Airship

The Academy City airships (飛行船) or Blimps are lighter-than-air aircraft that can be steered and propelled through the air using rudders and propellers or other thrust mechanisms. It is one of the most recognizable features of Academy City, probably second only to the Three-bladed Propeller wind turbines

Appearance
The third Toaru Majutsu no Index light novel where the airship first appears does not give a detail description of the airship's physical appearance. The first actual visual depiction of the blimp is in the 11th chapter of the Toaru Majutsu no Index manga, showing an ellipsical balloon, a gondola at the bottom, flanked with rudders, and a screen on the side.

It is not until the advent of the Toaru Majutsu no Index anime adaptation that the look of the airship is finalized. Here the airship retains its elliptical shape but tapers at the back where the tails is, under it are the gondola and two propellers, and on the top is another rudder. The screen on both sides of of the airship, though originally it did not show that it was screen and it was as if the skin of the balloon projected the images itself, though the anime would rectify this. The airship is colored beige and would later become incorporated later in both Index and Railgun manga.

Mechanics
The blimp uses solar power to heat carbon dioxide with a heater for lift and spin a large motor for thrust, as such it is an ecological craft that does not need fuel.

Purpose
According to Kamijou Touma, the Academy City Board of Directors wanted to have the students be more aware of current events. As such, the airships often broadcasted news, current events, weather reports, and announcements through its television screen and sound system. In the narrative, its purpose is to foreshadow or add backstory to the plot unfolding in the story, like the airships announcing that several research institutes related to muscular dystrophy have shut down, referring to Misaka Mikoto's actions behind the scenes during the Level 6 Shift prior to Touma's involvement.

The airship can apparently be either be borrowed or rented for private use, as seen when Kiyama Harumi's students decided to broadcast their birthday greetings to her.

The airships are apparently being tested to be used as part of an emergency air communications network through the use of the few antennae installed on them. The airships could sent out when normal towers were brought down in a disaster. This technology was developed from the Handy Antenna Service.

Christmas Day Arc
On Christmas Day, Anna Sprengel brought down an airship over School District 7 during her battle with Kamijou Touma, rupturing and igniting its gasbag from afar.