When an aircraft flies, air in the atmosphere rubs against the body. The air also contains particles such as dust, water droplets, ice crystals, etc. This results in the aircraft accumulating charge over a period of time. This potential can reach thousands of volts. In aircrafts, this can cause interference to the radio equipmen as this charge will bleed off other edges in the aircraft wings and tail. It can also result in sparks and flashes, when the aircraft touches land. Hence, this charge has to be safely discharged or bled off into the atmosphere.
Helicopters when used to rescue stranded people should be discharged to the ground before they are lifted in order to avoid an electric shock.
This is done by means of wicks in the aircraft wings. The wicks are pointed metallic rods about 8 inches long place in the airflow. The wicks have thin rods made of carbon which will keep discharging even as the aircraft builds up charge. This ensures that the potential of the aircraft is always low.
The wicks can be replaced periodically.