What happened
On March 6, 1976, at approximately 12:45 GMT, a Hughes 300C-269C helicopter, registration CS-HAO, was performing agricultural weeding operations over a 20-hectare wheat field at Monte Chaminé, near Ervidel, Portugal. The mission involved chemical application across a rectangular area measuring approximately 600 by 30/0 meters.
The flight path required the pilot to execute roughly thirty passes through a complex obstacle field. The terrain was intersected perpendicularly by two sets of high-voltage power lines and one telephone line. To maximize efficiency and minimize the number of passes, the pilot chose to fly along the longer axis of the field, which necessitated flying under one set of high-voltage lines, over the telephone cables, and under a second set of power lines.
During the final phase of the operation, after completing a turn for a new pass, the pilot lost sight of a marker. While attempting to relocate the marker, the pilot inadvertently lost altitude, causing the aircraft to strike the high-voltage power lines. The impact caused the main rotor to become entangled in the cables, leading to a loss of control and an inverted crash. The pilot sustained serious injuries, and the aircraft was totally destroyed.
The investigation
The investigation focused on the operational risks and the pilot's decision-making process. Investigators examined the terrain, noting that while flying along the length of the field was more efficient, it forced the aircraft into repetitive and dangerous maneuvers beneath and above utility lines. The investigation also reviewed the pilot's flight path and the specific circumstances of the loss of situational awareness regarding the field marker.
Findings
- The chosen method of executing the agricultural work was inherently risky due to the presence of multiple overhead obstructions.
- The pilot was aware of the significant dangers involved in the mission but proceeded with the operation.
- The accident was caused by a momentary lapse in attention while the pilot was attempting to re-establish visual contact with a field marker, leading to the fatal collision with the power lines.