Airplane holding patterns: What they are and why they happen

By CNN Newsource

Published February 14, 2024 3:03 AM

By Jacopo Prisco, CNN

(CNN) — If you’ve ever sat on a plane wondering why the pilot announced that you’d be landing soon but seemed to be flying in circles with no runway in sight, you’ve probably been in a holding pattern.

Holding patterns can happen for various reasons.

They’re primarily congestion and bad weather, often because of reduced visibility, strong winds or snow. But issues such as an equipment malfunction or an obstructed runway can cause them, too.

They are also one of the easiest ways to manage a surge of traffic around an airport.

“When you’re driving and there’s traffic, you get a red light and you stop — but for airplanes, there’s no such option,” explains Junzi Sun, a professor of air traffic management at the Delft University of Technology in the Netherlands.

“One can simply consider the holding pattern as a sort of traffic light for airplanes, used to control the inbound traffic landing at an airport.”

Technically, a holding pattern is a defined area of airspace in which an aircraft enters a circular or racetrack-shaped route that it repeats for a certain period to delay its descent toward the runway.

“The aircraft keeps circling until the air traffic controller gives clearance to descend and land,” Sun explains. “It’s a very standard procedure, and pilots don’t have to fly manually as their guidance systems help the aircraft maintain its holding pattern. It’s relatively easy from a pilot’s perspective.”

Holding patterns have become a mainstay around airports that are chronically congested — but are they inevitable? And do they come at a cost?

Air traffic flow

The first problem with holding patterns is that they are bad for fuel efficiency.

“Holding is a very inefficient way of flying, because you have to maintain a low altitude, so you burn quite a lot of extra fuel. For that reason, it is a last-resort method to control the air traffic flow. There are other ways to do it that can be more efficient,” says Sun.

Flying at a low altitude burns more fuel because of increased air resistance, leading to higher emissions.

Holding patterns are also noisy, because they exist between roughly 7,000 and 13,000 feet [2,130 to 3,960 meters] of altitude – with about 1,000 feet [about 300 meters] separating each plane vertically. That means engine roar can be heard on the ground below.

Lastly, because the plane is flying around the airport rather than landing, flight time is increased, usually by 10 to 30 minutes.

Options to circular holding

Alternatives to holding patterns, such as “linear holding,” essentially require planes to fly slower or to follow a longer path before arrival.

One type of linear holding is the “point merge” system, which was invented by Eurocontrol, the organization that coordinates air traffic management in Europe.

First used in Oslo in 2011, it is now in operation at about 40 airports worldwide, including Istanbul, Shanghai and Tokyo.

It works by keeping all the arriving aircraft at the same level and having them converge toward an arc with only horizontal separation. At the right time, they are cleared to enter the arc and proceed to a merge point, after which they all follow the same path toward the runway.

The big advantage of this over holding patterns is that the arc can be placed at a much higher altitude, up to 20,000 feet, or away from inhabited areas – such as over water – with a positive impact on pollution and noise. A 2023 study also found that planes placed in a traditional holding system use notably more fuel than those in a point merge system.

London’s a hot spot for holding patterns

Holding patterns are used worldwide, but they are somewhat less common in the United States.

“Here in the US, we have an agency called the Command Center, which spends months planning what traffic is going to look like,” says Margaret Wallace, a professor of aviation management at the Florida Institute of Technology.

“They predict what the flow is going to look like, and what anticipated weather may look like, and implement delays on the ground where they don’t let a flight depart at their scheduled time. We would rather hold an aircraft on the ground than get it in the air and have to go into holding, because that causes more issues for safety reasons,” she said.

Although holding patterns are safe and haven’t led to fatal accidents in decades, issues can arise if a plane is on hold for too long.

“The biggest risk is the fuel,” says Wallace. “If an aircraft is low on fuel for whatever reason, there’s two types of distress calls; one means that they’re not at the emergency level, but can’t accept any undue delays. The other means they need to get down immediately, because they don’t have enough fuel to participate in normal ops.” Planes in these situations usually declare an emergency and can either skip the line at the airport at which they were holding or they are given priority to divert to a nearby one.

One of the holding pattern hot spots of the world is London, which is served by six major airports, including one of the busiest in the world, Heathrow.

Holding patterns at Heathrow are so established that the airport has given names to the four “stacks” in which incoming aircraft can be placed — Bovingdon, Lambourne, Ockham and Biggin Hill, after the towns below.

The reason, other than the airport’s high traffic flow and only two runways, is the high number of international flights, according to Michael McCormick, a professor of air traffic management at the Embry-Riddle Aeronautical University. “Any airport that has a significant number of international arrivals and is subject to changing weather conditions is where you’re going to see holding more often,” he says.

“Many of the flights will have already gotten airborne prior to the constraint occurring, so it becomes an ‘ad hoc’ moment where you have to do something to stop the flow of arrivals. That’s why you get it in London, but you don’t necessarily get it in the United States.”