What happened
During a period of normal cruise flight, a helicopter suddenly descended steeply into a shallow tidal marsh and swamp area. Witnesses observing the aircraft from a boat reported that the helicopter was flying approximately 500 feet above the terrain and water. One witness observed the aircraft in straight and level flight over the water, noting that the engine sound seemed "tinny." This witness then observed the helicopter turn toward land, at which point the engine sound became quiet and the aircraft descended rapidly to the ground.
A second witness reported being alerted to the aircraft by a "pop" followed by a grinding noise. This individual noticed the rotors slowing down as the helicopter dropped while performing a left-hand turn or rotation.
The investigation
An examination of the rotor blades provided evidence of low rotor energy at the time of impact. The engine was removed from the airframe, cleaned, and placed in an instrumented test cell. During a 50-minute test run at various power levels following factory acceptance test protocols, no abnormalities were detected. Investigators established continuity throughout the control system, the fuel delivery system, and the drive train to both the main and tail rotors.
On the day of the accident, the pilot's partner had discussed an erratic fuel pressure issue with an aviation maintenance technician at the FAA Approved Repair Station where the helicopter was parked and serviced. This discrepancy remained unresolved at the time of the pilot's departure. There was no verified evidence that the pilot was aware of this issue.