What happened
On 16 June 2002, a Robinson R44, registration G-SYTN, was conducting a private flight from Denham to a landing site in Leicestershire. While cruising at 1,500 feet, the pilot observed that the engine needle on the dual engine/rotor tachometer had dropped to zero. Believing the engine had failed, the pilot initiated an urgent precautionary descent, which transitioned into an autorotative landing.
Landing options were limited because the helicopter was flying with a 12-knot tailwind and was positioned over large electricity transmission cables, making a turn into the wind unsafe. The pilot selected a field containing a standing cereal crop. As the aircraft made contact with the ground, it began to pitch forward. In an attempt to correct the pitch, the pilot applied aft cyclic control. During this maneuver, a main rotor blade struck the tail boom, severing it just ahead of the tail rotor assembly. There were no injuries to the two occupants.
The investigation
Investigators examined the engine and the tachometer unit to determine the cause of the perceived power loss. The engine was successfully run by the recovery organization and later underwent testing at an overhaul agent, including shock load and overspeed inspections; no mechanical faults were identified.
The dual tachometer was sent to the manufacturer in the United States for analysis. The investigation revealed that the engine tachometer section of the unit contained a capacitor with reflowed solder joints and a solder bridge on a printed circuit board. These defects caused the erroneous zero reading.
Findings
- A manufacturing defect in the tachometer caused the engine needle to indicate zero, leading the pilot to believe an engine failure had occurred.
- The pilot's primary concern during the event was the need to lower the collective to prevent a loss of rotor RPM, a critical procedure learned during initial training.
- Other indicators of actual engine failure, such as changes in engine noise, low oil pressure, or low rotor RPM audio warnings, were not present during the incident.