My first reaction to aircrafts was to think it was a typo, but I just checked usage on NGrams…
…and compared it to usage for the singular / collective noun form aircraft…
…which seems to indicate that the "regular" plural form is gradually being taken up. Does this represent a tendency for English speakers to enforce regularity on the language? Are there any other examples? I'm not expecting to find that "sheeps", for example, is displacing "sheep" for the plural meaning, but maybe there are other "less established" usages that could be changing.
EDIT Please note that I'm not asking whether "aircrafts" is correct, or common. I'm asking if there's any reason why it seems to be occurring more often over recent decades (even though it's still pretty rare). And whether this effect occurs with other nouns having "non-standard" plural forms.
Best Answer
Well, this is an example of why Google NGrams isn't a precise indicator. When we compare the two directly, aircrafts simply can't get off the ground:
Now, this result is also flawed since it is impossible to separate uses of aircraft (singular) and aircraft (plural). It is also impossible to factor out typos (aircrafts vs. aircraft's) and so on.
The point is, don't read too much into what an NGram shows (or at least take the graphs with a grain of salt), because a great deal of the time what you wind up with is this sort of thing:
(BTW, the huge spike in the use of aircraft in the early '40s is almost certainly due to the air war in Europe and the Pacific.)