Interlining Basics
What an interline agreement actually changes in the booking engine, how the distance prorate splits the fare, and why it is usually a small network's first signature.
An interline agreement is the simplest partnership in PlaneSim: two airlines agree to let passengers connect between their networks on a single itinerary. No shared branding, no pooled money, no fees. For a small carrier it is also the cheapest reach it will ever acquire, because it costs nothing at all.
What actually changes
Without an agreement, the booking engine builds connections only inside one airline's network. Passengers pick from itineraries a single carrier can sell alone. Once an interline goes active, itineraries that mix your flights with your partner's start appearing in the choice set, including two- and three-leg journeys neither of you could sell by yourself.
Nothing else moves. There is no through-branding: the passenger sees each leg under its operating carrier's name, and each leg sells at that carrier's own fare. You keep your pricing and your partner keeps theirs. If you want your brand across the whole journey, that is a codeshare, a separate agreement built on top of this one.
The money
Interline revenue splits by distance. Each leg's carrier receives the share of the total itinerary fare that matches its leg's share of the total distance flown. That is the entire settlement. There are no negotiated rates and no signing fees, and no money changes hands to set an agreement up.
The absence of commercial terms is deliberate. An interline is plumbing rather than a deal, and the game treats it that way: the fight over who keeps how much begins one rung up the ladder, with codeshares and their prorate slider.
Scoped or network-wide
An agreement does not have to cover everything. When you propose one, two scopes are available:
- Via airports: connections count only if the passenger transfers at one of the named airports, typically your hubs.
- Countries: only itineraries starting and ending in the named countries qualify.
Leave both empty and the agreement covers both networks entirely. A scoped agreement lets you route the partnership through the stations where you actually want transfer traffic.
Why a small network signs first
A startup's binding constraint is reach. Your metal touches a handful of airports, and every market beyond them is invisible to the booking engine. An interline with a larger carrier changes what you can sell without changing what you fly: your arrivals at the partner's hub become the first leg of journeys deep into a network you do not operate, and their passengers can end trips on your leg of the map.
The risk profile favors the small side. Nothing is prepaid, the prorate is mechanical, and either side can terminate an active agreement at any time, with connections dropping out from the next daily booking pass. Only one live agreement can exist per pair of airlines, so the decision is which partners, not how many contracts per partner.
AI-controlled airlines propose and accept interlines too, depending on their business model, so a small network can find willing counterparties even in a quiet world.
What can block a proposal
Alliances complicate the picture in both directions. If one of the two airlines belongs to an alliance whose charter forbids external interlining, proposals outside the alliance are rejected. Membership can also create interline agreements automatically; those appear on the Interlining page tagged with the alliance that produced them, and they end if the member leaves.
Where it sits in the stack
Setup lives on the Partners page, under Interlining. Choose a partner from the airline list (the Directory has a shortcut that pre-fills the form), set an optional scope, and send the proposal. Your counterpart sees an Offer received row and can accept or deny; you see Awaiting partner until they do.
Interlining is the foundation the other partnerships build on. Codeshares require the interline plumbing and will create it for you when accepted, and chartered alliances are, mechanically, a bundle of member interlines plus politics. The full reference lives at wiki.theplanesim.com; the worlds themselves are at play.theplanesim.com.