Agusta A109 N91NM accident near Alexandria, Minnesota

Probable cause

The pilot's excessive cyclic input during a missed approach maneuver in night instrument meteorological conditions, which resulted in a loss of control and spiraling descent into terrain.

NTSB analysis

The pilot and two medical crewmembers were conducting a night instrument flight rules cross-country flight to pick up a patient. During the instrument approach to the destination airport, the weather conditions deteriorated. The pilot was using the helicopter's autopilot to fly the GPS approach to the airport, and the pilot and the medical crew reported normal helicopter operations. Upon reaching the GPS approach minimum descent altitude, the pilot was unable to see the airport and executed a go-around. The pilot reported that, after initiating the go-around, he attempted to counteract, with right cyclic input, an uncommanded sharp left 45° bank . Recorded flight data revealed that the helicopter climbed and made a progressive right bank that reached 50°. The helicopter descended as the right bank continued, and the airspeed increased until the helicopter impacted treetops. The helicopter then impacted terrain on it's right side and came to rest near a group of trees.

Postaccident examinations of the helicopter and flight control systems did not reveal any malfunctions or anomalies that would have precluded normal operation. The helicopter was equipped with a GPS roll steering modification that featured a switch that allowed the pilot to manually select the heading reference source. In case of a malfunction or an erroneous setting, the helicopter's automatic flight control system had at least two limiters in place to prevent excessive roll commands. Further testing revealed that the GPS roll steering modification could not compromise the flight director and autopilot functionalities to the point of upsetting the helicopter attitudes or moving beyond the systems limiters.

Recorded helicopter, engine, and flight track data were analyzed and used to conduct flight simulations. The simulations revealed that the helicopter was operated within the prescribed limits; no evidence of an uncommanded 45° left bank was found. The helicopter performed a constant right climbing turn with decreasing airspeed followed by a progressive right bank with the airspeed and descent rate increasing. In order to recover, the simulations required large collective inputs and a steep right bank; such maneuvers are difficult when performed in night conditions with no visual references, although less demanding in day conditions with clear visual references. The data are indicative of a descending accelerated spiral, likely precipitated by the pilot inputting excessive right cyclic control during the missed approach go-around maneuver, which resulted in a loss of control.

Text from the NTSB. Open the NTSB docket for CEN16FA372 for the full record.