Might it be possible to make player-buildable modular airports if enough rules and safeguards are in place?
Airport construction pieces
Runways come in short and long versions as complete modules. Short versions only support small aircraft, long versions support all aircraft. Diagonal runways may or may not be possible.
- Take-off only
- Landing only
- Combined take-off and landing
Helipads also sort-of fall under runways.
- Helipad without gate
- Helipad with gate
- Control tower
- Terminal buildings (decoration)
Other functional stuff
- Terminal gate (loading bay)
- Airports are constructed by first buying land for the purpose, using a special buy land mode. The airport land becomes fenced in.
- Airports can only be constructed/modified while they are closed and there are no aircraft on the ground. (There may be aircraft in hangars, in that case those hangars are locked.)
- Airports can only be opened if all requirements below are met.
- Town noise is generated based on number and length of runways. Too much noise prevents the airport from being opened.
- If you attempt to open an airport and it cannot be opened, all problems are highlighted and/or listed.
- An airport must have at least one runway for take-off and one for landing, or a combined runway, or a helipad.
- If an airport has long runways it must have a control tower and a radar.
- It must be possible to move from any landing runway to any gate, and from any gate to any take-off runway.
- It must be possible to move from/to any helipad without loading bay to any gate.
- It must be possible to move from a hangar to at least one gate.
- It must be possible to move to a hangar from at least one landing runway.
- Runways connect one-way to adjacent taxiways at their ends.
- Helipads (both types) and gates connect two-way to adjacent taxiways.
- Taxiways are always two-way, but their route may make them effectively one-way.
- Taxiways are constructed similar to rail, however diagonal taxiways have a minimum distance to avoid wings clipping.
- If there are no helipads, the ends of runways function as helicopter landing sites.
- For simplifying movement rules (probably) it's not possible to have helicopters land at any location on a taxiway similar to the original city airport (helicopters landing at the taxiway crossing.)
- An approaching aircraft circles the airport's orthogonal tile bounds until it is assigned a landing runway and a gate.
- An aircraft heading for take-off is assigned a take-off runway and reserves a path to that runway. No other aircraft can enter or cross that path, but the path can have sections shared with other aircraft also going to take-off, as long as they move in the same direction on the taxiway.
- A landing-only runway is free as long as it is not reserved and there is no aircraft running/standing on it.
- A combined runway is free for landing if the landing aircraft can move away from it to the first taxiway tile towards its gate immediately, even if another aircraft has the combined runway reserved for take-off, as long as the taking-off aircraft is sufficient distance from the runway.
- A movement path may not cross a gate the aircraft is not stopping at.
The hope is to design a set of rules that can never cause a deadlock situation.
In case a deadlock on ground does happen, the best solution might be to teleport all the aircraft to a hangar. Undecided if they should be stopped then, or resume movement immediately.