AW139 N304FD accident near Tujunga, California

Probable cause

A yaw excursion and uncontrolled descent due to loss of tail rotor effectiveness during aerial firefighting operations resulting in an impact with trees. A contributing factor was the pilot’s failure to maintain sufficient airspeed, coupled with a quartering tailwind during aerial firefighting operations. An additional factor was the operator’s lack of guidance to the hazards associated with aerial firefighting operations near and around smoke columns.

NTSB analysis

The pilot of the Los Angeles Fire Department (LAFD) helicopter was conducting a water drop during an aerial firefighting mission when the accident occurred. As the helicopter neared the drop area, the pilot slowed to about 20 knots and the helicopter entered an uncommanded right yaw and began to descend. The pilot released the water and the helicopter continued to yaw to the right and descend rapidly. The pilot reported that he had little positive control of the helicopter, and the helicopter contacted trees. After the tree impacts, which resulted in substantial damage to the horizontal stabilizers and tail boom, the pilot regained control, departed the area, and performed an emergency landing nearby.

The pilot reported, and onboard data from the helicopter confirmed, that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.

During the water drop, it is likely that the helicopter encountered radiant heat from the fire and its associated smoke columns, which reduced helicopter performance. Additionally, the helicopter was operating with a quartering tailwind at the time, which was within the critical wind azimuth published by the manufacturer. It is likely that the combination of the increased heat and smoke, adverse wind conditions, a higher helicopter weight due to the water onboard, and the higher torque demand as the result of the helicopter's slower airspeed resulted in the yaw excursion and vertical descent due to a loss of tail rotor effectiveness. As the helicopter continued its flight path out of the area of the gases, heat, and wind, and dropped the water, the pilot was able to regain control of the helicopter.

Review of published LAFD training information and operating procedures revealed no guidance regarding water drop operations or the hazards associated with operating near and around smoke columns.

Text from the NTSB. Open the NTSB docket for ANC17LA051 for the full record.