Transmission tower inspection: checking every member of a steel tower without climbing it

Most Japanese transmission towers are angle-steel lattice structures protected by hot-dip galvanising. The zinc wears away over decades; once it is gone, the steel starts to rust, so older towers are life-extended with paint. Bolts, foundations, and the insulator assemblies and ground wires the tower carries are all part of inspection too.

Tower inspection starts with walking to the tower — sometimes hours up a patrol path in the mountains. On arrival, inspectors look up through binoculars; for detail they climb with fall-arrest equipment. Near the conductors, work requires an outage or strict live-line proximity procedures.

The pain is mountains, height and people. Getting there takes half a day; climbing up and down takes more. Summer brings heat and hornets, winter brings snow. Since February 2019 Japan has required full-body harnesses as the norm for fall arrest at height, with special training — safer, but no less effort. And the people who have watched these towers for decades are retiring all at once.

A tower has a known shape at a known position, so it suits drones that repeat exactly the same flight every time.

The biggest gain is seeing around the conductors without an outage. With no climb, inspectors may drive to a nearby point and launch rather than walk the whole path. And photos from the same angle, year after year, describe rust progression more precisely than words.

Around a tower you face a mass of steel plus high voltage. Compass and radio links are easily disturbed, and ridge tops are windy.

• Magnetic disturbance: the closer to the steel tower, the worse the compass. Take off away from the tower and be ready for heading errors. Magnetic disturbance
• Strong EM fields: fields are strong around conductors and insulators. Keep the utility’s stand-off distances and check how the link degrades in advance. Strong EM fields
• Height: towers range from tens of metres to well over a hundred. Near the peak, wind and visibility differ from the ground. Height
• Wind & turbulence: strong on ridges and valleys, and broken up by the lattice. Fly where a gust towards the conductors still means no contact. Wind & turbulence

Hundreds of photos per tower, thousands of towers: no person can review it all. AI grades rust and flags missing parts; 3D models measure tilt and bent members. Linking it all to tower and member numbers is the machine’s job; deciding the order of repainting and repairs is the human’s.

Japanese transmission towers are designed and maintained under the Electricity Business Act and the technical standards for electrical equipment, which set the wind and snow loads they must resist; after the 2019 collapses a government working group reviewed the cause and the standards, and they were amended in May 2020 (keeping the 40 m/s basic wind speed, now specified as a 10-minute mean, and adding a special-terrain category). Inspection frequency and method rest mainly on each utility’s own safety rules (self-managed safety). Towers 60 m or taller need aviation obstruction lights or daytime markings under the Civil Aeronautics Act, and climbers are covered by occupational safety rules such as full-body harness requirements.

For drones, towers and wires generally count as “property,” so close-up inspection is usually a specified flight within 30 m of property, requiring permission or approval. Abroad, the US National Electrical Safety Code (NESC) and Europe’s EN 50341 (overhead line design) are the foundations for towers and lines. Compare flight rules in world regulations.

• Bolt tension: a loosening bolt is almost invisible in photos. Missing or displaced bolts show, but checking torque takes hands and a wrench.
• Corrosion below ground: legs below the ground line and inside foundations are hidden; excavation is needed.
• Rust under paint and wall thickness: surface rust shows, but under-coating corrosion and section loss need contact measurement.
• Legs hidden by trees: mountain tower bases are often overgrown and invisible from above.
• Repairs and painting: replacing members and repainting are human work; painting drones are still in trials.
• Rating and accountability: whether a tower is strong enough, and when to rebuild, is an engineer’s decision.

What follows is the editors’ projection. From drone ports at substations and tower bases, drones visit tower after tower on fixed routes. Every tower has same-angle photos and a 3D model for every year, and changes in rust or tilt line up automatically.

The morning after a typhoon, checks begin with the towers in the windiest sections. People climb only to tighten bolts or replace members. Tower work has shifted from climbing to look, to foreseeing the life of thousands of towers and deciding the order of care.