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The world’s airports say aviation has a growing problem. Demand is rising. Big hubs are filling up. Yet many rules that decide who gets access still rest on ideas shaped in the 1970s.

Airports Council International World, or ACI World, has released a new white paper calling for airport slot reform. It calls for a system that is more open, more flexible, and better able to meet modern travel demand.

The issue sounds technical. It is not. Airport slots determine which airlines can launch routes, add flights, or enter busy markets. That can affect airfares and choice. It can also change the number of useful connections available to travellers.

ACI World says the pressure is growing fast. The number of congested, slot-restricted airports has risen by more than 25% in the past decade. Global passenger traffic is also expected to almost double by the mid-2040s.

Fewer than 10% of the world’s airports handle almost two-thirds of all passengers. ACI also says 85.5% of airports handling more than 40 million passengers a year are already at or near capacity.

That is a lot of aircraft chasing a limited number of gates, runways and useful times. Sadly, airports cannot simply order another Tuesday.

ACI says the old system needs an upgrade

Justin Erbacci, Director General of ACI World, summed up the argument neatly.

“We are trying to run a 2026 aviation industry on a 1970s operating system,” Erbacci said. “Airports are doing their best to operate within an outdated regulatory framework, but the reality is that these inflexible rules and practices are preventing the industry from adapting to modern passenger demands, providing competitive fares and building resilient networks. ACI World urges regulators to sit down with the industry and finally deliver the overdue upgrade that our passengers deserve.”

A key part of the debate is how historic slots are treated.

At a fully coordinated airport, known as a Level 3 airport, an airline must hold a slot before it can operate at the agreed time. Under the current Worldwide Airport Slot Guidelines, an airline can normally keep a series of slots for the next equivalent season if it used them at least 80% of the time.

That rule makes sense. Airlines need stable schedules. They sell seats months ahead. They plan aircraft and crews. They also build connecting networks around set departure and arrival times.

But ACI says that stability has a cost. New airlines, or smaller rivals, can struggle to gain access when a major airport is already full.

At the 50 most congested airports, ACI says the four largest airlines account for 67% of flights. It argues that historic rights can build up over time and are rarely released.

The result, ACI says, is a system that may not move quickly enough when demand changes. A city may want new links, more competition or better use of scarce airport space.

Reform, not demolition

ACI is not asking regulators to tear up the global slot system.

Instead, it wants airports to have a clearer role in managing scarce capacity. Independent slot coordinators would still make neutral decisions.

ACI also wants more transparency. Airlines, airports and governments should be able to see who asks for slots and who gets them. They should also see how slots are used, traded or exchanged.

The group also wants historical access reviewed periodically. It says local needs should have more weight when new slots become available.

Another proposal deals with slot transfers. ACI says these deals should be more open and subject to suitable oversight. If a slot deal creates financial value, ACI argues that some of it could help pay for airport upgrades or extra capacity.

That idea will attract attention. At some major hubs, a good slot can be a very valuable asset indeed.

Airlines have a case too

There is another side to the runway.

The Worldwide Airport Slot Guidelines, or WASG, are jointly published by ACI, the International Air Transport Association and the Worldwide Airport Coordinators Group. IATA says the system is updated over time. It says the rules aim to support fair access, consumer choice, competition and reliable schedules.

That qualification matters. It would be misleading to suggest that the slot framework has literally been untouched since the 1970s. ACI’s case is that its basic architecture and historic-access principles date from that era, even though the guidelines themselves have continued to be revised. ACI’s own policy page says the Worldwide Airport Slot Board, formed in 2020, gives airports, airlines and coordinators a formal role in developing the guidelines.

Schedule stability also has real value. If regulators move slots too freely, they can damage airline networks. They can also break connections and make long-term planning harder.

So the policy challenge is not simply to take slots from established airlines and hand them to newcomers. The task is to find a better balance between certainty and competition.

ACI says that is exactly the point. It is arguing for change within a global system, not the end of that system.

Erbacci has urged airlines and other industry bodies to work with airports on a more modern model. His message is that defending old rules simply because they are old is no longer enough.

Why travellers and travel advisors should care

Airport slots may look like an argument for lawyers, regulators and people who genuinely enjoy spreadsheets. In practice, they matter to what the travel industry sells.

A slot can help decide whether a new airline enters a route. It can affect whether an existing airline adds flights. It can also shape whether a regional city gets a useful link to a major hub.

For travel advisors, that flows through to price, schedule choice and available product.

For passengers, the impact is simpler still: more competition can mean more options, while poor access can leave a market with fewer flights and less choice.

With passenger numbers rising and many major airports already tight on capacity, the slot debate is moving from the back office to centre stage.

ACI World has now put a clear reform case on the table. Regulators must decide what should stay. They must also decide what should change, and how quickly.

One thing is certain. As global travel grows, the fight for a good airport slot will only become more valuable and much harder to ignore.