Confined spaces: how a drone flies where the walls are within arm’s reach

In a confined space, the first problem is collision: walls, beams, pipes, ladders, cables. Outdoors you can shoot from metres away; inside you thread gaps of a few tens of centimetres. The air the propellers push down has nowhere to go and bounces off floor, walls and ceiling back into the aircraft. Near the floor it floats (ground effect), near the ceiling it gets sucked upward, and near a wall it gets pushed sideways. On top of that, confined spaces are usually dark, GNSS-denied and poor for radio — several barriers stacked at once.

Aircraft built for confined spaces are now mostly designed to collide. A spherical or box-shaped protective cage surrounds the airframe so it can bounce off and roll along surfaces. That is combined with powerful lighting, LiDAR-based localisation and 3D mapping, and ways of getting data and video back outside. Example: cage-type inspection drones such as Flyability’s Elios series.

Confined spaces cluster in plants and underground: inside storage tanks, boilers and furnaces, the inside of chimneys, plant piping racks, vessels and columns in process plants, sewers, utility tunnels, the voids above tunnel linings, dam galleries, inside wind turbine blades and towers, ship ballast tanks and holds, and mine stopes. In each, simply getting inside is a job in itself.

• Decide from opening and interior dimensions: check on drawings the hatch size, the narrowest internal gap and ceiling height, then choose between caged, micro or ground robots.
• Light for shadows: frontal light flattens cracks and wall-thinning relief. Prefer oblique lighting or aircraft whose light direction can be changed. (Related: darkness)
• Account for your own downwash: over settled soot or dust, fly slowly and well clear of the floor. Agree in advance to stop and wait when it billows up.
• Plan recovery first: decide link-loss behaviour, how a downed aircraft is retrieved (poles, hooks, robots), and the procedure if a person must go in — before flying.
• Split roles with the people outside: the pilot watches the video; an attendant at the entry watches aircraft sound, tether and gas readings. Even if nobody enters, work near the opening needs atmospheric monitoring.
• Record where each image was taken: confined spaces look repetitive. Tie photos to a 3D map or to wall numbers and markers so repair crews can find the spot later.

An aircraft meant for tight spaces should first be flown with confidence in open ones. Airffic Training Hyperspace sketches how indoor and outdoor practice spaces could be rented by the hour (sample listings, in Japanese).

In Japan, indoor flights are outside the aviation flight rules. The government’s interpretation counts spaces whose inside is clearly separated from the outside as indoor even when doors or windows are open — tunnel and chimney interiors are given as examples — and most tank and pipe interiors fall into this category. But an aircraft that leaves through an opening is flying outdoors, so take measures to keep it inside and, if it does get out, end the flight or bring it back in at once. Where the boundary between inside and outside is unclear (such as pits with a wide-open top), check case by case. The heavier obligations are occupational safety: under Japan’s Industrial Safety and Health Act and the Ordinance on Prevention of Anoxia, work in spaces with oxygen-deficiency (below 18% oxygen) or hydrogen-sulphide hazards requires atmospheric testing before work, ventilation, a qualified supervisor and attendants. In the United States, OSHA regulates “permit-required confined spaces”.

Related: oxygen deficiency & toxic gas, GNSS-denied, dust & smoke, world regulations.

• Contact inspection: thickness gauging (ultrasonic) and hammer tapping need the probe pressed to the wall. Contact-capable aircraft exist, but holding steady contact in tight spaces is still hard.
• Truly narrow gaps: a cage makes the aircraft bigger. Gaps below a few tens of centimetres and small-bore pipes still favour ground robots and borescopes.
• Flight time: lights and cage weight shorten each flight. Large tanks rarely get covered in one go, and battery swaps at the hatch are assumed.
• Self-raised dust: in soot, ash or sludge, flying itself destroys visibility. There is no fundamental fix yet.
• Proving equivalence to human inspection: where statutory inspections require internal visual inspection, how far drone footage can substitute is still being worked out by industry and asset type.

When a tank or boiler is shut down, drones and robots go in through the hatch first and return with a 3D map, full-surface imagery and thickness readings. People enter only where repair is known to be needed. Overlaid on the previous map, progress of wall loss is highlighted automatically. Safety planning centres less on supervising entry and more on avoiding it. That is the editors’ outlook.