What happened
On July 29, 2024, an Eurocopter France AS 350 B2 helicopter was conducting vineyard spraying operations near Ockfen, Germany. During the fifth flight of the day, the pilot noticed an unidentifiable jolt in the aircraft. Although an initial inspection of the instruments and controls showed no abnormalities, the pilot continued the mission.
During the sixth flight, while maneuvering uphill through a vineyard, the pilot experienced a sudden loss of tail rotor control authority, noting that the pedals felt as though they were "moving into empty space." To maintain directional control, the pilot immediately increased airspeed to utilize the vertical fin. After attempting to troubleshoot the issue and finding the hydraulic test switch ineffective, the pilot decided to execute a controlled emergency landing at Frankfurt-Hahn Airport. To reduce weight, the pilot performed an emergency dump of the spray liquid before approaching the runway. The helicopter performed a landing on runway 03, which resulted in a 24-meter skid and a 180-degree rotation. There were no injuries to the pilot.
The investigation
The BFU investigation focused on the failure of the tail rotor pitch control assembly. Technical examinations of the T/R-Spider (the component responsible for tail rotor blade pitch adjustment) revealed that the assembly had fractured into two pieces. The non-rotating portion of the spider, including the ball bearing, showed no evidence of grease in critical areas, such as the bearing, lubricating grooves, or spacers. Conversely, the rotating portion of the spider exhibited marks consistent with the inner rings of the bearing rotating improperly, and the fracture surface showed signs of overheating.
While the operator's maintenance records indicated that the bearing had been greased during installation 151 flight hours prior, the physical evidence suggested otherwise. The investigation noted that the tightening torque of the cap screws was below specification, though this was not considered the primary cause of the failure.
Findings
- The primary cause of the tail rotor control failure was a lack of lubrication in the T/R-Spider ball bearing.
- The absence of grease led to overheating, which caused the material of the rotating spider to creep and eventually fracture.
- Although maintenance documentation stated the component had been greased, the physical condition of the bearing and spider confirmed that the lubrication was insufficient or absent.
Safety action
Following the incident, the operator reviewed its internal maintenance procedures and conducted retraining for technical personnel. As an immediate preventive measure, all helicopters in the operator's fleet were inspected and re-greased.