At Baltimore-Washington International Airport a helicopter approached a construction site on the airport property with a quartering left tailwind. The helicopter turned right, and slowed to a stationary hover at about 250 ft with a direct tailwind. Once in a hover, it made a right rapid 180 degree pedal turn. Stopped momentarily and then initiated another rapid pedal turn to the right. The helicopter turned at a faster rate than the initial turn and continued into a spinning vertical descent and collided with Alpha taxiway abeam Runway 15 Right. The FAA’s examination of the helicopter found no mechanical anomalies.
The NTSB determined the probable cause was the pilot’s improper decision to maneuver in an environment conducive to a loss of tail rotor effectiveness and his inadequate recovery from the resulting unanticipated right yaw.
According to FAA Advisory Circular AC90-95, any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. It also advises of greater susceptibility for loss of tail-rotor effectiveness in right turns and states the phenomena may occur in varying degrees in all single main-rotor helicopters at airspeeds less than 30 kt.
Bell’s Operations Safety Notice OSN 206-83-10, regarding loss of tail-rotor effectiveness in the 206B and similar airframes describes the phenomenon as an unanticipated right yaw. It contains the following warnings when maneuvering between a hover and 30 mph:
“Be aware that a tail wind will reduce relative wind speed if a downwind translation occurs. If loss of translational lift occurs, it can result in a high power demand and an additional anti-torque requirement. Be alert during hover (especially OGE) and high-power-demand situations. Be alert during hover in winds of about 8-12 kt (especially OGE), since there are no strong indications to the pilot [of] the possibility of a reduction of translational lift… Be aware that if a considerable amount of left pedal is being maintained, that a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw.”
The pilot at BWI was performing an aerial photography mission, the nature of which requires maneuvering at low altitudes and slow speeds. Add to that the distraction of trying to work with a photographer to line up the desired shot and the mission can become very demanding. Any pilot performing a similar mission needs to understand the aerodynamics and limitations of maneuvering at slow speeds.