What is the plural word for "aircraft"?

The plural of "aircraft" is "aircraft".
There are formatting rules for making singular nouns plural. Most nouns, you simply add an 's' but if the word ends in 's, sh, ch, x or z', you add 'es'. Other exceptions occur for words ending with 'o' and 'y'. Exceptions exist - sometimes you change other letters eg 'goose'/'geese', 'louse'/'lice'.
Other exceptions are words like 'sheep'. 'salmon' and 'aircraft'. In these cases, the plural is the same as the singular.
Unfortunately, there is no easy way of knowing which nouns follow which rules. You just have to know, learning by experience.

Interesting the plural of 'aeroplane' is 'aeroplanes'.

https://www.grammar-monster.com/plurals.htm

In addition to the answer above...

The spelling rule is: when the word has a vowel (a, e, i, o, u) before the letter 'y', you add the letter 's' and when the word has a consonant (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y, z) before the letter 'y', you remove the 'y' and replace it with 'ies'.

So, when the word has a vowel (a, e, i, o, u) before the letter 'y', you add the letter 's'.

Examples:

· Key = Keys
· Delay = Delays
· Holiday = Holidays
· Annoy = Annoys

And when the word has a consonant (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y, z) before the letter 'y', you remove the 'y' and replace it with 'ies'.

Examples:

· Accompany = Accompanies
· Emergency = Emergencies
· Embassy = Embassies
· Quality = Qualities

https://www.twinenglishcentres.com/blog/spelling-rules-changing-y-to-ies

Although the reference given above ...
https://www.twinenglishcentres.com/blog/spelling-rules-changing-y-to-ies
... does not make it explicit that it is only talking about words that are nouns, it does mention that the rules apply to items or things.
One of the examples above is "Accompany", and the plural given as "Accompanies".
My problem is that the word "accompany" is not a noun, it is only a verb, and verbs have entirely different rules for different case. Here the word "accompanies" is the third person singular of the verb "accompany"
I accompany you, he accompanies me.
If the example had been "fly", "flies" I wouldn't have worried, because although "fly" is a verb, it also happens to be a noun, and in that case subject to these rules. Or better, just "company".
I'm sorry to persist with this point, but I do think it a good thing to be accurate in our answers where possible.