Why do planes circle over my area? Holding stacks explained

Look up on a busy morning near a large airport and you may see several airliners turning slow ovals, one above the other. They are not lost, and they are not circling you. Most circling planes are waiting, and the rest are doing a job that needs them to stay in one place.

Holding stacks: planes waiting to land

A runway can only take one landing at a time, with a safe gap between each. When more planes arrive than the runway can take, air traffic control sends the extras to a holding stack: a fixed point where they fly an oval racetrack pattern, each at its own level, until there is space in the landing queue.

NATS, which runs air traffic control in the UK, describes how it works. Planes usually join at the top of the stack, around 15,000 ft, and leave from the bottom, around 7,000 ft. The levels are normally 1,000 ft apart, so up to eight planes can hold in one stack at once. Planes work their way down as the ones below them leave for the airport. The most common reason is simply a lot of flights arriving at the same time; bad weather that slows the landing rate adds to it.

Each lap takes a few minutes. In the FAA's standard pattern, the straight leg towards the holding point lasts one minute at or below 14,000 ft and a minute and a half above it.

Elsewhere, Manchester has three holds, and Stansted and Luton shared two until an airspace change in recent years gave Luton its own. Heathrow's stacks have been in the same places since the 1960s, so the places under them have heard holding planes for decades. Gatwick set its minimum of 7,000 ft to keep the noise on the ground as low as possible. At that height a plane is more than 2 km up, far higher than planes on the final approach, which are below 2,000 ft for the last 10 km or so.

Being guided in: the loops below the stack

After the stack, planes do not follow set routes to the runway. At Heathrow, controllers direct each one individually, turning and descending it to line up with the runway at the right distance behind the plane in front. During the day, planes may not join the final approach below 2,500 ft, about 14 km (7.5 nautical miles) from the runway; at night, not below 3,000 ft, about 18 km (10 nautical miles) out. Gatwick works the same way, with its own minimum heights.

From the ground, this looks like circling. Planes sweep round in wide arcs between about 2,500 and 7,000 ft, then turn onto the straight line to the runway. On a flight tracker, each plane's path is a little different, and together they fan out like a wide loop before the approach.

Go-arounds: a second attempt at landing

Sometimes a plane on its final approach abandons the landing, climbs away and comes round to try again. The crew or air traffic control does this whenever they are not completely sure the landing will be safe: the runway is still occupied, the approach is not stable, or the weather, such as wind shear, gets in the way. It is a routine safety manoeuvre that crews practise.

At Gatwick, a plane that goes around climbs straight ahead to 3,000 ft, turns south, and is then guided back into the queue to land. That can take it over areas that never normally see planes, at lower heights than people there expect. Go-arounds are rare: in most months of 2021, between 1 and 4 in every 1,000 arrivals at Gatwick went around. If you see a single plane circle once at a few thousand feet and then land, this is the likeliest reason. Why go-arounds and other low planes are allowed is covered in why planes fly so low.

Training circuits and calibration flights

Near a small airfield, a light aircraft going round and round is most likely a pilot practising take-offs and landings. The circuit is a rectangle around the runway, flown at about 1,000 ft above the ground in the FAA's standard pattern, with helicopters lower, at around 500 ft. A lesson can mean many circuits in a row.

Large airports have their own repeat visitors. The instrument landing system that guides planes onto the runway has to be checked regularly. At Gatwick, a small propeller aircraft does this about every six months, flying approach after approach to each runway, usually at night.

Police, air ambulance and survey flights

• Police. A police helicopter or aeroplane watching an incident circles over it to keep it in view. The National Police Air Service's helicopters can stay up for up to two hours and its aeroplanes for more than eight. The service says its crews fly higher at night to reduce noise.
• Air ambulance. Air ambulance crews circle to check the scene, find a safe place to land and work with the crews on the ground, then land in a field, a road or a park. Near Gatwick, police and air ambulance helicopters from Redhill sometimes hold over Horley before they are cleared to cross the runway.
• Survey and photography. Aircraft mapping or photographing an area fly long straight lines back and forth, like mowing a lawn, at a steady height. On the ground it can feel like circling, but on a tracker the parallel lines are unmistakable.

How to tell which it is on a flight tracker

Note the time, then find the plane on a flight tracker, the airport's own tracker such as Heathrow's WebTrak, or our free flight map. The shape of its track and its height usually settle it.

The callsign and aircraft type help too. An airline callsign on a jet points to holding or to a plane being guided in; a helicopter points to police, an air ambulance or a private flight.

Holding stacks are kept high to limit the noise on the ground. The planes that are loud over a house are the ones landing and taking off near it, and a Flight Noise report counts those for one address, day and night, from a full year of recorded flights.