CHI00GA160 · Schweizer 269C

The police helicopter was providing night airborne surveillance support to a Topeka, Kansas, Police ground unit, which had responded to an alarm at a building materials supply store. Witnesses on the ground said the helicopter was heading northwest when it "started spinning" and "the nose went straight down." An examination of the wreckage revealed no anomalies. The pilot had 148.9 total hours in helicopters, all within the 84 days preceding of the accident. The winds reported at Phillip Billard Airport, 8 miles east of the accident site were 180 degrees at 12 knots. Federal Aviation Administration Advisory Circular (AC) 90-95 states that loss of tail rotor effectiveness (LTE) is a critical, low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in a loss of aircraft control. Helicopters are subjected to constantly changing wind directions and velocity. 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. "There is greater susceptibility for LTE in right turns. This is especially true during flight at low airspeed since the pilot may not be able to stop rotation." The loss of translational lift is a flight characteristic that can create an LTE conducive environment capable of 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.

Verbatim NTSB analysis and probable cause from the NTSB dataset