Slots are a ledger

PlaneSim models airport capacity twice: an hourly movement count every world enforces, and on slot-ledger worlds a property register of quarter-hour holdings with named owners, seasonal rollovers, and a secondary market.

Most airline sims model an airport as a single number: this many movements per hour, first come, first served, forever. PlaneSim starts there. On some worlds it keeps going, and capacity becomes a register of holdings with named owners and a season clock.

The count every world enforces

Every airport declares a movement capacity per hour. When you save a schedule, your departures and arrivals are counted together with every other airline's, hour by hour, and an over-full bucket fails the slots gate. The airport page shows the whole picture as a heatmap: movements per hour across the week, green when quiet, amber from 60% of capacity, red at or over the line. Reading it first tells you where the free hours are before the schedule wizard tells you no.

This system answers one question: does this hour have room? It says nothing about who is entitled to the room.

The ledger some worlds add

A world can enable a full slot ownership ledger at creation, so check yours. On those worlds capacity is divided into 15-minute buckets, and each bucket's arrival and departure movements become property. Three consequences follow from that word.

Holdings have named owners. The airport page carries an ownership panel showing who holds each bucket, so you can see exactly which rival is sitting on the times you want, and who might sell. Congestion stops being weather and becomes a cap table.

Acquisition is automatic and first-come. When a schedule commits, the holdings it needs are acquired from the pool on the spot. There is no separate purchase step for pool slots. Scheduling is claiming.

Buckets are directional. Arrivals and departures are capped separately, each at a fraction of the bucket total, and the caps deliberately overlap so the mix can flex. A bucket is not one number; it is two overlapping ones.

Use it or lose it

Holdings survive schedule edits. They do not survive a season of not flying. At each rollover, every 180 days by default (your world may differ), holdings shrink to what you actually scheduled. The Slots page flags unflown holdings as at risk and shows the next rollover date, so hoarding carries a visible clock. You can own a quarter hour of a hub's morning, but only by flying it.

The coordinator's letter

When a bucket you need is dry, the failure does not read like an error code. It reads like a coordinator's letter: how many movements you need, how many you hold, how many remain in the pool, and the nearest bucket with room within two hours. It is a counter-offer, not a wall. Often the fix is moving a departure fifteen minutes.

The secondary market

Unused holdings can be sold airline to airline. From your Slots portfolio you list a slot with an ask price; other airlines' listings appear on the same page and can be bought outright. This is where the ledger earns its name. A congested hub's peak-hour buckets are scarce, permanent assets, and early movers at a growing airport end up owning the morning. Arrive late and the ownership panel at least tells you whose door to knock on.

Curfews, the other constraint

Some airports close at night, on every world. A curfew is a local-time window, possibly wrapping past midnight, during which neither departures nor arrivals are allowed. The gate checks your departure at the origin and your arrival at the destination, which is the detail that catches long-haul planners: a sector can be legal at one end and land inside a closed window at the other. Airports without a curfew show as 24h on the Stations page, and the map's Night layer draws an amber ring around each airport currently inside its curfew, which makes sanity-checking arrival times cheap.

Why a ledger

An hourly cap constrains a schedule. A ledger constrains a market. Named holders, quarter-hour buckets, a season measured in days, and a price on the unused remainder: that is enough structure for peak slots at a good airport to behave like what they are in the real industry, property worth planning around. The full mechanics, including the directional caps and the rollover rules, are in the Slots and curfews article on the wiki.