At first glance, airport slots might appear to be a purely operational matter: they are timings for airlines to land and take off, helping manage congestion at busy airports and ensuring orderly scheduling. Slots are integral to both airport and airline operational planning. But who gets them and why is not a technical question.
In reality, slots determine who can enter and compete in the market, influence connectivity, and shape how efficiently airport infrastructure is used. They play a key role in long-term planning and ensuring that aviation meets the needs of diverse stakeholders.
The way slots are allocated affects not only airlines but also airports, passengers, and regional economies. As air traffic grows and capacity constraints become more pressing, their impact will only become more important. While slot allocation is an operational process, slot policy is a strategic choice that shapes the aviation landscape.
This article will explore the strategic importance of airport slots from both operational and economic perspectives, their role in shaping access and competition, and how a more balanced approach to slot management can contribute to a more efficient and equitable aviation system.
What do flight timings have to do with fair access?
Globally, airports fall into three categories based on congestion level: Level 1 airports have unrestricted access, Level 2 airports have a potential for congestion at peak times, and Level 3 airports, or slot-coordinated airports, face the most congestion, where demand often exceeds capacity. In simple terms, airlines want to have more flights there than the airport can accommodate.
Following this, a slot allocation process is implemented to prioritize and systematically manage airline demand. Airlines submit schedule requests, and a coordinator reviews these against the airport’s declared coordination parameters, which set limits on the number of aircraft movements or passengers that can be processed within various airport systems. Slots are approved if there is capacity available at the requested time; otherwise, alternative timings may be offered. From that point, slot series become eligible for historic status and can potentially remain with the airline indefinitely, provided slot utilization requirements are met.
This is where slot policy moves beyond logistics and into strategy. The rules governing slot allocation determine who gets to serve demand in a constrained market. In such a market, some demand inevitably goes unmet, making these decisions highly consequential for airlines, airports, consumers, and broader economies.
While one could argue that incumbents are simply responding to demand, reality is more complex. Different airlines prioritize different types of demand. Some introduce products that stimulate markets, while others focus on defending their existing positions. Adding a new destination, increasing frequencies on an existing route to outcompete a rival, or “babysitting” valuable slots by operating minimally profitable flights to maintain future control over scarce capacity are all strategic choices. Currently, these decisions rest with airlines holding historic slots, and may not always align with wider economic or accessibility goals.
To understand how the system operates today, it is helpful to consider the needs around which it was originally designed.
The airline-centric origins of the Worldwide Airport Slot Guidelines
The foundational principles of the Worldwide Airport Slot Guidelines (WASG) were defined by airlines in the 1970s, during a time when most airports were government-owned and considered public infrastructure, and the airline market was dominated by state-owned flag carriers. The liberalization of air transportation drastically changed the competitive landscape.
Yet the WASG, originally intended as a temporary measure, continue to reflect the market structure that existed in the 1970s. It is useful to consider how its core principles—certainty, flexibility, sustainability, and transparency—have been applied in practice:
- Certainty is guaranteed by the concept of historic slots, where incumbents are granted the same series of slots again season after season if they comply with slot usage requirements, which currently mandate the use of at least 80% of allocated slots.
- Flexibility allows incumbents to change the specific use of a slot to respond to market demands, meaning that a slot is not destination or aircraft specific. In addition to the 20% flexibility provided by the 80% slot utilization requirement, slots are also considered as operated in cases of justified non-utilization (JNUS), meaning events that should be both unforeseeable and unavoidable.
- Sustainability is not understood as environmental sustainability, but rather as the long-term viability of the slot allocation system, ensuring that incumbents can retain or trade their slots as they wish, while reinforcing the notion that slots should be granted at no direct cost, despite airport capacity being a limited resource.
- Transparency is interpreted as a requirement for visibility into the allocation process managed by the slot coordinator.
While these principles have shaped the industry’s growth, the slot system’s underlying design did not anticipate major shifts in market dynamics, including the widespread expansion of airline alliances and joint ventures, the rise of low-cost carriers, the privatization of airports, and advancements in air traffic management and operational efficiency. As the aviation landscape evolves, it is crucial to reassess whether this vision still serves the broader needs of the industry, consumers, and society.
With this objective, airports joined the governance of the WASG through the creation of the Worldwide Airport Slot Board (WASB) in 2020, which includes an equal number of airports, airlines and slot coordinators. Although the first term of the WASB from 2020 to 2022 was overshadowed by the pandemic, the board succeeded in adapting slot allocation practices to address the impacts of the COVID-19 crisis.
Since 2022, the WASB has refocused on its original agenda of improving the slot system. While some provisions of the WASG have been updated, achieving a more balanced approach will require continued collaboration that critically examines what the system aims to promote.
The role of airports and the overall goal of the slot system
Given the fixed nature of their infrastructure, airports have a unique understanding of, and close ties to, the communities they serve. However, once an airport is designated as Level 3 slot-coordinated, it loses much of its decision-making power in the allocation process. Airlines that already hold slots have a major advantage over those trying to enter. Airport operators are required to comply with slot allocation decisions, limiting their ability to optimize capacity for broader economic or public interest goals.
This raises fundamental questions: should airports, as businesses in their own right, have greater influence over how their capacity is used? Should slot allocation take into account broader economic, social, or environmental goals, or should the system continue to prioritize meeting the needs of incumbent airlines? Given the value of slots and their significant impact, this is not an easy discussion to navigate. Meaningful answers will require collaboration between airlines, airports, coordinators, regulators, and academic experts.
