Hughes 369D N103SU accident near Tuckers Crossing, Mississippi

Probable cause

The pilot’s loss of helicopter control when the external saw contacted trees during an attempted emergency landing due to a main rotor transmission oil pressure warning. The reason that the pilot did not release the external saw from the helicopter during the attempted emergency landing could not be determined based on the available evidence.

NTSB analysis

The pilot departed for the first flight of the day to begin aerial tree trimming operations along a transmission line. The mission’s ground support specialist (GSS), who was in communication with the pilot via radio, observed the helicopter approach the trimming zone from the south then fly northbound to the east of the transmission line; the helicopter was equipped with an external saw assembly that was about 120 ft in length and about 40 to 50 ft above the trees. The GSS and the pilot discussed hazards located in the trimming zone. Shortly thereafter, the pilot announced over the radio that he would be making an emergency landing. The GSS reported that he did not see any smoke or hear “odd noises” coming from the helicopter and that “the helicopter made a quick forward descent until the saw disappeared into the east tree line.” The helicopter subsequently impacted trees and terrain, and a postimpact fire ensued.

A postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation, but the examination was limited due to the thermal damage to the helicopter. Annunciator light bulb filament analysis revealed that the main rotor transmission oil pressure light was likely illuminated at the time of the accident, and the emergency procedure is to land as soon as possible.

The pilot was likely performing an emergency landing due to the illumination of the transmission oil pressure warning light. The reason that the pilot did not release the external saw from the helicopter during the attempted emergency landing could not be determined based on the available evidence.

The reason that the warning light likely illuminated could also not be determined. Operator text communications showed that, about 2 weeks before the accident, another company pilot reported an issue with the accident helicopter, to which a mechanic responded, “trans oil pressure may be the cause.” However, the operator stated that the main rotor transmission oil pressure light could illuminate when ferrying the external saw at higher speeds due to the aerodynamic drag placed on the external saw in forward flight and the nose-low attitude required to pull the saw during flight. The operator also stated that slowing the airspeed or adjusting the helicopter into trim would extinguish the light and allow for normal operation.

Text from the NTSB. Open the NTSB docket for CEN21LA102 for the full record.