An airport slot is permission for an airline to arrive at or depart from a specific airport at a particular time. It is not a route right, a landing permit or a gate booking. Slots exist because, at a small number of very busy airports, airlines want to operate more flights at popular hours than the runways, terminals and gates can handle.
For an airline, a slot is often worth more than the aircraft that uses it. The sections below explain how slots are allocated, what the 80/20 rule does and why the system matters for competition.
Why most airports do not need slots
Most airports have enough capacity for airlines to schedule flights freely. Slots become necessary only where demand at certain hours exceeds what the infrastructure can safely and reliably handle. According to IATA, there are more than 200 Level 3 airports worldwide, and about half of all passengers depart from one.
The industry classifies airports into three levels:
- Level 1: capacity is generally enough for demand and no formal coordination is needed.
- Level 2: a facilitator proposes voluntary schedule adjustments to avoid conflicts.
- Level 3: airlines must hold a slot to operate at the airport, and an independent coordinator allocates them.
Who allocates slots
At a fully coordinated airport, the slots are handed out by an independent coordinator rather than by the airport, the government or the airlines. IATA’s description of the system says the coordinator applies the law that applies locally and the Worldwide Airport Slot Guidelines, known as the WASG. The guidelines are published jointly by IATA, Airports Council International and the Worldwide Airport Coordinators Group. Where national law differs from the guidelines, national law takes precedence.
Allocation starts with the capacity the airport declares. The coordinator then weighs historic precedence, requests from new entrants and existing airlines, and operational feasibility. Slots are normally allocated as a series for a scheduling season, such as the summer or winter timetable, so that airlines can publish reliable schedules and sell tickets months ahead.
Historic precedence and the 80/20 rule
Airlines do not own slots, but they can earn the right to keep them. This is called historic precedence, and it is often described as “grandfather rights.” An airline that operates a series of slots at least 80 percent of the time during a season is entitled to ask for the same series in the next equivalent season. If it falls below 80 percent, the series can return to the pool and go to another carrier. This is the 80/20 or “use it or lose it” rule.
IATA stresses that the rule does not mean airlines routinely cancel one flight in five. Certain cancellations caused by unforeseeable and unavoidable events outside an airline’s control can qualify as justified non-utilization and be treated as operated.
The European Union has its own legal version in Regulation 95/93, as amended, and the United States manages slot usage at a small number of airports under Federal Aviation Administration rules. Regulators have also suspended the 80 percent threshold in emergencies. Both the European Union and the FAA did so during the early months of the COVID-19 pandemic, so that airlines were not pushed to operate largely empty flights just to protect their slots.
Slots in the United States
The FAA limits operations at Reagan National Airport under 14 CFR 93.227, and at New York’s JFK and LaGuardia airports under operating-limitation orders. These airports are not formally coordinated in the way Level 3 airports are elsewhere, but the practical effect is similar: an airline needs an allocated operating authorization, and unused authorizations can be withdrawn. Policies change, so check the FAA’s own notices for current rules.
Why slots matter for competition
Slots are a barrier to entry. A new airline needs good times at a busy airport to run a viable schedule, yet the best times are often held by established carriers. The WASG tries to address this: new entrants receive priority for at least 50 percent of the slots in the pool, provided their requests can be accommodated.
Even so, the pool at the busiest airports can be small. A new carrier may be offered different times than it requested or placed on a waiting list. Airports Council International has argued that slot rules, whose basic structure dates from the 1970s, should be modernized, while IATA says the current system works well when consistently applied. Those are opposing positions and both are on public record.
What slots mean for airline strategy
An airline that holds valuable slots has to use them, and that shapes decisions. It may keep flying a route that is marginal in order to protect the slot series, swap to a larger aircraft to carry more passengers per movement, or return slots it does not plan to use. Network planners treat slot portfolios as strategic assets. That link between slots, aircraft size and how full each flight is explains why capacity decisions come up so often in airline economics. The blog collects related topics.
Frequently asked questions
Can an airline sell a slot?
Rules differ by country. IATA’s guidance describes slots as permission to use capacity rather than property. Some jurisdictions allow exchanges or transfers under conditions. Readers should check the rules of the airport and jurisdiction concerned.
What is the difference between a slot and a route right?
A slot is a time at an airport. A route right, usually granted through aviation agreements between governments, is permission to fly between two places. An airline needs both where slots apply.
Do all airports have slots?
No. Slots are used only at congested Level 3 airports. Level 1 and 2 airports do not require them.
Related reading
For the governance side of aviation companies, see the page on what independent directors do, which covers how boards oversee strategy and risk.