Answer: De-icing giant wind turbines.
A video (below) recently surfaced that shows a large unmanned aerial vehicle (UAV) on a tether de-icing a giant wind turbine. The drone was created by Aerones, a Latvian firm that builds large, heavy-duty UAVs. Aerones craft can lift just over 220 pounds (100 kilograms), have as many as 36 propellers, and are designed for use in scenarios such as deliveries, rescues, firefighting and industrial cleaning.
The drone being used in this case has a tether attached to it supplying both water and electricity. It uses the water to complete its task, spraying the turbine blades with 100 liters of it per minute. And the power supply means it can stay in the air until its task is completed without having to land for recharging.