Why Summer Flight Delays Are Worse in Europe (And What Slot Delays Really Mean)

Summer in Europe: blue skies, long days, packed airports… and, unfortunately, flight delays. If you’ve flown during the warmer months, chances are you’ve heard the dreaded announcement... maybe even by me, saying something like...
“Thank you for boarding so quickly, we're all ready to go up here. Unfortunately, we have just spoken to air traffic control and we currently have a slot delay for our flight back to London.”

But what does that actually mean? Why is it more common in summer? And is there anything passengers can do about it?

Let’s break it down.

European Skies Get Crowded — Fast

Unlike other parts of the world, European airspace is incredibly dense and fragmented. There are dozens of national air traffic control authorities, each managing a patchwork of routes. During the summer holiday rush, there's a maximum number of planes per hour that can be managed. This is often called a 'Flow rate'.

In July and August, daily flight volumes can increase by 30–50% at major European hubs. That means more aircraft in the sky, more crossing points, and more congestion in key choke points — especially over central Europe, France, Germany, and the Mediterranean. 

Imagine this: you're in charge of the entire UK motorway system. But there’s a catch — there can’t be any traffic jams, and every car must maintain a safe distance from the one in front. During quiet times, that might be manageable. But on a Friday evening at rush hour? Chaos.

The only way to control the flow is to assign each car a specific time to start its journey — a 'slot delay'

What Is a Slot Delay?

A “slot” is a specific takeoff or landing window allocated by Eurocontrol, the body that coordinates air traffic management across Europe. When airspace gets saturated or an ATC unit (like France’s or Germany’s) becomes overloaded, they impose flow control measures. That’s when flights are assigned a new slot — sometimes hours later than originally planned. You can also get a slot delay assigned by the arrival or departure airport.

Here are the most common reasons you might get a slot delay:

• Airspace congestion (especially over France, Germany, or central Europe)

• Airport congestion due to limited runway capacity or parking stands

• Weather — such as thunderstorms, strong winds, or low visibility

• Staff shortages in air traffic control, ground handling, or airport ops

• Strikes or industrial action, which are unfortunately common in Europe

• Late-arriving aircraft or tight turnarounds causing delays down the line

Summer Weather Makes It Worse

In summer, thunderstorms are a cause of slot delays across European airspace. Unlike steady rain or low cloud, thunderstorms can be dangerous and unpredictable — pilots must fly around them, often making wide deviations from the planned route.

Here’s a simple way to think about it:

Imagine a motorway with three open lanes. Now picture a section of that road under construction — one lane is closed, and all traffic has to squeeze into the remaining two. Things slow down fast.

The same thing happens in the sky. When thunderstorms pop up along busy air routes — like over France or the Alps — aircraft are forced to reroute around them. That creates “dead space” in the air: pockets of sky that can’t be used. Air traffic controllers respond by reducing the number of flights allowed through that airspace at any given time to maintain safe separation.

So even if the weather is sunny at your departure airport, a thunderstorm hundreds of miles away can still cause your flight to be delayed — all because the flow of traffic in the skies has been narrowed, just like on a motorway at rush hour.

Delays Throughout The Day Normally Build Up!

In summer, airports and airlines operate at near full capacity. Turnarounds are tighter, schedules are packed, and delays become harder to absorb. A 15-minute delay leaving Athens can ripple into a 2-hour delay later in the day for a plane landing in Amsterdam.

In general, an aircraft in the summer can take off at 5am and end up landing at 3am after a night flight. Here's an example schedule for one plane out of London.

London to Tenerife to London. London to Amsterdam to London. London to Dalaman to London. The aircraft leaves London at 06:30am not to finish the day until 03:45am.

As you can imagine, a few hold ups earlier in the day can knock onto the later flights.

France Is a Hotspot for ATC Delays

Due to geography, most northern–southern European traffic flies over France. So, if French ATC is understaffed, experiencing strikes (a frequent summer occurrence), or managing bad weather, delays build quickly across the continent.

In fact, Eurocontrol often lists France as the #1 contributor to en-route delays during summer months.

What Can You Do to Minimise Disruption?

While you can’t control the skies, you can plan smarter:

• Book morning flights. Aircraft and crew are fresh, and delays haven’t stacked up yet.

• Avoid overly tight connections, especially through hub airports like Frankfurt, Paris, or Heathrow.

• Use apps like FlightRadar24 or FlightAware to track slot delays and aircraft inbound status.

• Be realistic. If you're flying on a summer Friday afternoon from a major airport — expect queues and plan accordingly.

• Pack snacks, power banks, and patience. You may need all three.

What Do Pilots Do To Minimise Disruption?

Flying in Europe during summer is part of the adventure — just don’t be surprised if air traffic control asks your plane to wait its turn. Slot delays may be frustrating, but they’re a vital part of keeping European skies safe and efficient under immense pressure. Trust me, the cabin crew and pilots want to get home from work as quickly as possible.

As a pilot, if we get a slot delay, I use all of the resources I have to find out the reason for the delay and how to avoid it. Sometimes, it might mean we can change our route to avoid an airspace. This might mean taking off from Malaga, heading directly West to Portugal before heading North back to London.

Remember, if you're looking to take some Jet Bean Coffee abroad, get it grounded which makes it super easy to make in a French Press!

Nathan Raab
Airline Pilot | Better Coffee Advocate | Jet Bean Founder