What happened
On February 3, 1999, a Robinson R22 Beta helicopter, registration OH-HKP, was conducting a training flight near Sipoo, Finland. The flight, which included a check pilot and a trainee, had departed from Hernesaari for a series of maneuvers, including two autorotative approaches and two normal landings on ice.
While flying at an altitude of approximately 400–50 and 500 feet, the crew heard a clicking sound coming from the rear of the aircraft. Immediately following this noise, the pilot found that the anti-torque pedals had become ineffective. The crew promptly executed an autorotative landing on the ice near the shoreline. The landing was performed smoothly, and the aircraft sustained no significant damage during the touchdown.
The investigation
The investigation focused on the mechanical failure of the tail rotor control system. Technical examinations of the pitch control assembly (serial number 2683) revealed that one of the two bearings had seized completely. Upon disassembly, investigators found that the bearing was entirely devoid of grease and showed significant signs of corrosion on the outer race and inner race.
Further analysis by the Robinson Helicopter Company (RHC) suggested that corrosion likely originated from moisture entering the bearing, possibly during periods when the aircraft was parked. This corrosion caused uneven rotation, leading to fatigue cracks in the bearing's retainer ring. The subsequent failure of the retainer ring caused metal debris to accumulate, eventually leading to the seizure of the bearing. This seizure generated enough torque to break the pitch control lever's lug and displace the control rod end.
Findings
- The immediate cause of the incident was the seizure of a bearing within the pitch control assembly, which resulted in the loss of tail rotor controllability.
- The underlying cause was the lack of lubrication in the bearing at the time of the failure.
- Corrosion within the bearing suggests that moisture had penetrated the unit, potentially through the seals, which may have been exacerbated by the loss of grease over time.
- The failure was not detectable through standard maintenance inspections, as the existing maintenance manual did not specifically require checking these particular bearings for grease levels or leaks.
Safety action
- It is recommended that maintenance personnel strictly adhere to updated manual revisions (Revision 17) regarding monitoring for unusual noises during engine start/shutdown and inspecting seals for grease leaks.
- Operators should avoid using high-pressure washers or aggressive solvents near bearing areas during aircraft cleaning to prevent moisture ingress.