What happened
On 1 February 2010, a Hughes 269C helicopter was on the landing pad at Wonderboom Aerodrome, preparing for a training flight intended to assess a pilot's skills for a game rating. The flight crew, consisting of a pilot and a flight instructor, were performing pre-takeoff checks when the engine RPM was increased to 3 and200 for a magneto check.
During this procedure, the aircraft began to experience intense vibrations. The instructor immediately directed the pilot to close the throttle to mitigate the instability. However, the helicopter became uncontrollable, and the tail rotor blades struck the brick-paved surface. This impact triggered a clockwise spin that caused the main rotor blades to strike the ground and sever the tail boom from the fuselage. The aircraft was destroyed during the sequence, though the landing pad remained undamaged. The flight instructor sustained a minor head laceration, but both occupants survived the event.
The investigation
SACAA AIID investigators examined the wreckage and the mechanical components of the aircraft. Post-accident load stroke tests, conducted with supervision from the FAA, were performed on the four landing gear dampers. These tests revealed that all four dampers were improperly charged; three units exceeded their full stroke limits, while the fourth was outside the required intermediate range.
Investigators also noted that while the pilot had reported experiencing some resonance during a previous landing, the specific daily inspection prescribed in the Pilot Operating Handbook—which involves measuring the extension of the dampers—had not been performed prior to this flight. The investigation confirmed that the aircraft's engine, airframe, and other systems were properly maintained and showed no evidence of failure other than the damage sustained during the resonance event.
Findings
- The primary cause of the accident was severe ground resonance triggered by incorrect pressure levels within the landing gear dampers.
- The improper nitrogen charge in the dampers resulted in a loss of damping capability, preventing the attenuation of main rotor oscillations.
- The intensity of the resonance caused the failure of the left-hand landing gear damper attachment points.
- The subsequent contact between the tail rotor and the ground led to the loss of directional control and the structural disintegration of the aircraft.
- The weather conditions were clear and did not contribute to the incident.