On Towards the Stars: Planes and Anti-Air
[This post is part of a larger strategy guide for Civilization 6]
A lot of people really sleep on planes — I think because the AI never builds them — which is unfortunate because they're actually really powerful. They can never be attacked by existing land units, they have a massive range, and they do a fair bit of damage. Tldr: build planes, they're good.
Planes are interesting because you can't upgrade into them. The traditional strategy for maintaining a large standing army is to build a bunch of cheap ancient era units — warriors, slingers, horsemen — and upgrade them through the tech tree. You can't do that at all with planes. In other words, a defender with a few planes going against a warmonger without planes has a fighting chance (or, put another way, any warmonger also needs to have some production to build new units)1.
In the late game, fighters, bombers, anti-aircraft, and nukes have a bit of a rock paper scissors thing going on.
Bombers will instantly pillage districts and can carry nukes (though they will not be able to launch the nuke if a fighter or anti air gun brings its health down to 50 or less);
Fighters can do a lot of damage to bombers and have a chance of taking down nukes coming from bombers if the fighter is deployed in a one tile adjacent area;
Anti-air defends all tiles in a one tile radius from any kind of plane (and mobile SAMs also can protect against nukes) but that's all they do. Besides the actual anti-air class of units, you can get anti-air defense from battleships (and up) and GDRs (see below). The actual way anti-air gets calculated is pretty complex and almost never really matters; still, if you're interested, check out the civ wiki.
Generally, you want to treat your bombers as siege, fighters as ranged, and anti-air as counter spies protecting key assets. I tend to use bombers to go after aerodromes, factories, and commercial hubs to try and cripple production during wartime; and fighters to help on the ground or defend against enemy bombers.
It's worth mentioning that by the time you're slinging planes, there's a nonzero chance that there's a GDR or two in the game. GDRs also stop nukes, and are much more susceptible to fighter class planes than bombers. But they also carry anti air guns, and do a fair bit of damage themselves. If you're going against a GDR, make sure you have a lot of jet planes lying around.
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Note that, like all units, planes are less powerful the faster the game is. Playing on online speed will mean your planes are less valuable. It also means a war-monger might be able to win before planes really slow them down.