A holding pattern is a route that can be flown by an aircraft, typically in an oval formation, which allows the aircraft to remain in flight without leaving an area. This type of flight pattern is usually used when an aircraft has reached a destination but is not able to land, often due to excessive aircraft traffic or poor ground conditions. Multiple holding patterns in different airspace may be established for a single airport if it is busy enough, and each will have its own area and route course. A holding pattern can also be utilized to keep multiple aircraft in a single route pattern, each separated by vertical distance.
Typically used at major airports, a holding pattern can be any route that ultimately results in a full circuit that brings an aircraft back to a stationary starting point. The simplest sort of route, and the easiest to visualize, is a circle or oval, much like a race track used by a car. An aircraft will enter this route at a stationary point, called the fix, and then move through the pattern until returning to the fix. For an oval holding pattern, this typically involves each turn taking about one minute to complete, and each side being long enough to require another minute or more to complete.
Holding patterns with longer sides are typically used for longer holds, since frequent turning may make passengers on an aircraft uncomfortable. A holding pattern is usually used at a busy airport to keep incoming aircraft in the air around the airport while waiting for the landing area to be ready or accessible. This may be necessary if traffic is heavy or if ground conditions are poor and snow or ice must be removed before the aircraft can land safely. Multiple airplanes can utilize a single holding pattern; each one is placed into the pattern higher than the one before it, creating a vertical stack of aircraft.
When this type of holding pattern stack is used, each approaching aircraft is instructed to enter the stack at a higher position, and the bottom aircraft is the first to land. Once the bottom craft exits the holding stack, each aircraft moves down and the process continues as necessary. Since a holding pattern is fairly wasteful with regard to both time and fuel, it is typically preferable to avoid it; most airlines try to plan ahead for issues with landing, delaying take off rather than holding aircraft in the air. Holding patterns can also be used in certain emergency situations, such as a plane having to land without landing gear, to burn off excess fuel and to reduce the chance of fire or explosion from fuel.