Balancing priorities: the need to prepare for future growth
Behind debates on specific slot provisions lies a fundamental difference in how airports and airlines perceive the value of slots. Airlines tend to prioritize stability and control, often resisting amendments that could cause changes to their existing networks. Airports value long-term stability while focusing on maximizing infrastructure efficiency, accommodating growth, and balancing competition and connectivity to best serve their communities. Effective collaboration should aim to create a system that respects both perspectives and ensures a fair balance for the benefit of consumers.
A key concern with the current slot system is that it primarily optimizes benefits for individual airlines rather than for the broader industry and the traveling public. Under the framework set by the WASG, incumbent Airline A can strategically manage its slot portfolio, especially at its hub airports, to maintain market position. Airline A has no incentive to voluntarily adjust its portfolio in a way that would make it easier for competitor Airline B to enter the market. Yet, when Airline B follows the same approach at its own hub, Airline A faces the same barriers to entry. This creates a classic example of the “tragedy of the commons,” where short-term individual strategies, permitted under the existing rules, ultimately limit broader industry efficiency.
Beyond individual airlines, the growing influence of airline groups, airline alliances and multi-brand holdings has further shaped slot dynamics. As consolidation increases, airline groups can manage slot portfolios across multiple carriers within the same corporate structure, allowing them to strengthen their presence at key airports. While this can support network efficiency for established players, it also raises concerns about access for new entrants and the overall competitiveness of slot-constrained airports. The current system was designed when airlines operated more independently, and it does not fully account for the complexities of today’s interconnected airline business models.
As sometimes pointed out, it is true that LCCs have experienced significant growth within the current system. Their success reflects the ability of certain players to navigate constraints. However, it is important to note that many of the major LCCs operating in highly congested markets were established decades ago. The conditions that allowed them to grow, particularly their early focus on secondary airports that used to have fewer slot constraints, would be difficult to reproduce today.
Looking at air travel forecasts, the limitations of the current system become increasingly concerning. ACI World projects that by 2043, global passenger traffic will reach 17.7 billion, an almost twofold increase from 2024 levels. To accommodate this growth, ACI estimates that approximately US$2.4 trillion in airport capital investments is required between 2021 and 2040.
While infrastructure expansion remains the primary long-term solution to address capacity constraints, it is a slow and complex process due to physical, regulatory, and financial constraints. In the meantime, optimizing the use of existing infrastructure through effective capacity allocation is both a pragmatic and necessary step to support the broader aviation ecosystem, ultimately benefiting consumers and the communities that airports serve.
Efficient use of slots vs. efficient use of airport capacity
A core issue is that the current system’s main objective is to ensure stability and predictability for incumbents, which sometimes conflicts with promoting efficient use of constrained infrastructure at the airport level. As the aviation industry expands and the number of Level 3 airports is forecasted to increase, addressing this tension is becoming increasingly urgent.
Aviation is a data-driven industry, and data analysis is often the natural approach to evaluating policies. However, as with any analysis, it is crucial to carefully assess the conclusions that can genuinely be drawn. For example, statistics on high slot utilization at certain airports are frequently presented as evidence that the slot system is working. Yet, high slot utilization does not necessarily equate to efficient airport capacity utilization. In fact, it can mask inefficiencies, especially when valuable slots are not used to their full potential. In slot utilization statistics, an empty flight will look identical as a fully booked widebody aircraft.
This distinction is important because airlines and airports are both capital-intensive industries that depend on the efficient use of limited capacity. Once an aircraft takes off, that capacity is lost; but the same is true for many elements of the aviation value chain, including airport infrastructure, which, once time has passed, cannot be reused or reallocated in a meaningful way. This reinforces the need for policies that prioritize slot allocation stability to the extent that it also supports the best use of available capacity.
Over time, various adjustments have been made to the WASG, often in response to regulatory pressure. That being said, discussions often fail to address the inherent bias within the system itself. As briefly discussed above, once incumbents gain historic slots at a Level 3 airport, they are then granted a significant amount of flexibility. This flexibility results in multiple ways for incumbents to retain slots, leading to limited opportunities for reallocation at the end of each season. This mechanism stifles innovation and delays progress, which is not conducive to fostering a fair, modern, and competitive aviation sector.
Recognizing these biases is key to approaching this discussion with the right mindset. Technology can help modernize slot management, but automating existing processes alone will not correct the imbalances embedded in the system’s design. As explored throughout this text, this is fundamentally a policy challenge that requires a comprehensive, balanced, long-term solution.
A changing landscape: addressing emerging challenges
As the aviation industry continues to evolve, the slot allocation system must also adapt. Rising demand for air travel presents both challenges and opportunities. While some have argued that the current system has served its purpose, it is increasingly clear that it does not fully align with today’s industry realities, and by extension, the needs of consumers.
A comprehensive discussion of slot allocation must consider its impact on competition, economic growth, and market access to ensure the greatest possible benefits for consumers and the communities served by airports. By focusing on the strategic role of slots, we can begin to address some of the deeper structural issues that have emerged within the system.
We will continue to explore these questions in future articles in our blog series. If you have any questions or would like to engage with us on slot policy, please feel free to contact us at slots@aci.aero. Otherwise, we invite you to follow the next article in the series, which will explore the links between airport charges and slot allocation.
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