Circumstances:
A report was received that indicated that a jet aircraft struck power lines during an approach to land on Runway 33 at Cairns Airport. The resident of a house approximately 600 metres from the threshold of Runway 33 and on the extended centreline, claimed to have seen what she described as the wheels of the aircraft hitting the power lines after overflying her house during its approach to the runway. Examination of the particular aircraft showed no evidence of it having been in contact with the wires and the pilot reported that the approach was normal. The aircraft flight path was reconstructed using data from the Digital Flight Data Recorder, and this indicated that the height of the aircraft above the ground at the position of the failed powerline was approximately 50 metres. The height of the wires at mid span is approximately 8.6 metres, and the pole height is approximately 9.5 metres. Extensive checks did not reveal any evidence to support the resident's claim. The wires which were allegedly struck by the aircraft wheels showed no evidence of damage. The Far North Queensland Electricity Board (FNQEB) did receive a report of lines down in the area shortly after the arrival of the aircraft. However, the affected lines were not adjacent to the resident's house but were around the corner in the adjoining street some 75 metres away. The damaged wires had been in contact with each other resulting in an explosion, and a blackout in the area. Separators are sometimes fitted to prevent wires from touching and have since been placed on the affected wires when they were repaired. The adjoining span had been similarly dated in the past, and it all so had been fitted with separators to prevent a recurrence. The cause of the wires fusing together could not be positively determined but was probably the result of wake turbulence (wing tip vortices) generated by the aircraft passing overhead. Wingtip vortices are narrow horizontal whirlwinds rotating in opposite directions and streaming rearwards from each wingtip. The wingtip vortices of large aircraft are most intense when the aircraft is flying at low speed with high lift devices extended, such as during an approach to land. The vortices tend to settle behind the generating aircraft and then spread out laterally upon reaching the ground. Under the conditions prevailing on the night of the occurrence, a light wind straight down the runway, it is likely that the vortices developed and caused the power lines to come into contact.
