Planes

"Planes can't be trusted," Rill added. "Always been like that."

Whereas tanks rule the land, planes rule the skies. Or at least, they used to. They were driven close to extinction by the tanks, who realized how easily they could get rid of their counterpart soon after the humans were gone.
Planes have been deliberately and systematically demonized in cultural depictions for decades. They are said to be guileful, malicious, or at least cowardly. Stories and legends usually portray them as villains and criminals.
Similarly to ships, they are most commonly owned by clans that use them to transport cargo or as special forces, with only very few left outside the clan territories. The ones who don't enjoy this sort of protection live in constant fear.
On the ground, they are sluggish and vulnerable. They slow down travelling teams and are easy targets for attackers. Therefore, their usefulness is very limited for free teams, making them a burden.
In flight, they can make short work of even the strongest tanks. The terror they spread led to bitter hatred for their "ilk".

Only A Dead Plane ...

Not only are they unwelcome, they are outright hunted down in many cases. Most tanks have no qualms about murdering planes. Even though they have few means to harm the planes when they're in the air, they take advantage of the fact that every plane has to land eventually.
There are several guides for plane hunters that describe the most effective ways of killing them. A very well-known one details the popular ambush method in an easily understandable way:

To hunt a wing-scum
Killing a plane is an honourable endeavour and rewarding always. It takes a certain amount of work and preparation, but it is worth the effort!
Bring your friends if you can, especially if you are doing it for the first time, but doing it alone is also possible.
(This method describes how to hunt a solitary plane, however in some rare cases, they may live in pairs or small groups. It works the same, just be careful - they usually don't all land at the same time and may catch you off-guard.)
Start by selecting a target that stays within a certain area. It is a pointless waste of fuel to chase a nomadic plane; it will outrun you always. A day or two of observation may be necessary to determine whether it is a suitable target.
There are only certain places where a plane can land. It needs straight, relatively level stretches. Since they like to hide, you can narrow the possibilities down to places that offer cover nearby. Find these places and conceal yourself. Planes have very good sight, so you need to disguise yourself very carefully.
Planes cannot stay in the air for very long. It depends on the model, but you can expect them to land every few hours. Once it is on the ground, the rest is easy.
Shooting the hull with your main armament will kill it reliably. If available, make sure you have HE ammo loaded. It may be tempting to shoot the wings, but you don't want to give it a chance to shoot back.
As rewarding as doing a good deed is on its own, there is more to gain. Always look for a dead plane's lair. They need a lot of fuel, so you will almost always find vast amounts in close vicinity of their landing places.
In closing: Remember to never hesitate to kill a plane. A plane would never hesitate to kill you.

The original author is unknown. There are several versions, but they usually only differ in minor points. It has been translated into a number of languages and can be acquired as prints, heard on radio channels, and even seen written on walls.
Of course, not all tanks outside of the clan territories endorse the killing of planes. Especially in more recent times, there are many who are indifferent or even condemn it. The latter find themselves justifying their views often - there is usually no drastic repercussions, but it's a very easy way to make oneself unpopular with one's peers.

Daily Life

Their persecution makes them pariahs. Due to their high fuel consumption, they spend most of the day looting and raiding their surrounding area and hide the rest of the time. Once they have found an area that is rich with victims, they settle down and hunt there for as long as possible before moving on. Planes that are constantly on the move are comparably rare - they are only driven by fear, to the point of taking the risk of venturing into unknown areas.

The Planes As A Mystery

It is probably not wrong to assume that at least some of the hatred for planes of the average tank is not as much fear anymore, and rather a crucial lack of understanding how planes actually are and how they work. Even a tank who holds no ill will against planes per se may be easily swayed by those who loathe them, simply becacuse they don't know any better.

Even the reason why there are so few planes is not common knowledge everywhere. Many tanks have not seen a single plane in their whole life, only dead ones, or maybe one or two from a distance. In any case they may have heard about them, but only negative or untrue things. Common misconceptions for example are that all planes can lift off vertically, jump, or simply hover in place.

Production

There are very few factories that produce planes, as only a handful of Matchmakers are willing to protect them. Those Matchmakers are often regarded as rogues of some sort by the populace, but their own kind's stance on them is unknown.

Beliefs
A little known fact is that many planes have a different belief system from tanks. Unlike the ground vehicles, planes believe in a sort of cycle of life and death, where they are reincarnated after their death. Depending on how many good deeds a plane did in their life, their next form could be a bigger, more powerful plane, or, if they were cruel and unjust, even a tank.

Sometimes, this belief is inverted and doing good deeds is believed to make you a tank in your next life.

These beliefs may be a reaction to their poor living conditions. They are often accompanied by the belief in a sort of plane god. Others worship the - according to tanks, cursed - plane gods Hate and Scorn. The latter belief is usually considered to be reserved for planes that are hateful towards tanks.
Keeping Them In Line
A cruel but common punishment for misbehaving planes is to cut off their wings completely or partly. Even though they can be easily re-attached, grounding a plane even for short amounts of time puts them in severe distress.
Much less common, but all the more cruel is the practice of swapping bodies - tank minds and plane minds work in fundamentally different ways. Putting a plane's brain into a tank's body (or vice versa) is, in essence, comparable to removing a human's limbs. However, there is no recorded evidence of this punishment being actually used, so it may just be a horror story to scare uncooperative planes into compliance.

Where are the modern planes?
It is mostly unknown why advanced planes like jet fighters are seemingly completely gone nowadays. However it's safe to assume that, considering how powerful they were, their fate was a genocide.

"That's not the mystery we're trying to solve. The real mystery is what caused them to disappear."
"They fell into the ocean, dumbass."
"That's not the mystery we want to solve. What caused them to disappear."
"They fell into the ocean and we can't find them."
"How does it fall from the sky? Planes don't just fall into the ocean."
"It stopped working, idiot."
"How did they fall into the ocean?"
"The plane fell from the sky, it's not that hard to understand."

— Now stop asking.

See also