What happened
On 3 September 1999, an Agusta Bell 206B, registration G-SPEY, was conducting aerial work in the St Austell area of Cornwall. The mission involved low-level filming, for which the pilot had secured a specific regulatory exemption. After completing the third of four planned filming sequences, the pilot transitioned to a fourth location.
During this transit, the aircraft struck a 33kV powerline spanning a valley. While the pilot was aware of a separate 11kV line running parallel to a nearby road, the 33kV line went undetected. Following the strike, the pilot performed a precautionary landing in a nearby field. Although an initial inspection by an engineer suggested the damage was limited to the external camera mount and windscreen cracks, subsequent technical investigations revealed significant electrical damage to the aircraft's avionics and structural components.
The investigation
Investigators examined the aircraft and identified damage consistent with a high-voltage electrical discharge. Specifically, the main rotor gearbox exhibited pitting on the surfaces of several meshing gears, and there was evidence of internal bearing damage. Additionally, one main rotor blade showed signs of electrical arcing on both the trailing edge near the root and at the blade tip.
Regarding the visibility of the hazard, the pilot noted that the 33kV line was extremely difficult to identify because the supporting pylons were obscured by dense woodland and the cable itself was green. The pilot of a second helicopter also struggled to spot the line upon arrival at the scene.
Findings
- The primary cause of the incident was the collision with a 33kV powerline that was not visually prominent.
- The pylons supporting the line were below 80 feet, meaning they were not depicted on the standard CAA 1:250,000 aeronautical charts.
- The green color of the cable and the location of the pylons within heavy vegetation significantly reduced the line's conspicuity.
- At the time of the accident, there was no national requirement to increase the visibility of such powerlines, leaving the responsibility for hazard avoidance solely with the pilot.