What happened
The incident began when a Robinson R4-4 Clipper, registration G-KAZZ, experienced a lateral vibration during a flight to Sywell Airfield for scheduled maintenance. The pilot managed to land the aircraft safely, but the vibration recurred with greater intensity during a subsequent hover at the airfield.
To investigate the cause, an engineering test flight was conducted the following day. During a high-load vertical climb, the pilot experienced a severe vibration and a sudden increase in engine manifold pressure. The aircraft began to yaw uncontrollably, eventually rotating through approximately two full turns before the pilot successfully performed an emergency landing. Upon inspection by ground personnel, it was discovered that the tail rotor was no longer rotating.
The investigation
The AAIB investigation focused on the mechanical integrity of the drive system. Examination of the aircraft revealed that the tail rotor drive shaft had suffered a complete fracture just aft of the whirl mode damper bearing. This failure caused the unsupported rear section of the shaft to strike the tail cone and support frame, resulting in damage to the tail cone, the drive shaft flexible plate, and the engine clutch assembly.
Metallurgical analysis determined that the fracture was the result of a torsional overload, where the rear portion of the shaft was stopped while the forward section continued to rotate. Investigators also noted that the friction levels in the whirl mode damper bearing hangar and link were significantly lower than the recommended maintenance manual limits. While the main rotor system was thoroughly inspected, no defects were found to explain the initial vibrations reported by the crew.
Findings
- The tail rotor drive shaft failed due to a torsional overload.
- The physical damage was consistent with the rear of the shaft being arrested while the engine continued to drive the forward section.
- The vibration experienced during the initial flight and the test flight could not be definitively explained by the physical evidence.
- The low friction levels in the damper bearing hangar were noted, though testing suggested this alone was insufficient to cause the failure.