NTSB has released a preliminary cause for the DHC-3 crash. The horizontal tail actuator had become disconnected - not failed - disconnected.
They said the separation of this component occurred when a clamp nut that should have been fixed in place by a circular wire lock ring unthreaded and rotated.The NTSB said that while the lock ring was not located in the wreckage, they found that three of five holes drilled in the clamp nut to accept the lock ring were damaged “such that they would not allow for the full insertion of the lock ring.”
“This suggests that it may be possible for a lock ring to be partially installed … not fully seated in a hole in the clamp nut,” the NTSB said. “Further, it might be difficult to visually determine if the lock ring is fully engaged in the clamp nut hole” depending on conditions such as lighting, viewing angle and the presence of dirt or grease.
Whidbey seaplane crash investigators identify possible cause | The Seattle Times