Amateur-built Scorpion 133 N51017 accident near Andale, Kansas

Probable cause

The pilot’s abrupt control inputs, which resulted in a low-G condition, main rotor mast bumping, and subsequent severing of the tailboom by the main rotor blades.

NTSB analysis

The pilot departed from his residence on a local area flight to practice maneuvers in the experimental helicopter. A witness heard a loud “bang” and observed debris separating from the helicopter as it descended to the ground. Surveillance camera audio captured the sound of the helicopter’s experimental engine at a constant high-power setting, followed by a momentary reduction in power. Shortly thereafter, the audio captured multiple impacts and the helicopter came into view inverted, then descended to the ground. A postimpact fire consumed the wreckage.

Postaccident examination of the wreckage found that the aft tailboom had separated from the helicopter and was located about 150 yards from the main wreckage. An impact mark consistent with the shape of a main rotor blade was noted at the right side of the tubular structure at the separation area, consistent with a main rotor blade impact.

While no mechanical anomalies were found during the airframe and engine examination, the examination was limited due to extensive fire damage.

The throttle control cable was found thermally separated from the collective control and the snap-out ball joint was not in the safety position; it was found separated from the carburetor. However, the surveillance audio of the engine’s reduction and increase of rpm before the main rotor’s impact with the tailboom are consistent with an intact throttle control and indicate that the throttle cable likely separated from the carburetor during the impact with terrain.

The pilot did not hold a helicopter rating. His last solo endorsement was October 07, 2022, and was valid until January 05, 2023. The accident occurred 18 months after the expiration of the last solo endorsement, and the pilot was not authorized to conduct solo flight operations at the time of the accident.

The pilot previously reported that he felt uncomfortable performing autorotations and requested additional training from a flight instructor; however, it was unknown what maneuvers he was performing during the accident flight.

Teetering main rotor systems, such as the one on the accident helicopter, are particularly susceptible to main rotor mast bumping during the low-G conditions that result from abrupt forward (nose-down) cyclic inputs. Based on the available evidence the pilot likely applied abrupt control inputs while performing an unknown maneuver, which resulted in main rotor mast bumping during a low-G condition and subsequent main rotor blade contact with and subsequent separation of the tailboom..

Text from the NTSB. Open the NTSB docket for CEN24FA248 for the full record.