a misconfiguration of the yaw hydraulic isolation switch or a failure in the lower hydraulic system that was not evident during postaccident testing. Although the specific cause of the absence of hydraulic boost to the pedals could not be identified, there was no evidence of either abnormal functionality of the lower hydraulic system or a tail rotor hydraulic circuit misconfiguration. Additionally, by design, the helicopter's caution panel does not provide a warning indication when the yaw hydraulic isolation switch is activated.

The manufacturer had originally equipped the helicopter with a cockpit imaging system; however, the operator had removed the system. The removal of this system precluded a determination of the configuration of the hydraulic control switches before takeoff. Further, due to the lack of available cockpit images, the investigation was unable to verify the pilot's actions before takeoff, including whether he moved the hydraulic isolation to "off" before the loss of control.

Verbatim NTSB analysis and probable cause from the NTSB dataset