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In my medieval fantasy world, airships are efficient and entrenched in the economy of trade and commerce. However, coordinating air travel on certain routes and prevailing winds seems difficult, especially considering that this world lacks much of what modern air traffic control uses to coordinate travel in airspace (ie. radio communication, high powered light, etc).

How would a medieval society coordinate/communicate with these airships? Would this process be possible on a wide scale? Is there real-life precedent for this?

For a basis on the general technological level, refer to late Imperial China (Ming/Qing Dynasty). This world also has a strong light-weight substance which the airships are made out of and an abundance of non-flammable gas that is lighter than air.

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    $\begingroup$ What era of Imperial China are we talking about? The term covers more than a thousand years. $\endgroup$
    – Karst
    Jan 16 at 20:04
  • $\begingroup$ @Karst I have edited the post for clarity. $\endgroup$ Jan 16 at 21:42
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    $\begingroup$ Semaphore, or something like this. And I'd drop the magical substances. Hydrogen is fairly usable for airships. There are plenty of mundane substances that work well for reasonable amounts of time to hold hydrogen. $\endgroup$
    – NomadMaker
    Jan 16 at 22:38
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    $\begingroup$ These are not airships, they are balloons the way you describe them. What kind of engine do they have? If they don't have an engine they will just drift with the wind. $\endgroup$ Jan 17 at 16:29
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    $\begingroup$ If they travel on the winds, they are all moving along at the same speed and in the same direction (unlike sailing ships). This means that the risk of collision is very low during flight. They should be spaced out at take-off sufficiently that, when they land they have time to avoid each other. $\endgroup$ Jan 17 at 18:12
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During Middle Age ships cruised the seas with no communication whatsoever with both land and other ships. If there was any communication it was only when the ship was in sight or ear distance.

Despite this they were able to trade and travel.

Same can go for your airships: travel by sight and have always some eyes on the watch. Forget about systematic communications.

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    $\begingroup$ I agree. Airships are much more like watercraft than aircraft, they generally have enough lifting capability to accommodate a crew which can stand nautical-style watches and the pilot rarely needs to wrestle with the controls with fractional-second response. Navigation could be by coloured beacons at waypoints, with fallback to fragrance in fog (a little vanilla can be smelled for miles downwind). $\endgroup$ Jan 17 at 13:05
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For congested airspace: Flags, Candles, coloured glass, and a codebook.

So Semaphore Code is probably the best option (using flags to communicate), but also modern air traffic control has a backup system using coloured lights for when the radio fails (Eg flashing green means either "Cleared for Taxi" or "turn around and come back to airport" - depending on whether you're airborne or not). Both coloured flags and coloured lights give a way of transmitting a few pre-arranged simple messages long distance day or night.

To achieve bright coloured light at night in medieval time, you need a flame and coloured glass. There's a trick to making candle light much brighter that was used in early lighthouses - put a glass cylinder above the flame, which creates and then amplifies an updraft, which makes the candle burn much brighter. By having a few different coloured glass cylinders, you can transmit coloured light at night.

Outside of controlled airspace

Your going to have to use "visual flight rules" - look for others, and stay out of clouds so others can see you. If you see someone else, pass on the "left/right".

If there are lots of airships, you would specify a convention for routes - either one way routes, requesting routes in advance from a central authority, or, if you have accurate barometers, altitudes are assigned for different directions.

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    $\begingroup$ And even in real life, when radio already existed, but was too bulky to equip smaller planes with it, there were interesting solutions, like building giant arrows on the ground out of rocks and concrete. Another real life example: older people at my glider club said that long ago gliders weren't equipped with radio, and ground control used two long white strips of fabric, which they laid down on the grass in various configurations, to give orders to gliders in the air. $\endgroup$
    – vsz
    Jan 17 at 12:56
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    $\begingroup$ Visually transmitted Morse code comes to mind as an option here. It’s actually been used historically in aircraft with damaged radios, and could be easily achieved in a medieval setting by combining the aforementioned updraft technique with a simple shutter system, or during the day by simply using a system of mirrors. $\endgroup$ Jan 17 at 14:28
  • $\begingroup$ Even in 2019, when I was in a soaring club, not every glider was equipped with a radio. Pilots simply had to look out the window and avoid each other. If you wanted to communicate with a glider in flight and they didn't have a radio... too bad. $\endgroup$ Jan 18 at 4:01
  • $\begingroup$ Just to add to the idea of one-way routes. You can also have controlled routes that use tokens to ensure that the way is clear (see en.wikipedia.org/wiki/Token_(railway_signalling)) for more info $\endgroup$
    – Dancrumb
    Jan 18 at 18:49
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Flying can be tricky on occasion. Flying something as ungainly as an airship even more so. Even just navigating becomes a new skill from up there. So you need someone who knows the lay of the land so to speak.

Harbor Pilots

They've been a thing since ancient Greece and is the most reliable way to navigate without all the fun bits and bobs we have today. You'd have to start with establishing a holding point or points outside of the controlled airspace. The airship comes to a stop or enters a holding pattern at the station and a harbor pilot is sent up via a small airship to board. the harbor pilot, fully updated on current traffic, winds, and where to dock, then proceeds to take the airship in.

A significant portion of the crew should also be on lookout for rogue or out of control airships, because you can never be too careful, and idiots exist.

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  • $\begingroup$ How do you send a pilot up to the airship? Rope ladder over how much distance? Something else? $\endgroup$
    – SRM
    Jan 17 at 3:39
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    $\begingroup$ @SRM Teak suggests sending the pilot up in an airship, and I think that largely reflects real-world practice (i.e. the harbor pilots stay at the harbor and board each vessel briefly to guide it in), but it could make for some very interesting storytelling if the common practice were for the harbor pilot to join the crew at the point of departure and come for the whole voyage. Imagine if Obi-Wan & Luke needed hire Han not for his ship but because only he knew the proper landing procedure at their destination. Mysterious supercargo a la Edgar Wallace. $\endgroup$
    – Tom
    Jan 17 at 6:00
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    $\begingroup$ @Tom That’s actually the type of thing I would envision happening here, though I imagine it would end up generalized to a greater degree. You’d end up with dedicated pilots for regularly used routes who know the route as well as docking/landing procedures at each end (and each layover, and possibly even each designated backup airfield for the route), and then a much smaller group who specialize in flying ‘new’ or ‘charter’ routes. $\endgroup$ Jan 17 at 14:32
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In addition to optical signals mentioned in other answers, for near and mid-range communication you might also use horns to broadcast acoustic signals. Note that this is used even today by ships navigating in certain weather conditions (real life precedent: foghorns!) where optical communication is impossible. Messages sent can be quite basic ("Watch out, I'm here!") but could be complex in theory, e.g. a few letters of Morse code makes for a rich message catalog (real life precedent: Q-Code).

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  • $\begingroup$ Bugles and bagpipes are other options. This was how armies used to be coordinated after all. $\endgroup$ Jan 17 at 15:47
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    $\begingroup$ I like the idea of drums mounted on the airships. Maybe automatic drums beaten by a winddriven engine. Or probably just some shirtless muscly oiled guy in a loincloth beating on the drums. You could hear a ship coming. Ships could communicate via drums. If you got so high up that it was very cold the guy could wear a really warm loincloth. $\endgroup$
    – Willk
    Jan 17 at 18:04

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