Introduction
On 25 August 2021 at 1618 hours UTC, an Enstrom 280FX helicopter, registration G-OJBB, sustained a hard landing near Rhobell Fawr mountain in Dolgellau, Gwynedd. The helicopter was engaged in a private flight with a single pilot on board. The pilot received minor injuries, and the helicopter was damaged beyond economic repair.
History of the Flight
The pilot, who also owned the helicopter, departed Hawarden Airport at 1509 hours for a local flight. The planned route included a westward leg through Snowdonia National Park, then south to Barmouth, followed by a turn northeast toward Bala Lake before returning to Hawarden. Weather conditions were dry and sunny with visibility exceeding 9 km, few clouds at 2,900 ft amsl, temperature 21 °C, and QNH 1027 hPa. The flight was uneventful until the helicopter approached Rhobell Fawr. The pilot saw a stone obelisk near the summit and decided to investigate. He approached from the northeast and brought the helicopter into a hover about two to three feet above gently sloping ground. Without pilot input, the helicopter rapidly yawed to the right and touched down heavily on its skids. The pilot shut off fuel and electrical systems and exited via the right door. The helicopter sustained substantial damage to the main rotors, tail boom, and skids. The pilot, with minor injuries to his right arm and left leg, was airlifted by a Search and Rescue helicopter to Caernarfon for treatment.
Aircraft Information
G-OJBB was an Enstrom 280FX, a single-engine light helicopter powered by a Lycoming HIO-360-F1AD piston engine (1 engine). It had a three-bladed main rotor and a two-bladed teetering tail rotor driven by a tail rotor gearbox (TRGB) mounted at the aft end of the tail boom. The TRGB case contains an input and output shaft supported by bearings, with bevel gears providing drive. At nominal engine speed of 3,050 rpm, the main rotor rotates at 351 rpm and the tail rotor at 2,514 rpm. The TRGB is splash lubricated by oil, with a sight glass for oil level indication. G-OJBB was fitted with an original flat sight glass. The helicopter also had a TRGB chip indicator on the cockpit panel.
Examination Findings
The helicopter was recovered from the mountain approximately four weeks after the accident. Damage indicated that the main rotors had struck the tail boom and left vertical stabiliser, causing deformation and failure of the tail rotor driveshaft coupling. The tail rotor blades were intact but had minor indentations. The TRGB magnetic plug was covered in metal particles. When removed, only a few drops of oil dripped from the drain plug. Further examination revealed that the interior of the sight glass appeared covered in dark brown oil, but tilting the TRGB did not change the appearance. After draining, a total of 17 ml of oil was collected from the TRGB. The oil had a metallic appearance with small ferrous particles. Borescope inspection showed extensive damage to the bevel gear teeth, allowing the input and output shafts to rotate independently. Disassembly confirmed damage to the bevel gears and failure of a bearing, consistent with a lack of lubrication. The investigation found inconsistencies in how maintenance was performed on the TRGB compared to the manufacturer's instructions, and it is likely that insufficient oil was added to the TRGB when it was serviced 25 flying hours before the accident. Three Safety Recommendations were made regarding the helicopter Maintenance Manual's information on oil quantity and servicing interval for the TRGB.
Probable Cause
The probable cause is loss of tail rotor thrust due to damage to the bevel gears and bearing failure caused by lack of lubrication, likely resulting from insufficient oil added during servicing 25 flying hours before the accident.