The Permission to Use a Runway at Eight in the Morning
At congested airports the scarce resource is permission to use a runway in a given time window. Those rights were mostly given away, are traded for enormous sums, and function as a barrier to entry.
A Runway Is a Capacity Constraint
An airport can handle a limited number of movements per hour, determined by runway configuration, air traffic control capability, terminal capacity, and noise restrictions.
At most airports demand fits within that limit. At a small number of major hubs it does not, and access must be rationed.
A slot is permission to use the airport infrastructure for an arrival or departure within a defined time window on a defined day. Without one, an airline cannot operate the flight regardless of aircraft or crew availability.
How They Are Allocated
Allocation at most congested airports outside the United States follows international guidelines administered by a coordinator, and the mechanism heavily favours existing operators.
The central principle is grandfather rights: an airline that operated a slot in the equivalent season the previous year is entitled to it again. Remaining slots go to a pool, with a share reserved for new entrants, allocated administratively.
The entitlement is conditional on the use it or lose it rule, requiring the slot to be operated at least eighty percent of the time to retain it.
| Rule | Effect |
|---|---|
| Grandfather rights | Incumbents retain slots indefinitely |
| New entrant pool | Limited access for others |
| Eighty percent usage requirement | Prevents pure hoarding |
A slot is allocated free, held indefinitely provided it is used, and at some airports sold for tens of millions of dollars. The allocation and the value have never been reconciled, which is the entire policy argument.
The Ghost Flight Problem
The usage requirement produces a recognisable failure. An airline holding a valuable slot on a route that has become uneconomic faces a choice: operate the flight at a loss or surrender an asset worth a great deal.
The rational answer is to operate. The result is flights with very few passengers, sometimes none, flown to satisfy the retention threshold.
The practice attracted attention during the pandemic, when demand collapsed and airlines faced losing slots accumulated over decades. Regulators suspended the usage requirement temporarily and then reintroduced it at reduced thresholds, which was an explicit acknowledgement that the rule can compel economically and environmentally pointless flying.
The Trading Market
Whether slots can be sold differs by jurisdiction and the practice developed ahead of the rules.
At London Heathrow, secondary trading became established through arrangements initially structured as exchanges with cash consideration, and courts confirmed their legality. Individual pairs of slots at attractive times have sold for tens of millions of dollars.
Those transactions are the clearest available evidence of what the administrative allocation is worth. An asset given away for free that sells for that much represents a substantial transfer from the public to whichever airlines happened to hold the rights when the system was formalised.
Proposals to auction slots, or to auction a rolling share of them as leases, have been made repeatedly and consistently opposed by incumbent carriers, whose balance sheets carry the slots as assets and whose networks depend on them.
The American Difference
The United States restricts slots at a small number of airports rather than applying the international system broadly, and its rules have evolved through a mix of regulation and antitrust remedies.
Slot divestitures are a standard condition of merger approval, requiring the combined carrier to surrender slots at congested airports to competitors, which is the mechanism through which new entrants have most often obtained access.
That approach uses merger review to accomplish what allocation policy does not, which works occasionally and depends on there being a merger to condition.
Why It Matters Competitively
Slot scarcity is one of the most durable barriers to entry in aviation. A competitor with aircraft, capital, and a viable route cannot enter if it cannot land.
The barrier is strongest at exactly the airports where entry would be most valuable, since congestion correlates with demand. And because slots are held indefinitely, incumbency at a major hub compounds over decades.
Airlines value them accordingly, and slot portfolios are frequently a substantial component of what an acquirer is buying in an airline transaction, sometimes exceeding the value of the fleet.
The Bottom Line
Airport slots ration a genuinely scarce resource and were allocated to whoever was operating when the system was created, held indefinitely on condition of use, and traded privately for very large sums where trading is permitted. The usage requirement prevents hoarding and compels empty flights, which is a poor trade nobody designed on purpose. Every proposal to auction the rights runs into the fact that they are already on incumbent balance sheets, which is a strong constituency for leaving the allocation exactly where it is.