Air Evac EMS Bell Helicopter Textron Canada 206 L4 accident

The commercial pilot was departing for a positioning flight from a helipad bordered on three sides by buildings and parking structures. The pilot reported that, after lifting off, as he translated the helicopter from behind one of the buildings and into the prevailing wind, the nose began yawing right. He applied full left pedal, but the helicopter may have fully rotated once while moving back toward the helipad before the rotation stopped. The low rotor speed warning sounded, and the helicopter then began rotating rapidly right. The pilot lowered the collective and maneuvered toward the helipad. He subsequently raised the collective while at 25 ft above ground level, but the helicopter landed hard. A postaccident examination of the helicopter revealed no preimpact mechanical failures or malfunctions that would have precluded normal operation.

A loss of tail rotor effectiveness can be encountered while hovering under certain wind conditions, which may be encountered unexpectedly near buildings due to rapidly changing wind conditions. However, the pilot's report that the low rotor speed warning sounded and engine data provided by the operator indicated that the main rotor speed decayed during the takeoff with a corresponding decrease in the tail rotor speed. A significant reduction in the tail rotor speed could result in an uncommanded yaw and a loss of directional control. The investigation was unable to determine if the pilot's loss of directional control was due solely to the decrease in rotor speed during takeoff, the varying wind conditions, or a combination of both.

Verbatim NTSB analysis and probable cause from the NTSB dataset