What happened
A police helicopter was performing night airborne surveillance support for a Topeka, Kansas, Police ground unit. The ground unit had responded to an alarm at a building materials supply store. According to witnesses on the ground, the helicopter was traveling northwest when it began spinning and the nose pointed straight down.
The investigation
An examination of the wreckage revealed no anomalies. At the time of the accident, the pilot had 148.9 total hours in helicopters, with all flight time occurring within the 84 days prior to the event. Winds reported at Phillip Billard Airport, located 8 miles east of the accident site, were 180 degrees at 12 knots.
Findings
Analysis of the flight conditions suggests the potential for loss of tail rotor effectiveness (LTE). Per FAA Advisory Circular 90-95, LTE is a critical, low-speed aerodynamic characteristic that can cause an uncommanded rapid yaw rate. If not corrected, this can lead to a loss of aircraft control.
Helicopters are subject to changing wind directions and velocities, which modify the required tail rotor thrust. An uncommanded yaw may occur if wind conditions reduce effective tail rotor thrust. There is a higher susceptibility for LTE during right turns, particularly at low airspeeds where rotation may be difficult to stop. Additionally, the loss of translational lift can create an environment conducive to LTE by increasing power demand and anti-torque requirements. When operating at or near maximum power, this increased demand could result in a decrease in rotor rpm.