An airport slot is a scheduled permission granted to an airline to use a specific airport's runway or gates at a defined date and time. Slots are a capacity management tool used at airports where demand for takeoff and landing times exceeds the available infrastructure.
How It Works#
Airports become slot-coordinated when their capacity cannot absorb all requested flight movements. A governing body, called a slot coordinator, allocates time windows to airlines. Each slot covers a narrow band, typically 5 to 10 minutes, during which an aircraft may arrive or depart.
The global framework for slot allocation follows IATA's Worldwide Slot Guidelines (WSG). Under this system, airlines that hold slots from a previous equivalent season can retain them the following year. This right is called grandfather rights or a historic slot. An airline must use each slot at least 80% of the time to keep it, a threshold called the 80/20 rule or use-it-or-lose-it rule.
Slots are treated as a finite resource. At heavily congested airports, airlines sometimes trade or sell slots through secondary market transactions. At London Heathrow, for example, a single slot pair has sold for tens of millions of dollars.
New slot requests go through a coordination process at each IATA scheduling conference, held twice yearly. Slots not covered by grandfather rights are redistributed from a slot pool, with half going to new entrants and half available to existing carriers.
Example in Aviation#
A regional carrier wants to launch a new morning service from London Heathrow (LHR). Heathrow is a Level 3 airport, meaning it is fully slot-coordinated. The airline submits a request through the UK slot coordinator, Airport Coordination Limited (ACL). Because no slots are available at the requested time, the carrier is offered an alternative window from the slot pool or placed on a waiting list.
If the airline accepts and then operates that slot fewer than 80% of the allocated days in a season, it forfeits the slot. The slot returns to the pool and becomes available for redistribution.
Why It Matters#
Slot coordination directly shapes which airlines can serve the world's busiest airports and at what times. For passengers, it affects schedule options, connection times, and fares. At constrained airports, slot scarcity limits competition and keeps prices higher than they might otherwise be.
For student pilots and aviation enthusiasts, understanding slots explains why an airline may operate a route at an inconvenient hour or fly a nearly empty aircraft. The flight may exist primarily to protect a valuable slot rather than to fill seats.
Key Takeaways#
- Airport slots grant permission to use a specific runway at a defined time.
- Slot-coordinated airports follow IATA's Worldwide Slot Guidelines (WSG).
- Airlines must use slots at least 80% of the time to retain them.
- Grandfather rights allow airlines to keep slots from previous equivalent seasons.
- Slots at congested airports are scarce and can hold significant commercial value.