Air traffic control delays: what "ATC restrictions" means for your flight

Few delay reasons are as vague as "air traffic control restrictions". The weather at your airport can be perfect, the plane can be at the gate and the crew can be ready, and you still sit and wait. This post explains what is going on behind that phrase, why it happens, and what it means for you as a passenger.

What an ATC restriction actually is

Air traffic controllers keep aircraft safely apart. Each piece of airspace, and each runway, can only handle a certain number of flights in a given period. When more flights want to use a sector or an airport than it can safely take, controllers limit the flow. Flights heading into that area are given a later time, and the simplest place for a plane to wait is on the ground at its departure airport, with its engines off.

In Europe these waits are usually managed centrally, and your flight is given a take-off slot: a window of a few minutes in which it has to leave. In the United States, the Federal Aviation Administration uses ground delay programmes and ground stops for the same purpose. The names differ, but the idea is the same: it is safer and cheaper to wait on the ground than to circle in the air.

Why it happens on a clear day at your airport

The problem is often somewhere else. The restriction can be on your route or at your destination, hundreds of miles away. Common causes include:

• Thunderstorms along the route, which force planes to fly around them and squeeze more traffic into less space.
• Strong winds, low cloud or fog at the destination, which can reduce how many planes can land each hour.
• Staff shortages or equipment problems at a control centre, which lower how much traffic a sector can accept.
• Industrial action by controllers, which can affect flights that only fly over the country involved.
• Busy periods, when airspace simply runs near its limit for much of the day.

Because of this, two flights leaving the same airport at the same time can have very different days. One may leave on time while the other waits an hour, simply because it is heading into a busier or more restricted area.

Slots, and why boarding can happen and then nothing moves

A take-off slot can be given, moved or cancelled at short notice. Sometimes the crew boards everyone quickly to be ready for an earlier slot if one opens. Sometimes the slot moves later after boarding, and the plane waits at the gate or near the runway. If the flight misses its slot, for example because boarding took too long, it may need a new one, which can mean another wait.

This is also why announced delays can change in steps. The airline often only knows its new time once it has a confirmed slot, and that slot can change as the situation ahead of you changes.

The knock-on effect later in the day

A plane held by a restriction in the morning is late for every flight it operates afterwards, unless there is spare time on the ground to catch up. So an ATC delay on one route can turn into a delay on a completely different route in the afternoon. Our guide on late arriving aircraft explains how that chain works, and why the incoming plane is often the best early warning for your own flight.

What you can do

• Check your flight before you leave home, and again at the airport. See will my flight be delayed? for the signs to look for.
• Stay close to the gate once boarding time nears. A slot can move earlier, and flights sometimes board quickly when it does.
• Keep an eye on the airline app for gate and time changes rather than the departure board alone.
• If you have a tight connection, tell the airline early. Rebooking is easier before the connection is missed.
• Keep receipts for food and drinks during a long wait. Our flight delay help guide covers what to keep and where to ask.

Does an ATC delay affect what the airline owes you?

Rules depend on where you fly and which airline you fly with. In some places, delays caused by air traffic control decisions are treated differently from problems the airline caused itself, while care during a long wait, such as meals, may still apply. The details are set by each region, so check the official passenger rights pages for your trip, and your airline's own conditions. We list the official bodies for the EU, UK and US in our airline support page.

In short

An ATC restriction means there is not enough room in the sky or at an airport for every flight that wants to be there at that time. The wait is usually spent on the ground because that is the safest place for it. You cannot speed it up, but you can stay informed, stay near the gate and keep your options open for any onward journey.