What happened
On 13 March 2018, a Robinson R22 helicopter, registration ZS-RNW, was conducting a Part 141 training flight departing from Rand Aerodrome. The flight, which included an instructor and a student with very limited flying experience, was performing attitude and power change exercises.
While flying approximately 500 feet above the ground near the general flying area, the aircraft experienced a sudden increase in engine RPM. During this period, the student pilot unintentionally increased the throttle, causing the engine to overspeed. The aircraft responded with a rightward yaw and a nose-up pitch. In an attempt to regain control, the instructor closed the throttle. This action caused the engine RPM to drop and triggered the low rotor RPM warning horn. The helicopter entered a rapid descent, and the instructor was unable to execute a successful flare before the aircraft struck the ground. A small fire ignited following the impact due to contact between the hot exhaust and dry grass.
The investigation
SACAA AIID investigators examined the wreckage and the engine components. The investigation confirmed that the helicopter had been properly maintained and that all documentation was in order. Analysis of the main rotor blades showed minimal damage, which supported the conclusion that the aircraft impacted the terrain with low rotor RPM.
Physical evidence from the engine revealed that grass had entered the cylinders, confirming the engine was still running at the moment of impact. The investigation also noted that the governor switch was in the off position during the accident. The weather conditions at the time of the occurrence were found to have no bearing on the event.
Findings
- The primary cause of the accident was improper use of engine controls, specifically the instructor's decision to abruptly close the throttle to manage the overspeed.
- The student pilot's unintentional increase in throttle led to the initial engine overspeed.
- The instructor's technique in "chopping" the throttle caused a rapid decay in rotor RPM, making a controlled landing impossible.
- The accident resulted in 1 serious and 1 minor injury.
Safety action
Following the investigation, the training organization held a meeting to address the incident. Key safety points emphasized included the necessity for instructors to use small, controlled throttle inputs to recover from overspeed scenarios rather than abrupt movements. Additionally, the importance of clear and unambiguous control handover procedures between instructors and students was highlighted to prevent future occurrences.