Holding patterns: aircraft stacked in racetrack holds

When an airport or route is busy, or weather closes a runway, air traffic control can tell aircraft to hold: to fly a racetrack pattern over a fixed point until they are cleared onward. A standard hold uses right turns, and its inbound leg takes about a minute at or below 14,000 ft and a minute and a half above that.

The site finds a hold when a track circles for at least ten minutes and closes three or more loops. A single aircraft's hold only adds context to another story about it. When three or more aircraft circle at the same time within 10 nautical miles of one another, at least 500 ft apart vertically and at 5,000 ft or higher, the site treats them as a stack and tells one story about the group, listing the other aircraft and their altitudes.

A racetrack shape does not show why an aircraft held. The reason can be traffic, weather, a closed runway or a training exercise, and controllers' instructions are not part of this data. Loops close to an airport can also be practice circuits, which the site labels separately so they do not crowd out the rarer holds. Gaps in reception can hide part of a hold, so the number of loops describes the saved positions.