Holding patterns: why planes circle

Watch a busy airport such as London Heathrow on a flight tracker and you will often see aircraft flying slow, oval loops in the sky, sometimes for several minutes, before they turn onto the approach. Pilots call this a holding pattern. It is one of the most useful tools air traffic control has, and it is simple to understand once you know what it is for.

This guide explains why aircraft hold, how the pattern is built, how a crew flies it and what a holding stack is. It also shows how the same idea works in the game.

What a holding pattern is

A holding pattern is a racetrack-shaped loop. It has two parallel straight legs joined by two 180-degree turns. The aircraft flies over a fixed point, called the holding fix, then turns and flies the pattern again and again until it is cleared to leave.

Holding patterns are published. Airports and airspace designers draw them on instrument charts, with the fix, the direction of the turns and the altitude to hold at. An aircraft that holds at a published pattern always knows where it is, and so does the controller.

The standard turns are right-hand turns. A pattern with left-hand turns is non-standard and must be published or given by the controller. The rule is simple: check which way the pattern turns before you look for the aircraft.

Why planes hold

An aircraft holds because the next step is not yet available. The usual reasons are these.

• Runway capacity. A runway can take only so many landings an hour. When arrivals are bunched together, some must wait.
• A runway is closed or blocked. An aircraft on the runway, a vehicle crossing it or a runway inspection can stop landings for a time.
• Weather. Low cloud, low visibility, thunderstorms or strong winds can slow the flow of arrivals, or change which runway is in use.
• An emergency. Other aircraft may be asked to hold while an emergency is handled.
• Traffic management. Some airports use time-based flow control. An aircraft given a slot later than its planned arrival may hold until its slot time.

When an aircraft is put into a hold, the controller usually gives an "expect further clearance" time, which is the time the crew should expect to be released. The crew does not have to guess. They fly the pattern until they are cleared.

How the pattern is flown

Flying a hold is mostly a matter of timing, and the timing is set by rules. In the United States, the FAA's Aeronautical Information Manual says that the outbound leg is normally timed at one minute below 14,000 feet, and one and a half minutes above it. Crews use a timer or the aircraft's flight management system to keep their legs the right length.

Speed is limited, so that aircraft do not overrun each other in the pattern. Under the FAA rules, the holding speed is at most 200 knots indicated up to 6,000 feet above sea level, 230 knots from 6,001 to 14,000 feet, and 265 knots above 14,000 feet. The ICAO limits are different in some bands. Crews always check the chart for the limit that applies. FAA Aeronautical Information Manual

There are three ways to enter a pattern: direct, parallel and teardrop. The crew chooses the entry from the direction they arrive. The aim in all three is to join the pattern on the correct side, so the aircraft does not drift into the other traffic. Once inside the loop, the crew watches the timer, the wind and the altitude, and reports to the controller when required.

Wind matters too. It pushes the aircraft off the track it would fly in still air, so pilots correct for it and the loop keeps its shape. The timer then keeps each leg consistent, and the pattern stays inside the airspace it was designed for.

Holding stacks

Where traffic is heavy, several aircraft can hold at the same fix at once. They do this at different heights, usually a thousand feet apart. That arrangement is called a holding stack.

A stack lets the controller hold more aircraft without moving them sideways. The aircraft at the lowest level is first to be released, and the others move down one level as each one leaves. Each aircraft flies its own pattern at its own level, and the controller keeps the layers apart.

Stacks need fuel. Every minute spent in a hold burns fuel, and crews plan for it. Airlines carry reserve fuel for exactly this reason, so a long delay is never an emergency by itself. Even so, a crew that cannot get a clearance in time may have to divert to another airport, and that decision is taken with the controller.

Holding in Short Final

Short Final has holding stacks as a named feature. Two fixes near the edges of the airspace are marked with a triangle, and each has a published racetrack hold. When you draw a path to one of those fixes, the aircraft joins the stack at the lowest free level, between about 5,000 and 8,000 feet. Up to four aircraft can hold there at once without touching each other.

The game shows the holding pattern around the fix when you draw towards it, so you can see where the loop will be before you commit. Drawing a new path takes the aircraft back out of the stack. The game also has a simpler hold: tap an aircraft to make it circle in place, or draw a closed loop and it flies that loop until you give it a new path.

These are simplifications. Real holds have published fixes, timed legs and charted speeds, and the game does not show those details. What it keeps is the idea that a hold buys time and space, and that using it well is part of the job.

What the radio sounds like

You can hear a hold on the radio. A controller might say "hold at the fix as published, maintain five thousand, expect further clearance at one one three zero". The crew repeats the instruction back, flies to the fix and enters the pattern.

Controllers may also say "hold present position" when an aircraft is already close enough to the hold to circle over where it is. The aim is always the same: to tell the crew exactly where to be, at what height, until the next instruction arrives.

What to remember

• A holding pattern is a published racetrack loop. Standard turns are to the right.
• Aircraft hold for runway capacity, closures, weather, emergencies and traffic flow.
• Under FAA rules a standard leg is timed at one minute below 14,000 feet and a minute and a half above.
• A holding stack puts several aircraft at different levels over one fix.

Sources

• FAA Aeronautical Information Manual, chapter 5 (Air Traffic Procedures)
• FAA Air Traffic Control handbook, JO 7110.65
• SKYbrary: the aviation safety knowledge base
• ICAO: the International Civil Aviation Organization