ERA13LA242 · Sud Aviation SA 318C

During a deer population survey flight over swampy terrain, the pilot descended the airplane from 200 feet to a hover about 25 feet above ground level and then side-stepped the helicopter right to maneuver over an island. As he did so, the helicopter began to yaw right and descend. The pilot corrected the yaw and increased collective pitch; however, the helicopter continued to descend. He then moved the cyclic forward in an attempt to fly through effective translational lift, but the descent continued, and the helicopter impacted the swamp and came to rest on its right side. Examination of the wreckage revealed that the helicopter had adequate fuel and was about 400 pounds below its maximum gross weight. No evidence of any preimpact mechanical malfunctions were found that would have precluded normal operation. However, the examination did reveal that the main rotor pitch angle cockpit indication was at 20 degrees (16 degrees when power was applied), which was beyond the redline of 15 degrees.

The helicopter would have required more power to hover in sideways flight, below effective translational lift, than it would have required for a stationary hover. One of the common errors in hovering sideways flight is the failure to maintain proper rotor rpm. The excessive main rotor blade pitch angle and the pilot's report that the helicopter continued to descend after he increased the collective pitch and that he had to correct a right yaw indicate that it is likely that he failed to maintain proper rotor rpm during the hovering sideways flight.

Verbatim NTSB analysis and probable cause from the NTSB dataset