An airport slot is a formal permission that allows an airline to use an airport’s full infrastructure — runways, terminals, and gates — to arrive or depart at a specific time and date. The International Air Transport Association (IATA) defines it as “a permission given by a coordinator for a planned operation to use the full range of airport infrastructure necessary to arrive or depart at a Level 3 airport on a specific date and time.”
Slots exist because demand for flight times at busy airports far exceeds the available capacity, making a fair and structured allocation system essential for global aviation. The slot system affects airlines, passengers, and airport operators around the world. As of summer 2025, there were 215 slot-coordinated airports worldwide, and approximately 43 percent of all passengers globally depart from a slot-coordinated airport. On average, nine new airports are declared Level 2 or Level 3 each year, reflecting the growing mismatch between airport capacity and passenger demand.
Allocating Airport Slots Began After World War II
Airlines have coordinated their schedules under the auspices of IATA since 1947, initially through bilateral timetable discussions aimed at improving interline connections and handling arrangements. The first biannual Timetable Coordination Meeting was convened by IATA in 1948, attended by just three airlines: BEA, Alitalia, and Air France. With the rapid growth of commercial aviation, congestion emerged at major airports in the early 1960s.
IATA broadened the scope of slot discussions to manage anticipated delays to an acceptable level. The Worldwide Airport Slot Guidelines (WASG), which is the cornerstone of global slot coordination, were first published in 1976 and have since been revised 33 times to reflect changing industry needs.
Understanding Level 1, 2, and 3 Airports to Understand Slots
IATA classifies all airports worldwide into three levels based on their capacity relative to demand. The three levels are defined as follows:
Level 1 (non-coordinated):
Capacity is adequate to meet demand. No slot allocation is required.
Level 2 (Schedules Facilitated):
There is potential for congestion, particularly at peak times. Airlines voluntarily submit schedules to a facilitator to assist with planning and avoid bottlenecks. Participation is not mandatory, but non-participants may face penalties if the airport is later reclassified as Level 3.
Level 3 (Coordinated):
The airport is highly congested. Every airline must hold an allocated slot for both arrival and departure. Independent slot coordinators manage all allocations. As of the summer 2022 scheduling season, there were 156 Level 2 airports and 195 Level 3 airports globally. By summer 2025, the number of Level 3 coordinated airports had grown to over 215.
- Well-known Level 3 airports include:
New York John F. Kennedy International Airport (JFK) - London Heathrow Airport (LHR)
- Amsterdam Airport Schiphol (AMS)
- Paris Charles de Gaulle Airport (CDG)
- Melbourne Airport (MEL)
- Beijing Capital International Airport (PEK)
Who Governs Airport Slot Allocation?
Three key stakeholders govern the global slot coordination process. According to IATA, those three stakeholders are airlines, airports, and independent slot coordinators.
The Worldwide Airport Slot Board (WASB) — made up of seven members each from IATA, Airports Council International (ACI), and the Worldwide Airport Coordinators Group (WWACG) — oversees the WASG and ensures it reflects globally accepted best practices.
Independent slot coordinators sit at the centre of the process. At London Heathrow Airport (LHR), slot allocation, exchange, and trading are managed by Airport Coordination Limited (ACL). In the United States, the Federal Aviation Administration (FAA) governs slots at the three designated Level 3 US airports: JFK, New York LaGuardia Airport (LGA), and Ronald Reagan Washington National Airport (DCA). The mix of public and private institutions illustrates that slot governance varies significantly by country and region.
The IATA Slot Conference (ISC) serves as the main industry forum for slot allocation. The conference has taken place since 1948 and today attracts over 1,300 delegates representing more than 250 airlines and 300 capacity-constrained airports. It is held twice yearly:
- in June for the upcoming northern hemisphere winter season
- in November for the following summer season.
How Are Airport Slots Allocated?
Slot allocation begins approximately one year before the relevant scheduling season. According to IATA, the process follows three main rules applied in sequence.
