Robinson R44 loses control during low-altitude game tracking flight

Casualties unknown • Cypherkuil Private Farm, South-West of Colesberg, Northern Cape, ZA

A commercial helicopter flight in the Northern Cape resulted in substantial aircraft damage after the pilot experienced a loss of tail rotor effectiveness during a turn.

What happened

On 1 August 2020, a Robinson R44 Raven II helicopter, registration ZS-RDI, was engaged in a commercial game culling operation in the Northern Cape Province. The flight departed from De Poort private farm, heading toward Cypherkuil private farm to track springbok. While flying at an altitude of less than 500 feet above ground level, the pilot executed a right turn, during which the aircraft experienced a loss of tail rotor effectiveness (LTE).

As the aircraft began to spin clockwise, the pilot attempted a recovery by abruptly increasing engine power. This action caused the helicopter to lose control, resulting in a hard landing on the skids. The impact was severe enough that the main rotor blades severed the tail boom, and the aircraft subsequently rolled onto its left side. Despite the substantial damage to the airframe, skids, and rotor system, the two occupants on board escaped without any injuries.

The investigation

An investigation by the SACAA AIID established that the aircraft was airworthy and had undergone a mandatory periodic inspection approximately 45 flight hours prior to the accident. The weather at the time of the incident was clear with excellent visibility and minimal wind. The investigation focused on the aerodynamic sequence that led to the loss of control, specifically examining the relationship between engine power, main rotor RPM, and tail rotor thrust.

Findings

  • The pilot attempted to increase airspeed during a turn by abruptly increasing engine power.
  • This sudden increase in collective pitch caused the power required by the main rotor to exceed the power available from the engine, leading to a decrease in main rotor RPM.
  • Because the tail rotor speed is directly linked to the main rotor RPM, the tail rotor's ability to provide counter-torque was significantly reduced.
  • The resulting loss of tail rotor effectiveness induced a clockwise spin that the pilot could not arrest.
  • The low altitude of the flight left the pilot with insufficient height to execute an effective recovery technique.

Probable cause

The loss of control was caused by a loss of tail rotor effectiveness resulting from low main rotor RPM, which was triggered by an abrupt increase in engine power following a decay in airspeed.

Frequently asked questions

What happened in the 2020-08-01 Robinson R44 Raven II accident near Cypherkuil Private Farm, South-West of Colesberg, Northern Cape, ZA?

A commercial helicopter flight in the Northern Cape resulted in substantial aircraft damage after the pilot experienced a loss of tail rotor effectiveness during a turn.

What aircraft was involved and where did it happen?

The accident on 2020-08-01 involved a Robinson R44 Raven II, registration ZS-RDI, at Cypherkuil Private Farm, South-West of Colesberg, Northern Cape, ZA.

What was the probable cause of the accident?

The loss of control was caused by a loss of tail rotor effectiveness resulting from low main rotor RPM, which was triggered by an abrupt increase in engine power following a decay in airspeed.

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