What happened
On 16 December 2006, a Robinson R44 Raven II, registration G-CEFR, was performing a private flight near Ballymena, Northern Ireland. The aircraft, carrying one pilot and three passengers, was on a right-hand circuit approach to land when it suddenly began to oscillate in pitch. This was accompanied by high-magnitude vibrations that were felt through the cyclic control.
As the aircraft descended at approximately 700 feet agl with an airspeed of 75 to 80 KIAS, the pilot found it impossible to arrest the oscillations using standard control inputs. Fearing for the structural integrity of the helicopter, the pilot executed an expedited run-on landing. The aircraft came to a stop without any injuries to the occupants or apparent damage to the airframe, though the vibration persisted during engine shutdown.
The investigation
The investigation examined the aircraft' and its maintenance history. It was noted that the aircraft had previously experienced heavy vibration in November 2006 when it was operated at a weight and center of gravity (CG) near the forward limit. Following that incident, the maintenance organisation had performed inspections and flight tests, but the vibration issue had not been detected during those checks.
Technical examinations of G-CEFR revealed a distorted aluminium lower rib within the mast fairing assembly. This damage was attributed to the excessive rocking of the main rotor gearbox. Additionally, investigators found that the main rotor hub teeter friction was 3 lbf above the specified limit, and the rotor blade coning hinge frictions were unevenly set.
Findings
- The primary cause of the vibration was the excessive fore and aft rocking of the main rotor gearbox.
- This movement was permitted by the use of new, softer main rotor gearbox mounts.
- The manufacturer confirmed that the specific mounts used in the aircraft were softer than previous versions, which allowed for the observed instability.
- The manufacturer has since implemented a process of conditioning mounts with heat and load to simulate in-service hardening, and has transitioned to using stiffer mounts in new production aircraft.