The pilot of a Bell 206B helicopter approached a construction site located at Baltimore-Washington International Airport (BWI) and brought the helicopter to a 250-foot out-of-ground-effect hover with a quartering left tailwind. Once in a hover, the aircraft made a rapid right 180-degree pedal turn, stopped momentarily, and then began another rapid pedal turn to the right. The helicopter continued turning at a fast rate and entered a spinning vertical descent impacting Alpha taxiway abeam Runway 15R. 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 loss of tail rotor effectiveness (LTE) and his inadequate recovery from the resulting unanticipated right yaw.
So what exactly is LTE? According to FAA Advisory Circular AC90-95, any maneuver that requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where an 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 to varying degrees in all single main rotor helicopters at airspeeds less than 30 knots.
Allowing a loss of translational lift results in a high-power demand with low airspeed and can set the helicopter up for LTE when certain wind conditions are present. Using the nose of the helicopter as a 0-degree reference, main rotor vortex interference can occur with a relative wind of 285 degrees to 315 degrees and cause erratic changes in tail rotor thrust. Moreover, be aware of tailwinds from a relative wind direction of 120 degrees to 240 degrees as this can cause the helicopter to accelerate a yaw into the wind. A tail rotor vortex ring state can also occur with a relative wind of 210 degrees to 330 degrees and cause tail rotor thrust variations.
To recover if a sudden unanticipated yaw occurs, apply full pedal to oppose the yaw while simultaneously moving the cyclic forward to increase speed. If altitude permits, power should be reduced.
Tags: Tim McAdams