Circumstances
The aircraft, a Robinson R44 with registration G-SYTN, was on a private flight from Denham to a landing site in Leicestershire. Approximately 12 minutes after takeoff, while cruising at 1,500 feet, the pilot observed that the engine needle of the dual engine/rotor tachometer indicated zero. He confirmed the throttle governor was engaged and initiated an urgent precautionary descent, which developed into an autorotative landing.
The selection of a landing field was influenced by the helicopter flying downwind with a tailwind of about 12 knots and passing over substantial electricity transmission cables. The pilot considered a turn into wind impracticable due to the proximity of the cables. The chosen field contained a standing cereal crop. During touchdown, the helicopter began to pitch forward, and the pilot applied aft cyclic control. The tail boom was severed by a main rotor blade strike immediately ahead of the tail rotor assembly. Neither the pilot nor passenger sustained injuries.
Subsequent Investigation
The maintenance organization that recovered the aircraft started and ran the engine successfully prior to recovery. The engine was subsequently tested at an overhaul agent, including shock load and overspeed inspections; no faults were found. The dual tachometer was sent to the helicopter manufacturer for investigation. The repair report noted 'reflowed' solder joints on a capacitor within the engine tachometer section and a 'solder bridge' on a printed circuit board. A reflowed joint is one remade by melting existing solder without flux, which can increase resistance. A solder bridge is an unintentional connection between adjacent conductors. The unit was rectified and released. The maintenance organization stated they were unaware of similar tachometer failures.
Discussion
It was apparent that a manufacturing defect had resulted in the failure of the engine tachometer, causing the pilot to believe the engine had failed. However, other indications of an engine failure—such as sudden yaw, change in engine noise, low oil pressure, and a low rotor rpm audio warning—were absent in this case. During his initial training some ten years prior, the pilot had been instructed on the urgent need to lower the collective lever in the event of an engine failure to prevent rotor speed loss. This action was foremost in his mind at the time of the tachometer failure. Following the event, the pilot undertook refresher training and now considers regular recurrent training in emergency procedures essential for maintaining flying proficiency.