What happened
On 25 August 2021, an Enstrom 280FX helicopter, registration G-OJBB, was conducting a private flight near Rhobell Fawr, Gwynedd. The pilot, who was also the aircraft owner, intended to observe a stone obelisk near the mountain summit. While maintaining a hover approximately two to three feet above the ground, the aircraft suddenly and uncommand to the pilot yawed rapidly to the right. This loss of control caused the helicopter to strike the ground heavily on its skids.
The impact caused significant damage to the main rotors, tail boom, and skids. The pilot sustained minor injuries to his arm and leg and was evacuated from the site by a search and rescue helicopter.
The investigation
An investigation into the wreckage and the tail rotor gearbox (TRGB) revealed that the internal components had suffered extreme degradation. Examination of the gearbox showed that the bevel gears were severely damaged, appearing almost toothless, which allowed the input and output shafts to rotate independently. Metallurgical analysis of the bearings indicated evidence of overheating, specifically blueing and oxidation, suggesting the components had operated at temperatures exceeding their design limits.
Investigators also examined the maintenance history and the oil levels within the TRGB. Upon draining the gearbox, only 17 ml of oil was recovered. This volume was significantly lower than the manufacturer's specified requirements. Furthermore, the investigation looked into the maintenance practices of the engineer, noting that the oil level in the sight glass was obscured by dark deposits, which could have led to an incorrect reading during pre-flight inspections.
Findings
- The primary cause of the accident was a loss of tail rotor thrust resulting from the mechanical failure of the tail rotor gearbox.
- The failure was driven by insufficient lubrication within the gearbox, as the recovered 17 ml of oil was inadequate to cool and lubricate the bearings and gears.
- The input shaft's inner bearing failed due to overheating, which subsequently caused the bearing cage to disintegrate and the bevel gears to deform.
- Maintenance inconsistencies contributed to the event, including a discrepancy between the engineer's target oil level and the manufacturer's instructions.
- The dark deposits on the oil sight glass likely prevented an accurate assessment of the oil level during pre-flight checks.
Safety action
- It is recommended that Enstrom Helicopter Corporation clarifies the minimum and maximum oil levels required for tail rotor gearbox operation in the Maintenance Manual.
- The manufacturer should ensure consistent references to the required oil quantity across all manual sections to prevent confusion.
- The manual should clearly state the intended frequency for changing the tail rotor transmission oil during 100-hour and annual inspections.