On March 14, 1997, a Hiller UH-12E (registration N545HA) was involved in an aviation accident near Glide, OR. Investigators recorded the probable cause as: The pilot's failure to see and avoid a power transmission line due to reduced visibility caused by overcast sky conditions during aerial application operations. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter pilot conducting aerial application of chemicals on a coniferous tree field struck a power transmission line due to poor visibility under overcast skies.
What happened
The incident occurred while the pilot was engaged in an aerial application operation targeting a field of coniferous trees. The aircraft was maneuvering within the作业 area to complete the chemical treatment when it collided with a power transmission line that spanned across the center of the field.
According to the pilot's account, he was aware of the presence of the power line prior to the collision. However, the ambient lighting conditions were compromised by an overcast sky, which significantly reduced visibility and made the structure difficult to discern during the maneuvering phase of the flight.
The investigation
The primary factor identified in the operational narrative was the interaction between the aircraft and the infrastructure within the作业 zone. The pilot acknowledged knowledge of the hazard but cited environmental lighting conditions as a contributing factor to the inability to maintain visual separation from the line.
Findings
Helicopter struck a power transmission line during aerial application operations.
Pilot was aware of the power line's location but experienced difficulty seeing it.
Overcast sky conditions created poor ambient lighting, reducing visibility.
The collision occurred while maneuvering to complete the chemical treatment of the coniferous field.
Probable cause
The pilot's failure to see and avoid a power transmission line due to reduced visibility caused by overcast sky conditions during aerial application operations.