If you’ve played any airline simulator more serious than a tycoon game, you’ve run into airport slots. You go to add a new flight to London or New York, and the game tells you a slot is required. The route exists. The aircraft is available. The demand is there. But the game won’t let you fly without permission to land at a specific time of day.

This guide explains what an airport slot actually is, where the rules come from, why most sims model them shallowly, and what a more honest version looks like.

It’s also the explainer that sits behind the slots system we built for SkylineSim. If you want the build-narrative version with screenshots of our market screen and auction results, that’s the post to read after this one.

What an airport slot actually is

A slot is permission to use a specific airport at a specific time. More precisely: permission to perform one arrival or one departure, in a 15-minute window, on specified days of the week, during a six-month season.

That’s it. A slot isn’t a route. It isn’t an aircraft type. It isn’t a passenger booking. It’s a single line in the airport’s coordination schedule that says: at 08:15 on Mondays through Fridays this summer, this airline is allowed to land here.

You can hold a slot you don’t use (badly, see “use it or lose it” below). You can sell it. You can lease it. You can swap it. In some jurisdictions you can spend tens of millions on a single slot at a congested airport. London Heathrow slots have publicly traded for over £75M each. They’re real, transferable assets on airline balance sheets.

Why slots exist

Every major airport has finite capacity. There are only so many runways, only so many gates, only so many tower controllers, only so many movements per hour the airfield can safely sustain. When more airlines want to land than the airport can handle, somebody has to decide who gets what.

The global aviation industry settled on a coordination system run mostly by independent airport coordinators (an “ACL” or “Airport Coordinator”), governed by IATA’s Worldwide Slot Guidelines (WSG). The WSG is updated annually and is the document the rest of this guide is, broadly, summarizing.

Coordinated airports fall into three levels:

  • Level 1, non-coordinated. Demand stays well below capacity. Airlines self-schedule. Most airports in the world are Level 1.
  • Level 2, schedules facilitated. Demand is approaching capacity at peak times. A facilitator works with airlines to avoid conflicts, but airlines still hold the schedules. Voluntary coordination.
  • Level 3, fully coordinated. Demand routinely exceeds capacity. Slots are mandatory and allocated by the coordinator. Every operation needs an approved slot, every season. This is what most people mean when they say “slot-controlled airport.”

Heathrow, JFK, LAX-international, Frankfurt, Haneda, Hong Kong, Singapore Changi, every major European hub of any size, all Level 3. If your airline wants to operate there, slots are not optional.

The 80/20 use-it-or-lose-it rule

This is the single most important slot rule, and the one airline sims most often skip.

A slot you hold isn’t yours forever. It’s yours for the current season. To keep it for next season (to make it historic, “grandfathered”), you have to actually use it.

The threshold is 80%. If you operate the slot on at least 80% of its scheduled days during the season, the coordinator confirms it as historic for next year. You carry it forward automatically. Below 80%, the slot goes back into the pool and gets reallocated to whoever wants it most at the next auction.

The 80/20 rule is the reason airlines flew ghost flights during COVID, half-empty or completely empty flights operated solely to maintain slot retention. When passenger demand collapsed in 2020, the alternative was abandoning slot positions at Heathrow and JFK that had taken decades to accumulate. Regulators temporarily relaxed the rule for that crisis; under normal conditions, it’s binding.

Two implications most sims miss:

  1. A slot has a usage rate over a season, not a binary “in use / not in use” state. You can fly a slot at 75% usage and lose it the moment the season ends. Most sims either ignore usage or treat it as on/off.
  2. Slots compete with the financial logic of operating an unprofitable route. A real airline running a marginal route to a small destination from Heathrow isn’t just deciding whether the route makes money, it’s deciding whether losing the slot costs more than running the route at a loss.

The seasonal cadence

Slots run on the global aviation calendar: summer (last Sunday in March through last Saturday in October) and winter (everything else). Each season is denoted by its year and a letter: 2026S, 2026W.

The schedule, simplified:

  • Roughly six months before the season starts: airlines submit slot requests for next season.
  • Several months before: the coordinator publishes the Initial Coordination Report with provisional allocations.
  • Slot conference (IATA SC): twice a year, all the airlines and coordinators meet in person to swap, return, and adjust. Slot trading happens here at scale.
  • Final allocation: issued, airlines build their published schedule against it.
  • During the season: airlines operate, usage gets logged, and 80/20 calculations accumulate.
  • End of season: retention determined. Slots either roll to historic or go back to the pool.

If you want to model a Level 3 airport without modeling this cadence, you’re modeling something other than slots. The fact that retention is decided once per season, six months at a time, is the engine that gives slot strategy its weight.

How slots get allocated to new entrants

When a slot returns to the pool, either because a holder fell below 80%, because an airline went bankrupt, or because the airport increased declared capacity, it doesn’t go to the highest bidder. The allocation rules are public, structured, and biased toward maintaining competition.

