Why does a plane keep circling?

Most aircraft seen circling near an airport are in a hold. Air traffic control assigns a holding pattern, a racetrack-shaped circuit flown at a set altitude around a navigation fix, whenever arrivals exceed what the runway can accept, or when weather, a blocked runway or a spacing problem delays the sequence. Standard holds use one-minute legs and right-hand turns, so from the ground the aircraft appears to loop over the same spot every four minutes or so. Controllers stack several aircraft at different altitudes in the same hold and release the lowest first. At busy hubs such as London Heathrow holding is routine in the morning peak; elsewhere it is rare and usually means thunderstorms or fog at the destination.

A hold can also be flown by choice. A crew arriving before the airport's night curfew ends, or before a closed runway reopens, will hold rather than divert. In low visibility, waiting twenty minutes for fog to thin is often cheaper than landing at another city.

When an aircraft returns shortly after take-off, it may circle to lose weight. A long-haul jet departs well above its maximum landing weight; some types can jettison fuel, others must burn it off. Circling for an hour at low altitude with gear or flaps extended is the standard way to do that. Crews also circle to run checklists after a technical fault, to let the cabin crew prepare, or to allow the airport to position fire and rescue vehicles for a precautionary landing.

Not every loitering aircraft is an airliner. Police helicopters, survey and mapping aircraft, aerial photography flights and pipeline patrols fly tight circles or grids by design. Military tankers and surveillance aircraft orbit for hours. Gliders circle to stay inside a rising column of warm air, a thermal, climbing at a few metres per second without an engine. Training flights practise steep turns and holding entries over the same area repeatedly.

A circling aircraft is very rarely in trouble. Holding is designed into the airspace system, crews carry reserve fuel specifically for it, and regulations require enough on board to hold, divert to an alternate airport and still land with a margin. When fuel does become tight, the crew declares a minimum-fuel state and receives priority, and the circling stops.