What happened
On the evening of 15 February 2024, a Bell 407 helicopter, registration ZS-RLS, was conducting a type conversion training flight from Ultimate Heliport to Grand Central Aerodrome in Gauteng. The flight, operated by National Airways Corporation, was intended to practice simulated engine failure maneuvers.
During the final approach to Runway 3/5, the pilot was performing a simulated engine failure exercise. As the pilot lowered the collective pitch, the flight instructor rolled the throttle to the idle position to enter an autorotation. During this maneuver, the engine experienced an unexpected flameout, triggering an audible warning. The instructor attempted to restart the engine by opening the throttle, but the attempt failed. The instructor then took control to execute a forced landing. Due to the darkness, the instructor initiated the flare at approximately 80 feet above ground level, which was higher than standard procedure. This resulted in a hard impact with the landing gear skids, causing the main rotor blades to strike the tail boom and sever it. The helicopter sustained substantial damage, though both occupants escaped without injury.
The investigation
SACAA AIID investigators examined the aircraft's Engine Control Unit (ECU) and Incident Recorder (IR) data. The analysis of the IR snapshots revealed that the engine flameout occurred while the Power Lever Angle (PLA) was at approximately 4°, which is significantly below the required ground idle position of 36°. The data showed a rapid deceleration of the gas generator speed (Ng), which was too fast for the ECU to initiate an automatic relight sequence.
Investigators also inspected the mechanical throttle rigging. They discovered that the HMU (hydromechanical unit) rigging holes were misaligned by approximately 5°, with the actual position at 30° instead of the required 35°. However, testing on a separate test stand indicated that the engine flameout point (7° PLA) was not directly caused by this specific rigging discrepancy, as the flameout in the accident occurred at 4° PLA.
Findings
- The primary cause of the accident was an unsuccessful autorotation following an engine flameout.
- The engine flameout occurred while the throttle was positioned below the ground idle stop.
- The instructor's attempt to restart the engine was unsuccessful due to the extreme rate of Ng deceleration.
- The hard landing was exacerbated by a high-altitude flare initiated due to a lack of visual reference in the dark.
- Contributing factors included a lack of situational awareness and the pilot's fixation on the engine restart attempt during the descent.