A missing man formation is an aerial maneuver which is intended to honor dead or missing members of the military, specifically pilots in the air force. This military honor is also accorded to astronauts and sometimes to high profile politicians such as Presidents as well. Many people find the formation quite evocative and moving, as it is a somber reminder of the dead or missing.
The origins of the missing man formation lie in the First World War, when Royal Air Force (RAF) crews got into the habit of doing an organized flyover when they returned to their home airfields, to alert ground crews that they were coming in. During the flyover, ground crews would also take note of how many men had returned from the mission, and since the layout of a tight flight formation is very rigid, the ground crews could figure out who was missing.
According to RAF history, the first official missing man formation as a military honor occurred with the death of the Red Baron, a famous flying ace of the First World War. Pilots decided to enact a spontaneous tribute to him, executing a flyover, also known as a flyby, in which an aircraft was obviously missing, symbolizing the Red Baron's departure from the world of the living. By 1938, the United States had picked up the practice, and it has since become common at prominent military funerals.
There are several ways to perform a missing man formation. In some cases, planes fly in a formation which is lacking one aircraft. In other instances, one pilot pulls away from a formation as it flies over the site of a funeral or memorial, acting as a metaphor for the fallen or missing pilot. Such formations can be used to commemorate tragic events like the attack on Pearl Harbor as well as individual deaths.
As with other military honors, there is a strict etiquette to flying the formation, and only certain people are permitted this honor. However, civilian pilots sometimes practice their own missing man formations to honor fellow pilots or beloved local figures. Flying in formation is extremely challenging and demanding, so pilots usually practice this maneuver extensively before they perform it in front of an audience.