Step 1 — Historic Precedence (Grandfather Rights)
An airline that used a slot series in the previous equivalent season retains the right to that slot in the new season. These are called grandfather rights, and they are automatically renewed provided the airline meets the usage threshold. Historic slots may not be withdrawn to accommodate new entrants or any other category of operator.
Step 2 — The 80/20 Rule (Use It or Lose It)
To retain a historic slot, airlines must operate it for at least 80 percent of the scheduled season. If an airline falls below this threshold, the slot returns to the pool for reallocation. At highly congested airports such as Heathrow, utilization rates typically reach 95–98 percent, well above the minimum requirement, reflecting the intense demand for slots.
Step 3 — The Slot Pool
Slots that are returned, surrendered, or newly created form a pool. Of this pool, 50 percent must be allocated to new entrant airlines, and the remaining 50 percent to non-new entrant requests.
A new entrant is currently defined as an airline holding fewer than seven slots at that airport on a given day, a threshold raised from five slots under a recent update to the WASG to improve access for smaller carriers.
The WASG also includes a secondary criterion giving priority to requests that have spent longer time on the waitlist, further helping carriers that have been unable to gain entry at an airport in previous seasons.
The Worldwide Airport Slot Guidelines
The WASG is jointly published by IATA, ACI, and the WWACG and sets a single global standard for slot management. It is continuously updated to reflect best practices, with the most recent edition incorporating significant changes to new entrant definitions, performance monitoring, and allocation priorities.
The prime objective of the WASG, as stated by IATA, is “to ensure the most efficient declaration, allocation and use of available airport capacity in order to optimize benefits to consumers, taking into account the interests of airports and airlines.”
The WASG applies the same principles to all business models — legacy carriers, low-cost carriers, and cargo operators alike. Major low-cost carriers including easyJet, JetBlue, Vueling, GOL, and AirAsia are firm supporters of the WASG, as it has enabled them to enter congested markets and build reliable schedules.
Within the European Union, slot allocation is also governed by EU Council Regulation No. 95/93, which is legally binding and applies to all community airports. While the EU rules broadly align with the WASG, there are minor differences; the WASG remains the globally accepted standard outside EU jurisdiction.
Here are the objectives of the slot allocation as noted by IATA:
| Core objective | Explanation |
|---|---|
| Improve passenger and cargo choices | by boosting connectivity and competition at congested airports for passengers and cargo. |
| Provide convenient and reliable schedules | that meet demand, remain consistent across seasons, and are operationally reliable. |
| Ensure fair and transparent slot allocation | through an independent slot coordinator acting in an open, non-discriminatory manner. |
| Maximise airport capacity utilisation | and promote regular reviews of capacity and demand for seasonal slot allocation. |
| Balance access between airlines | ensuring fair opportunities for both existing carriers and new entrants. |
| Enable operational flexibility for airlines | to respond to regulatory changes and shifting market demand conditions. |
| Reduce congestion and delays | at busy and capacity-constrained airports. |
Heathrow is The World’s Most Contested Slot Market
London Heathrow, which has seen talks of third runway in recent times, is the most heavily slot-restricted airport in the world and the site of aviation’s most valuable slot transactions.
With a combined movement limit of approximately 10,500 weekly movements — averaging 40–45 takeoffs and landings per hour — Heathrow operates at approximately 99 percent capacity, leaving virtually no room for additional movements.
Slot coordination at Heathrow is managed by Airport Coordination Limited (ACL). A 1999 ruling by the UK High Court established that slots at UK airports could be bought and sold on a secondary market, making Heathrow unique globally.
The most expensive publicly reported slot transaction took place in 2016, when Oman Air (WY) purchased a prime slot pair from Air France-KLM (AF/KL) for $75 million. The previous record was $60 million, paid by American Airlines (AA) to Scandinavian Airlines (SK) in 2015.
The value of a Heathrow slot varies significantly by time of day. According to IBA, an aviation market intelligence firm, slot value is shaped by market conditions, regulatory environments, and timing. Early morning arrival slots command the highest prices because they allow transatlantic flights arriving from North America to depart back within the same day.
