From a UK perspective. We are required to inspect all power lines annually. In the 500 sqmiles in my patch this equated to 10 men walking the lines for a year to achieve this. The decision was made to do 80% by air and the remainder by foot. In this patch that worked out to 6hrs/day for 5 days (wx permitting)
In the early days of this, company staff operated as observers with heli company and was usually done in jet rangers. Since changes in rules we employ a company to do it all usually with twin squirrels. It was also part of the deal to have heli company available for emergency operations (one of these led to a rotor turning refuel at Walney in a force 4) that they got some "normal" flying
Technique for wood pole lines put the observer (who also navigated) about 40ft diagonally away from the line with speeds around 30mph (IAS)
this allowed plenty of time to check and note defects. Any thing that required a closer look the pilot would go round to the pole and hover if reqd.
They were fun times with some brilliant pilots from around the country especially (with the low fly permissions) 20ft runs down local beaches as we repositioned (much to the shock of two topless sunbathers as we turned inland one day) or 15 mile runs across country at low level returning to base.
Learned a lot about pilot attitudes to Horses and Turkeys
