What are airport slots and why do ghost flights happen?
A slot is permission to use an airport's runway and terminal infrastructure at a specific time. At congested airports, slots are the scarcest and most valuable resource an airline holds.
Why they exist. A busy runway can handle a limited number of movements per hour. Demand at major hubs exceeds that capacity substantially, so access must be allocated rather than left to whoever turns up.
How they are allocated. At Level 3 coordinated airports — Heathrow, Gatwick, most major European hubs — an independent coordinator allocates slots twice yearly under the IATA Worldwide Slot Guidelines, with historic holdings taking priority.
Their value. Slots at capacity-constrained airports are traded between airlines for very large sums — individual Heathrow slot pairs have reportedly changed hands for tens of millions. They are effectively the airline's most valuable asset at that airport, and they are not owned outright but held subject to use.
The use-it-or-lose-it rule. This is the crux. Under the historic precedence system, an airline must operate a slot for a defined proportion of the season — traditionally 80% — to retain the right to it the following year. Fall below the threshold and the slot returns to the pool for reallocation.
Which produces ghost flights. If an airline holds a valuable slot but demand collapses, operating a near-empty aircraft can be commercially rational — the flight loses money, and losing the slot permanently would cost far more.
This became a public issue during the pandemic, when collapsed demand met an unchanged rule and airlines flew empty aircraft to protect holdings. Regulators granted waivers suspending the requirement, and the threshold has since been adjusted, but the underlying incentive structure remains.
The criticisms: the system entrenches incumbents, since new entrants cannot obtain slots at desirable times; and it creates an environmentally indefensible incentive.
Proposed reforms include auctioning slots and stricter environmental conditions, both resisted by incumbents.