Accident Details
A police helicopter was conducting night airborne surveillance in support of a Topeka, Kansas, Police ground unit that had responded to an alarm at a building materials supply store. Witnesses on the ground observed the helicopter heading northwest when it "started spinning" and "the nose went straight down." The aircraft subsequently crashed.
Wreckage and Pilot Background
An examination of the wreckage revealed no anomalies. The helicopter pilot had accumulated 148.9 total hours in helicopters, all within the 84 days preceding the accident.
Weather Conditions
The winds reported at Phillip Billard Airport, located 8 miles east of the accident site, were from 180 degrees at 12 knots.
Helicopter Aerodynamic Characteristics
Federal Aviation Administration Advisory Circular (AC) 90-95 describes loss of tail rotor effectiveness (LTE) as a critical, low-speed aerodynamic flight characteristic. LTE can result in an uncommanded rapid yaw rate that does not subside on its own and, if not corrected, can lead to a loss of aircraft control. Helicopters are subjected to constantly changing wind directions and velocity, and the required tail rotor thrust is modified by the effects of the wind. If an uncommanded yaw occurs in flight, it may be because the wind reduced the tail rotor effective thrust.
According to AC 90-95, there is greater susceptibility for LTE in right turns, especially during flight at low airspeed, as the pilot may not be able to stop the rotation. The loss of translational lift is a flight characteristic that can create an environment conducive to LTE, adversely affecting aircraft controllability. The loss of translational lift results in increased power demand and additional anti-torque requirements. When operating at or near maximum power, this increased power demand could result in a decrease in rotor rpm.