N373SPBell 206B · 1979

On October 6, 2021, about 1430 central daylight time, a Bell 206B3 helicopter, N373SP, sustained substantial damage when it was involved in an accident near Gonzales, Louisiana. The student pilot sustained minor injuries. The helicopter was operated as a public aircraft flight.

The student pilot of the Louisiana State Police Air Support Unit helicopter reported that he was making a short cross-country flight from Baton Rouge, Louisiana to Louisiana Regional Airport (REG), Gonzales, Louisiana. About 10 miles out from REG, he obtained meteorological data and determined that runway 35 was the appropriate runway for the current wind conditions. He entered the traffic pattern, and during the downwind leg, he completed the before landing checks and observed no abnormalities with the helicopter. The student pilot then reduced power and began to slow the helicopter to about 60 knots. After completing the turn to final approach, he maintained 400 ft above ground level (agl) and then started to descend. The student pilot applied a small amount of power to sustain the desired approach angle.

Upon reaching 40 knots and 175 ft agl, the helicopter started to yaw to the right. The student pilot applied left pedal, and the helicopter continued to yaw to the right. The student pilot observed the trim indicator and confirmed that the helicopter was in trim; however, he still had to apply left pedal. Shortly thereafter, the helicopter “aggressively” yawed to the right. The student pilot applied forward cyclic input, and the helicopter rolled to the left while it continued to rotate “very aggressively” to the right. The student pilot attempted to regain control using cyclic input; however, he was unable as the helicopter was “rotating so fast.” The student pilot decreased the throttle to idle; the helicopter began to settle; and he applied collective input to cushion the landing. The helicopter experienced a hard landing and came to rest partially upright on a flat grass field about 200 ft short of the approach end of the runway. The student pilot performed an emergency shutdown and was able to egress from the helicopter.

After the accident, the student pilot reported to a responding law enforcement officer that the helicopter experienced a loss of tail rotor effectiveness (LTE) during the approach. The helicopter sustained substantial damage to the main rotor system, the fuselage, the tail boom, and the tail rotor system. A postaccident examination of the airframe and the engine revealed no preimpact mechanical malfunctions or failures that would have precluded normal operation of the helicopter. Due to the extensive airframe damage sustained during the impact sequence, pre-accident tail rotor rigging settings could not be determined. The helicopter was equipped with standard tail rotor blades that were manufactured by Bell. An examination of the helicopter’s maintenance records revealed no evidence of uncorrected mechanical discrepancies with the airframe and the engine.

At the time of the accident, the student pilot reported he had accumulated 42 flight hours total in helicopters (all in the accident make and model helicopter).

At the time of the accident, the student pilot reported the helicopter weighed 2,661 pounds, which was 593 pounds below the helicopter’s certified maximum gross weight. The estimated density altitude for the accident site was 1,948 ft.

A review of meteorological data at REG showed that at 35 minutes before the accident, the wind was from 260° at 3 knots. At 15 minutes before the accident, the wind was from 030° at 4 knots. At 5 minutes after the accident, the wind was from 330° at 5 knots.

The Federal Aviation Administration (FAA) has published Advisory Circular (AC) 90-95 titled Unanticipated Right Yaw in Helicopters. This document discusses LTE and states, in part:

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 the loss of aircraft control.

LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots. LTE is not necessarily the result of a control margin deficiency. The anti-torque control margin established during FAA testing is accurate and has been determined to adequately provide for the approved sideward/rearward flight velocities plus counteraction of gusts of reasonable magnitudes.

The AC states that flight and wind tunnel tests have identified four relative wind azimuth regions and helicopter characteristics that could, singularly or in combination, result in LTE. The four regions are:

Weathercock stability (120° to 240°): The helicopter will attempt to weathercock its nose into the relative wind. The helicopter will then make an uncommanded turn to the right or left, depending on the wind direction.

Tail rotor vortex ring state (210° to 330°): The vortex ring state will cause tail rotor thrust variations, which result in yaw rates.

Main rotor disc vortex interference (285° to 315°): Main rotor vortex passes the tail rotor, reducing the tail rotor angle of attack. This causes a reduction in thrust and a right yaw acceleration will commence.

Loss of translational lift (all azimuths): Results in increased power demand and additional anti-torque requirements. If this occurs during a right turn, the turn will be accelerated as power is increased unless corrective action is taken.

The FAA Helicopter Flying Handbook FAA-H-8083-21B discusses LTE and states in part:

Some helicopter types are more likely to encounter LTE due to the normal certification thrust produced by having a tail rotor that, although meeting certification standards, is not always able to produce the additional thrust demanded by the pilot.

While information on LTE is not published in the Transport Canada-approved rotorcraft flight manual for the Bell 206B3, Bell has published Operations Safety Notice 206-83-10 and Information Letter 206-84-41 that discuss the topic of LTE.