Airport slots explained: coordination, the 80/20 rule and slot value

Why does the long-haul jet to Asia take off at almost exactly the same time year after year? Why does an airline fly a small regional jet to a congested airport when it actually has bigger equipment? The answer lies in an unassuming word: slot. This guide explains what airport slots are, who allocates them, how the famous 80/20 rule works, why time slots at airports like London Heathrow are worth millions – and what you as a spotter can take away from it.

Airport slots: the short answer

A slot is permission to land or take off at a coordinated airport on a particular date at a particular time. It is allocated not per flight but usually as a series for a whole scheduling period – for example “departure every Monday at 7:35 a.m.” throughout the entire summer. The slot covers the complete infrastructure a flight needs: runway, apron positions, terminal and handling.

Slots are only needed where demand exceeds capacity. At a quiet regional airport, an airline can simply agree with the operator when it comes. At a congested hub, on the other hand, without allocation more aircraft would want to arrive at peak times than runways and gates can handle. A neutral airport coordinator therefore distributes the time slots according to fixed rules.

By the way, a daily route with an outbound and a return flight uses two slots per day at a coordinated airport – one landing and one takeoff. Over a week, a single daily route thus occupies 14 slots.

Level 1, 2 and 3: how strongly an airport is regulated

Worldwide, the Worldwide Airport Slot Guidelines (WASG) apply, published jointly by IATA, the airports association ACI and the association of slot coordinators. They divide airports into three levels:

Level	Designation	What it means
Level 1	non-coordinated	Capacity is generally sufficient. Airlines agree their times directly with the airport.
Level 2	schedules-facilitated	At times it gets tight. A facilitator reconciles schedule requests and proposes shifts; participation is based on cooperation.
Level 3	coordinated	Demand clearly exceeds capacity. Without an allocated slot, an airline may not take off or land as scheduled.

According to IATA, there are more than 160 Level 2 and more than 200 Level 3 airports worldwide. More than half of all passengers depart from a fully coordinated airport. Among the best known are London Heathrow, Paris Charles de Gaulle, Amsterdam Schiphol and, in Germany, Frankfurt and Düsseldorf, among others.

The classification depends on the declared capacity. For each scheduling period, it is set how many movements per hour – partly also per shorter period – an airport can handle. Not only runways count, but also apron, terminals and operating restrictions such as night flight rules. How these key figures come about is explained in more detail in the article Airport capacity and movements. In Germany, the time restrictions described in the guide on the night flight ban at German airports also apply.

Who allocates the slots?

Allocation is handled by an airport coordinator. In the EU, Regulation (EEC) No. 95/93 stipulates that this coordinator must work independently – meaning it may be controlled neither by the airport nor by an airline. In Germany, this is the Flughafenkoordination Deutschland (German airport coordination), whose legal basis is the Aviation Act (Luftverkehrsgesetz). Basics are in the article Aviation Act and Air Traffic Regulations.

Besides the coordinator, every coordinated airport has a coordination committee in which the airport, airlines, air traffic control and other parties sit. It advises the coordinator, for example on capacity and local rules.

The annual rhythm of slot allocation

Slots are allocated per scheduling period. The IATA summer schedule begins on the last Sunday in March, the winter schedule on the last Sunday in October. How this shows up at the fence is covered in the article on the summer and winter schedule. The process follows a similar pattern every year:

1. Historic rights are confirmed: The coordinator determines which airline may keep which series from the previous season with sufficient use.
2. Applications come in: Airlines submit their requests for the new season – old series, time shifts and new routes.
3. First allocation: The coordinator distributes the slots by priority. First come historic rights, then changes to existing series, then new applications from the pool.
4. Slot conference: Twice a year, airlines and coordinators meet at the IATA Slot Conference to align and swap times worldwide. After all, a long-haul flight needs matching times at both ends.
5. Return and fine-tuning: Until shortly before the season starts, airlines return slots they don’t need, and others move up.

Use it or lose it: the 80/20 rule

The heart of the system is the historic rights rule, often called “grandfather rights” in English. An airline that gets a slot series in a season and uses it sufficiently may keep it in the next equivalent season. Sufficiently means: at least 80 percent of the series must actually be operated. Up to 20 percent may be dropped. Hence the name 80/20 rule, in industry jargon also “use it or lose it”.

If usage falls below the threshold, the airline loses its priority, and the series goes back to the slot pool. Time slots are redistributed from this pool. The EU regulation provides that a portion of the pool slots goes preferentially to new entrants – airlines that so far hold only a few slots at that airport. This counteracts a few large carriers dividing up an airport among themselves permanently.

Not every cancellation counts against the airline. Cancellations for reasons beyond its control can be counted as operated. Typical examples are closures of an airport or extraordinary events. How exactly these exceptions apply is set out by EU law and the WASG.

Ghost flights and the COVID exceptions

The 80/20 rule has a downside: in weak times, it can pay for an airline to fly nearly empty aircraft just so as not to lose its valuable slots. Such flights are called ghost flights. The problem became especially visible during the COVID-19 pandemic, when demand collapsed. The EU therefore first suspended the usage rule and then phased it back in step by step with lowered thresholds. In the meantime, the full 80 percent mark applies again.

