What happened
On 2 October 2006, a Robinson R44 Raven II, registration ZS-RAX, was involved in an incident involving mechanical vibration during a private flight. The pilot, who also owned the aircraft, initially flew from Grand Central Aerodrome to Cullinan. During this outbound leg, the pilot noticed a low-frequency vibration but attributed the sensation to potential turbulence.
During the return flight to Grand Central later that afternoon, the vibration persisted and appeared to increase in amplitude. Upon landing safely at Grand Central Aerodrome, the pilot requested a maintenance inspection to address the ongoing concern.
The investigation
An inspection of the aircraft's main rotor blades revealed that one specific blade (Serial # 2981C) had suffered severe deformation characterized by a visible bend and wrinkling on the upper skin surface. The investigation also identified damage to the teeter stop within the main rotor assembly linked to the affected blade. The second rotor blade showed no signs of distress.
Consultation with the manufacturer provided insight into the nature of the damage. The manufacturer indicated that such damage is not an isolated occurrence and is typically caused by improper handling. Specifically, the damage aligns with a failure to follow the warning placard located on the underside of the blade tips, which prohibits pulling down or pushing up on the opposite blade.
Findings
- The primary cause of the vibration was the structural deformation of main rotor blade Serial # 2981C.
- The deformation was likely caused by improper manual loading of the blade at an undetermined time.
- The physical damage—specifically the stretching of the upper skin—resulted in the blade bowing upward under in-flight lifting loads, creating the low-frequency vibration noted by the pilot.