British Airways (BA) holds over 50 percent of all slots at Heathrow, having built its position through grandfather rights, secondary market trading, and leasing. In the winter 2023–2024 season, BA held 4,779 slots, far ahead of Virgin Atlantic (VS) in second place with 392 slots and Lufthansa (LH) in third with 290.
This concentration has drawn criticism that grandfather rights create barriers to competition, locking in dominant carriers and limiting consumer choice.
In a related development covered by Simple Flying, IBA’s Manager of Markets and Sustainability, William McClintock, noted that “slot value is shaped by market conditions, regulatory environments, and timing, making them critical strategic assets for carriers.“
Ghost Flights is The Unintended Consequence of the 80/20 Rule
The use-it-or-lose-it rule has a well-documented unintended consequence: ghost flights.
Ghost flights are commercial aircraft operating with few or no passengers, flown primarily to meet the 80 percent usage threshold and retain a slot for the following season. The issue gained widespread public attention during the COVID-19 pandemic when demand collapsed.
In January 2022, Lufthansa (LH) publicly stated that it wanted to cancel 33,000 flights but would still have to operate 18,000 “unnecessary” flights to retain its slots under EU rules.
Around 500 ghost flights per month departed from UK airports between October and December 2021, even after regulators had temporarily lowered the usage threshold to 50 percent.
The European Commission suspended the 80/20 rule entirely in March 2020 and gradually reinstated it with reduced thresholds — 50 percent in 2021, then 64 percent by late 2022. The FAA extended partial waivers for international carriers through the 2024 summer season.
IATA has argued that concerns about ghost flights “are disproportionately focused on a small number of super-congested airports, such as London Heathrow, Amsterdam Airport Schiphol (AMS), and Hong Kong International Airport (HKG)” and that the focus should instead be on expanding airport capacity.
Critics, including climate groups, counter that ghost flights are a symptom of outdated rules not designed with climate objectives in mind. Global aviation accounts for approximately 2.5 percent of all CO₂ emissions, and campaigners argue that any avoidable emissions represent a policy failure.
The Debate Over Slot Reform
The current slot system faces mounting pressure for reform, particularly around grandfather rights and their effect on market access.
Some observers argue that the dominant position of home carriers at their major hub airports — combined with the automatic renewal of grandfather rights — limits opportunities for competing airlines, reducing consumer choice.
Proposals for reform include slot auctions, where airlines bid financially for access rather than relying on historic usage.
IATA has strongly opposed slot auctions, arguing that “auctioning adds costs and uncertainty to the slots process with potentially disastrous outcomes for consumers” and that “any time that it has been attempted, it has failed.”
The UK’s Department for Transport noted in a 2018 draft aviation strategy that the current allocation system “is not designed to stimulate a competitive market environment and has no means of taking into account broader objectives.”
Despite this, meaningful reform has been slow to materialise. IATA maintains that the root problem is not slot rules, but a fundamental lack of airport infrastructure — and that the number of capacity-constrained airports continues to grow year on year.
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Airport Slots and Nepal Aviation
Nepal’s aviation sector operates a small number of international routes, many of which connect to major slot-coordinated hubs in the Middle East, South Asia, and Southeast Asia.
Tribhuvan International Airport (TIA), Kathmandu, Nepal’s primary international gateway, faces its own capacity challenges rooted in infrastructure constraints rather than slot coordination mechanisms. As Nepali carriers seek to expand internationally and codeshare with global partners, an understanding of the slot system at destination airports — particularly Level 3 hubs like LHR, CDG, and AMS — becomes operationally critical.
Airlines seeking to launch routes from Kathmandu to slot-restricted airports must engage with independent coordinators well in advance of a scheduling season and compete for limited pool slots as new entrants. The 50 percent new entrant pool reservation under the WASG provides a route to access, but the scarcity of new slots at super-congested airports means that even new entrant protections offer no guarantee of access at the most in-demand times.