What happened
On 12 March 2006, a Schweizer 300 helicopter, registration EI-DDI, was conducting a training flight at Weston Airport. During an advanced takeoff and landing exercise, the instructor was performing a downwind approach and reducing forward airspeed. As the aircraft descended to approximately 50ft AGL, the airframe shuddered and the helicopter experienced a sudden, rapid yaw to the right.
Upon recognizing the loss of tail rotor control, the instructor immediately applied full left pedal, which proved ineffective. The pilot then initiated an emergency autorotation. The aircraft landed on the grass, sliding approximately 7 metres. There were no injuries to the instructor or the student passenger, and the helicopter sustained no apparent damage.
The investigation
The AAIU examined the tail rotor pitch change mechanism to determine the cause of the directional instability. The investigation established that the double row ball bearing within the tail rotor swately bellcrank had disintegrated. This failure allowed the bellcrank to slip off its pivot bolt, effectively disconnecting the control from the tail rotor gearbox.
Physical examination of the components revealed evidence of corrosion on the inner surface of the bearing. The manufacturer's analysis suggested that this corrosion likely led to the breakage of the bearing balls, which in turn facilitated the separation of the assembly. The investigation noted that the maintenance manual did not require periodic lubrication or specific inspection intervals for this particular sealed bearing.
Findings
- The primary cause of the loss of tail rotor control was the disintegration of the tail rotor swashplate bellcrank attachment bearing.
- Corrosion on the bearing's inner surface likely contributed to the structural failure of the bearing balls.
- The failure of the bearing allowed the bellcrank to physically separate from its fixed mounting on the pivot bolt.