What is a holding pattern?
A holding pattern is a racetrack-shaped oval that an aircraft flies around a fixed point while it waits for clearance to continue, usually to land. Planes hold at different altitudes, stacked 1,000 feet apart, and leave from the bottom one at a time.
The shape
A hold is an oval with two straight legs and two half-circle turns, flown around a fixed point called the holding fix. The fix can be a radio beacon or a GPS waypoint. The aircraft flies over the fix, turns, flies the outbound leg, turns again and comes back inbound to the fix. A standard hold uses right-hand turns, and controllers can ask for left-hand turns when the airspace calls for it. Every published hold is drawn on the charts, so the crew knows its exact shape before they get there.
In the United States, the inbound leg is timed to take one minute at or below 14,000 feet above sea level, and one and a half minutes above 14,000 feet. A full lap at the lower levels takes about four minutes. There are speed limits too: in the US, 200 knots up to 6,000 feet, 230 knots from there to 14,000 feet, and 265 knots above. Slower speeds keep the oval tight, so it fits in its protected piece of sky.
The stack
When several aircraft are waiting at the same fix, they hold on top of each other, separated by at least 1,000 feet. New arrivals join at the top. When the controller is ready for the next landing, the aircraft at the bottom leaves for the approach and everyone above steps down one level. From the ground it can look like a slow carousel of airliners.
Why planes hold
- Traffic: more aircraft are arriving than the runways can take. At a very busy airport, a short queue in the air keeps the runway from ever sitting empty.
- Weather: low cloud, strong wind or thunderstorms slow down landings, or close a runway for a while.
- Runway closures: an aircraft stuck on the runway, debris, an inspection or a snow clearance stops arrivals until it is clear.
- Waiting on something else: a crew working through a problem, or an emergency arrival getting priority, can push everyone else into the hold. What a 7700 squawk means.
How long holds last
Usually a few minutes, often one or two laps. Bad weather or a closed runway can stretch it to half an hour or more. In the US, a controller who sends an aircraft into a hold gives the crew an “expect further clearance” time, so they know how long to plan for. If the wait gets long enough, the crew can choose to divert to another airport instead.
Holding fuel
Airliners carry fuel for this. Flight plans include extra fuel for the expected delays at the destination, plus fuel to reach an alternate airport, plus a final reserve on top. In Europe the final reserve for a jet is 30 minutes of flying in a hold 1,500 feet above the airport, and it is not meant to be touched. When delays are forecast, airlines simply load more. A plane in a hold is using fuel it planned to use.
Famous stacks: London Heathrow
Heathrow’s stacks are the most famous example. It has four, one for each direction arrivals come from:
- Bovingdon (BNN), northwest of London, for arrivals from the northwest.
- Lambourne (LAM), in Essex, for arrivals from the northeast.
- Biggin Hill (BIG), southeast of London, for arrivals from the southeast.
- Ockham (OCK), in Surrey, for arrivals from the southwest.
Each stack runs from about 7,000 feet to 16,000 feet, in 1,000-foot steps. On a busy morning you can see A320s, 777s and A380s turning slow ovals over the edges of London.
Holding over your house
If you live 20 to 50 miles from a big airport and see the same jets circling high and quiet, you may live under a holding fix. Other reasons a plane circles include police helicopters, flight training and survey work, and the shape of the trail tells them apart.
