What happened
On 21 December 2017, a UH-1H helicopter, registration ZS-HLI, was conducting a firefighting demonstration flight near the Bredasdorp Fire Station in the Western Cape. After completing its demonstration, the aircraft landed at the station's wash bay helipad. Upon landing, the pilot realized the right-hand skid was caught on a Bambi bucket strop and attempted to reposition the aircraft using hand signals from a ground marshal.
While hovering just inches above the ground, the helicopter entered an uncontrollable vertical oscillation. The pilot attempted to regain control by increasing power and pulling up on the collective, but the instability worsened, causing the aircraft to yaw sharply to the and lose control. The helicopter struck a perimeter fence, causing the main rotor blades to separate and the skids to break. The aircraft eventually came to rest on its belly. During the impact, a piece of a rotor blade struck the marshal, causing serious injuries. The pilot remained unharmed. Following the crash, the engine failed to shut down despite the pilot cutting the fuel supply and pulling circuit breakers, necessitating the use of water from the fire department to force an engine stop.
The investigation
SACAA AIID investigators examined the wreckage and the mechanical state of the aircraft. The investigation focused on the phenomenon of "collective bounce," a known issue in this aircraft type. Investigators also reviewed the pilot's recent flight history, noting a similar oscillation event had occurred eleven days prior to the accident. The investigation also looked into the engine's failure to shut down following the impact and the extent of the damage to the surrounding fire station property and nearby vehicles.
Findings
- The primary cause of the loss of control was pilot-induced vertical oscillation (collective bounce).
- This oscillation was triggered because the collective friction was not tightened sufficiently, falling below the required 7 pounds (hand tight).
- The aircraft's engine remained running after the pilot attempted standard shutdown procedures, requiring external intervention from firefighters.
- The ground marshal sustained serious injuries from flying debris during the impact sequence.