The WSG rule of thumb: at least 50% of newly-available slots are reserved for new entrants at that airport. The exact definition of “new entrant” varies by airport but typically means an airline holding fewer than five slots per day at that airport before the allocation.

Inside that constraint, allocation considers:

  • Wishlist ranking. Airlines submit a prioritized list. Higher-priority requests get evaluated first.
  • Historical usage at that airport. An airline that already operates reliably gets weighted up.
  • Type of service. New international routes, year-round versus seasonal, frequency, aircraft size.
  • Bid value in jurisdictions that allow monetary auctions (note: most coordinators don’t run cash auctions, slots are technically allocated administratively. Secondary-market sales are a separate transaction between the holding airline and the buyer).

Once allocated, the slot becomes yours. If you fly it ≥80% next season, you keep it. If not, you’ve just given a competitor a foothold.

The secondary market

A slot you already hold can be:

  • Sold, full transfer of ownership. The buyer pays the seller directly; the coordinator records the change. This is where the £75M Heathrow numbers come from.
  • Leased, you keep ownership; the lessee uses the slot for a defined term (months to a season) and pays rent. The slot reverts at term end.
  • Swapped, you trade one of your slots for one of theirs, with optional cash to balance.

Real-world slot trading is a serious business. Some airlines run dedicated slot strategy teams. Sales between airlines need regulatory clearance in many jurisdictions (the EC and DOT both review specific transfers). Anti-competitive trades get blocked.

When a major carrier collapses, their slot portfolio is the largest single asset in liquidation. Monarch, Flybe, Norwegian Long-Haul, every airline failure in the last decade triggered a slot scramble. The aircraft can be repossessed; the slots are often more valuable than the metal.

How sims model slots (or don’t)

Now the part that matters if you’re shopping for an airline simulator.

The cheap version, used by most tycoon-style games:

  • One slot per route, granted on creation, free forever.
  • No 80/20 retention.
  • No seasonal allocation.
  • No secondary market.

This isn’t really modeling slots. It’s putting the word “slot” on a route license.

The medium version, used by some serious sims:

  • Slots are bought once at scenario start, lost only on bankruptcy.
  • No periodic re-allocation, no use-or-lose.
  • Maybe a market, maybe not. If yes, it’s usually a flat fixed-price exchange.

This is better, at least the player has to think about the asset, but it leaves out the dynamic where a marginal route’s slot equity outweighs its operating loss.

The deep version, used by AirlineSim and what we built for SkylineSim:

  • Slots have explicit owners, airports, directions, 15-minute windows, day-of-week patterns, and seasons.
  • 80/20 retention runs every six months at the rollover.
  • New allocations happen in periodic auctions/wishlists with new-entrant carve-outs.
  • A secondary market with sale, lease, swap, and fair-value pricing.
  • Visible risk surfacing, your slot dashboard tells you which slots are tracking under 80% before you lose them.

If you’re picking an airline simulator and slots matter to you, the question to ask is: does retention happen each season, and can I see my usage tracking in real time? Anything below that and slots are decoration.

How SkylineSim models them

SkylineSim runs the deep version, calibrated to the WSG. Every slot in the simulation has the full structure described above. Retention runs at the April 1 and November 1 rollovers. New allocations clear through a wishlist auction with the new-entrant carve-out preserved. The market supports sale, lease, swap, and pricing reflects the real fair-value structure (peak departures at major hubs trade for tens of millions; off-peak weekends-only arrivals at tier-3 airports trade for a fraction).

We also model slot pressure shocks, if your share of declared departure capacity at a hub exceeds 85%, the airport’s effective capacity scales down for everyone, including you. The point isn’t punishment; it’s that dominant hub positions create their own constraints, and the game should reflect that.

For the detailed build-narrative version of how this came together, including the auction results screen, slot risk surfacing, and the parts we know are shallow in v1, read the slots and treaties devlog.

Glossary

  • Slot, permission to perform one arrival or departure in a 15-minute window at a coordinated airport.
  • Level 3 airport, a fully-coordinated airport where slots are mandatory.
  • Historic slot, a slot retained for the next season by virtue of meeting the 80/20 usage threshold.
  • 80/20 rule, the use-it-or-lose-it rule: operate the slot on ≥80% of scheduled days to retain it next season.
  • New entrant, an airline below a small slot threshold at a given airport, with a reserved share of newly-allocated slots.
  • WSG, IATA’s Worldwide Slot Guidelines, the global rulebook.
  • ACL / Airport Coordinator, the independent body that allocates slots at a Level 3 airport.
  • Slot rollover, the seasonal moment (April 1 for summer, November 1 for winter) when retention is calculated and new allocations clear.

If serious slot modeling is a thing you care about in an airline sim, SkylineSim is in development and the Discord is where the slot system gets stress-tested first. The single-player airline simulator landscape covers what else is out there.