The rule was also relaxed in earlier crises, for instance after the attacks of September 11, 2001, during the SARS epidemic and in the financial crisis. These phases were exciting for spotters: suddenly patterns and equipment showed up that you wouldn’t otherwise see at an airport, because airlines served routes with small or simply available aircraft.

What is a slot worth?

Because slots at heavily congested airports are scarce and can be held for years, they are a real asset for airlines. Legally, the EU regulation doesn’t provide for a sale: slots are allocated but may be exchanged or transferred between airlines. In the UK, a market has developed from this in which such swaps come with high compensation payments.

Some transactions at Heathrow that became known:

Year	Buyer	Transferring airline	Reported value
2008	Continental Airlines	among others GB Airways	about US$209 million for four pairs
2015	American Airlines	SAS	about US$60 million for one pair
2016	Oman Air	Air France-KLM	about US$75 million for one pair

A “pair” here means a landing slot and a takeoff slot for daily use. The figures come from reports about the deals; there are no official price lists. But they show why established airlines like British Airways hold a considerable share of the time slots at Heathrow and why new entrants like Oman Air dig deep into their pockets just to get in at all.

Slots as leverage in mergers

Because slots mean market access, they also play a large role in competition law. In takeovers and mergers, competition authorities repeatedly require airlines to give up slots at certain airports so that competitors can take over routes. For large groups like the Lufthansa Group, such conditions are part of everyday life in consolidations. In insolvencies, too, the question regularly arises of what happens to the slots – when Air Berlin ceased operations in 2017, its time slots at German airports in particular were an important part of what interested other airlines.

Airport slot or ATC slot: two different things

In everyday spotting, you also hear the word slot in another context: “The aircraft is waiting for its slot.” What is usually meant then is not the airport slot but an ATFM slot of European air traffic flow management.

Feature	Airport slot	ATFM slot (CTOT)
Allocation	Airport coordinator	Network Manager of Eurocontrol
Period	whole scheduling period	a single flight on the day of operation
Purpose	distribute the airport’s capacity	avoid overload of airspace or destination airport
Form	time for landing or takeoff in the schedule	calculated takeoff time with a short tolerance window

If an airspace sector is overloaded, for example because of thunderstorms or a shortage of air traffic control staff, a flight gets a calculated takeoff time, the Calculated Take-Off Time (CTOT). The aircraft then waits on the ground rather than in a holding pattern. You can find more in the articles on Eurocontrol, DFS Deutsche Flugsicherung and the reasons for flight delays. Important: a delayed flight doesn’t automatically lose its airport slot. For the 80/20 rule, what counts is whether the flight took place, not whether it was on time. However, anyone who systematically and deliberately deviates from their slot times must expect sanctions.

What slots mean for spotters

The slot system is not a dry administrative topic but directly shapes what you see at the fence.

• Stable times: At coordinated airports, many connections stay at almost identical times for years, because airlines defend their historic slots. Once you know when the morning long-haul bunch lands in Frankfurt, you can count on it.
• Waves at hubs: Hub airlines bundle their slots into waves so that connecting passengers have short walks. The article Hub waves and banking explains that. For you, it means: there are peak times with dense traffic and gaps in between.
• Unusual equipment: If an airline has to operate a series but has no suitable aircraft available, it sometimes deploys other equipment or wet-lease partners at short notice. Such swaps are often the chance for rare photos.
• New airlines only after winning slots: When a carrier announces a new route to a fully coordinated airport, a slot swap or an allocation from the pool is usually behind it. The first flights are popular spotting targets.
• Season change as a deadline: Most changes fall on the switch between the summer and winter schedule.

At Heathrow you see this especially clearly: the airport has long been extremely busy, and at spots like Myrtle Avenue one approach follows the next. In Frankfurt, the Zeppelinheim viewing platform shows you how dense the program of a coordinated hub is at peak times. And at the Düsseldorf observation deck you experience an airport where capacity is limited mainly by operating rules.

Criticism of the slot system

The system is old, and it is criticized again and again. The main points:

• Vested rights instead of competition: Historic rights protect established airlines. At fully congested airports, new entrants find it hard to get attractive times. The British government has described the system as poorly suited to promoting a competitive market.
• Environment: Ghost flights burn fuel with no benefit. That is why exceptions are granted in crises, but they have to be negotiated politically every time.
• Gray area of trading: Whether and how slots may be transferred for money is not regulated uniformly within Europe.
• Capacity remains the bottleneck: No allocation system creates additional runways. The real lever remains expanding infrastructure – which in turn collides with noise protection and residents’ interests.

You can read more about the EU framework in which airlines may fly freely in the single market but are still bound by slots in the article on open skies agreements and the EU single aviation market. How low-cost carriers handle the topic and why they often move to less congested airports is explained in the piece on the business model of low-cost carriers.

Conclusion

Slots are the distribution system for scarce airport capacity. Where more flights want to arrive than runways and terminals can handle, an independent coordinator allocates the time slots for each season. The 80/20 rule rewards airlines that use their slots, but also makes them a valuable and fiercely defended vested right – up to sums in the millions at Heathrow and ghost flights in times of crisis. For you as a spotter, knowing this is practical: it explains stable departure times, the waves at hubs and the surprises around the schedule change.