What happened
On 18 September 2018, a Bell 206B helicopter, registration ZT-RBF, was engaged in a commercial crop spraying operation in the Melmoth area of KwaZulu Natal. The flight, conducted under part 137 regulations, was intended to spray an avocado orchard with a nutrient mixture.
Shortly after departing from a private farm, the aircraft was flying at an altitude of only 5 to 10 meters above the ground. During this low-level phase, the pilot was alerted by the low main rotor RPM warning horn. In an attempt to recover the lost engine power, the pilot applied throttle and lowered the collective; however, these maneuvers failed to restore the necessary rotor speed. Due to the extremely low altitude, there was insufficient height to perform a standard flare. The pilot instead executed a forced landing between two tree lines within the orchard. The aircraft's left skid struck the ground with significant force, and the main rotor blades subsequently collided with an avocado tree, causing the helicopter to roll onto its left side.
The investigation
An investigation by the SACAA AIID examined the aircraft's configuration and the pilot's flight profile. At the time of the accident, the helicopter was carrying 350L of pesticide and 20L of fuel. While the calculated takeoff weight of 3,022lbs was below the maximum takeoff weight of 3,200lbs, the pilot noted the actual weight was approximately 3,185lbs, placing the aircraft very close to its limit.
Investigators also analyzed the aircraft's position on the height-velocity diagram. At a speed below 40 knots and an altitude of less than 10 meters, the helicopter was operating within a high-risk flight envelope, often referred to as the 'dead man's curve,' where emergency maneuvers are severely limited by the lack of altitude and airspeed.
Findings
- The primary cause of the accident was a sudden loss of main rotor RPM triggered by the pilot rapidly raising the collective during takeoff.
- This rapid increase in blade pitch angles caused the rotor speed to decay faster than the engine could compensate.
- The aircraft was operating in a dangerous portion of the height-velocity diagram, which left the pilot with insufficient margin to recover from the power loss.
- The helicopter sustained substantial damage to the transmission system, skids, main rotor, and the spraying equipment, though the pilot escaped without injury.