Accident Details
A helicopter was destroyed when it struck an overhead road sign, a privacy/guard fence, and the terrain adjacent to an interstate highway. The public use flight was being conducted to survey a tollway.
Pilot's Account
The pilot reported that the helicopter experienced a sudden right yaw with possibly a raising of the tail. The aircraft then yawed left, possibly in response to pedal input, and subsequently began what seemed to be a slower right rotation. The helicopter continued rotating to the right, descended, and impacted the sign structure, fence, and ground.
Meteorological and Radar Data
Reported winds in the vicinity were from 290 degrees at 15 knots. Radar track data showed the helicopter traveling northwest parallel to the interstate, passing a toll plaza, and initiating a left turn. During the left turn, the relative wind direction entered a region defined in the Federal Aviation Administration's "Rotorcraft Flying Handbook" (FAA-H-8083-21) as conducive to weathercock instability. During this period, the helicopter's calibrated airspeed was between 4 and 20 knots.
FAA Handbook References
The FAA-H-8083-21 states that in this region, the helicopter attempts to weathervane its nose into the relative wind. Without resisting pedal input, a slow uncommanded turn can develop. If a right yaw rate is allowed to develop and the tail moves into this region, the yaw rate can accelerate rapidly. The handbook also notes that unanticipated yaw is an uncommanded yaw rate that does not subside on its own, which, if not corrected, can lead to loss of control. This is referred to as loss of tail rotor effectiveness (LTE) and occurs to the right in helicopters with a counter-clockwise rotating main rotor.
Aircraft Examination
No anomalies were found with respect to the helicopter.