On November 18, 2005, a Schweizer 269C (registration N400PD) was involved in an aviation accident near Manhattan, KS. Investigators recorded the probable cause as: The binding of the main rotor blade's tachometer cable during cruise flight resulting in an observed split tachometer indication/aural alarm and perceived mechanical emergency and the unsuitable terrain the pilot encountered during the emergency landing. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter nosed over in a rocky field during a precautionary landing after the pilot observed a split in tachometer readings and a low rotor rpm alarm. Examination revealed an open circuit in the control box warning circuitry and excessive wear on the tachometer cable.
Incident Overview
During a public use solo instructional flight, the helicopter sustained substantial damage when it nosed over in a rocky, uneven field. The pilot performed a precautionary landing after observing a split between the rotor and engine tachometer readings and hearing an audible low rotor rpm alarm while in cruise flight. The pilot perceived a mechanical emergency and initiated the landing.
Examination of Components
Following the incident, the control box, tachometer cable, and tachometer were examined. The control box warning circuitry was not operational when first checked. After repair of an open circuit found on the circuit board, the circuitry operated correctly. No anomalies were detected with the tachometer itself. Visual examination of the tachometer cable revealed indications of excessive wear and possible binding, with windup of the outer strands. Binding of the cable is consistent with a bouncing tachometer needle, which could produce the split readings observed by the pilot.