On June 4, 1994, a Hiller FH1100 (registration N746FH) was involved in an aviation accident near Tyonek, AK. Investigators recorded the probable cause as: The pilot's failure to maintain directional control during landing on unstable terrain, which resulted in dynamic rollover and the helicopter rolling onto its right side into the Theodore River. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
The pilot landed on a river bank which collapsed under the right skid, causing the helicopter to roll over into the Theodore River due to dynamic rollover.
What happened
The pilot reported landing the helicopter on the bank of the Theodore River. Upon reducing the collective pitch control to a flat position, the ground beneath the right skid gave way and sloughed off. This loss of support caused the aircraft to roll onto its right side and enter the river.
The pilot stated that he had already reduced engine power prior to the incident. He noted that he was unable to correct for the rapid roll rate because the helicopter experienced dynamic rollover, a condition where the pivot point shifts beyond the limits of control authority.
Probable cause
The pilot's failure to maintain directional control during landing on unstable terrain, which resulted in dynamic rollover and the helicopter rolling onto its right side into the Theodore River.