Why is the plural of "aircraft" not "aircrafts"?

Question

I came along this sentence:

Today, we have used a large number of assets, comprising of 34 aircraft, 40 ships, hundreds of men, thousands of man-hours has been deployed

I consulted dictionaries and forum threads that explained that aircraft, just like sheep, builds its plural without an -s.

For sheep, however, that can be explained by its evolution from an old word. Not so for aircraft, which is a relatively new word with no such legacy. I found it stated that it is simply a concatenation of air and craft.

I found this post about the possible increasing usage of aircrafts (probably among non-native speakers), but it didn't explain the etymology and I found nothing else to explain it.

So, what is the explanation for the plural of aircraft not being aircrafts? Does it come from craft used as a mass noun? If so, what would be the meaning conveyed by that?

Answer 1

It is because craft is a collective term and OED mentions that it might be originated as an elliptical expression. Craft itself is used as aircraft as well. OED includes the following explanation for the fifth definition of craft:

V. Applied to boats, ships, and fishing requisites.

These uses were probably colloquial with watermen, fishers, and seamen some time before they appeared in print, so that the history is not evidenced; but the expression is probably elliptical, sense 9 being = vessels of small craft, i.e. small trading vessels, or of small seaman's art, and sense 10 = requisites of the fisherman's craft. It is not impossible that the latter was the earlier: cf. quot. 1704 at sense 10. The want in English of any general collective term for all sorts of ‘vessels for water carriage’ naturally made craft a useful stop-gap.

_{(emphasis mine)}

For reference^[OED]:

9.
a. collect. (const. as pl.) Vessels or boats.
(a) originally only in small craft n.
(b) Hence, without small, in same sense; later, in the general sense of vessels of all kinds for water carriage and transport.
b. (with a and pl.) A small vessel or boat; any sailing or floating vessel.
c. An aircraft or spacecraft.

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10. collect. Implements used in catching or killing fish; in mod. use chiefly in Whale-fishery: see quot. 1887.

Craft, is a Sea word signifying all manner of Lines, Nets, Hooks, &c. which serve for Fishing; and because those that use the Fishing Trade use Small Vessels..they call all such little Vessels Small Craft.

_{1704 J. Harris Lexicon Technicum}

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The harpoons, hand-lances, and boat-spades, are usually called ‘craft’, and the other implements ‘gear’.

_{1887 G. B. Goode Fisheries U.S.: Hist. & Methods II. 241}

However, OED mentions that the plural form aircrafts is rare and there is one citation that aircrafts is used:

His world-famed aircrafts.

_{1903 Aeronaut. Jrnl. 7 81/1}

OED also adds that the word can be understood, esp. when used in the plural, to include other kinds of heavier-than-air machine, such as helicopters. The word is often preferred to aeroplane or airplane in official and military contexts:

Will you please make the following terminology effective in all British official correspondence: For ‘aeroplane’ the word ‘aircraft’ should be used.

_{1943 W. S. Churchill Telegram 15 June in Second World War (1952) V. 566}

Answer 2

The word craft is obviously related to German Kraft (plural Kräfte), meaning might, power (or in physics force). It still had this meaning in Middle English. Etymonline explains the connection to boats:

Use for "small boat" is first recorded 1670s, probably from a phrase similar to vessels of small craft and referring either to the trade they did or the seamanship they required, or perhaps it preserves the word in its original sense of "power."

As an ellipsis for "vessel[s] of small craft", a (singular and) plural word "small craft" makes perfect sense. Once the ellipsis has become a fixed expression whose origin people don't remember, small will easily be re-interpreted as referring to physical size (which, after all, is strongly associated with a boat's power), so that the rarely used "[vessels of] great craft" will look irregular and "large craft" will be said instead. At this point it was only natural to drop the adjectives and use craft as a collective term for boats which, however, continued and continues to be used also for singulars. Then the apparent singular form of the word facilitates regularisation to crafts when used as a plural. Language change takes centuries, so the last step is where we are now.

I have extrapolated all of this from the one sentence quoted above without checking any further sources, so the actual historical details may have been slightly different.

PS: I am not claiming that when people started using "small craft" for boats the word craft was still used in the original sense of power. It may not be known what the precise original meaning of "small craft" was, and I certainly don't know. It seems possible to me that the power meaning of craft survived for longer in nautical terminology, both because such terminology is often a bit detached from evolutions in the remainder of the language, and because the seafaring part of the population presumably had more contact with German-speakers. But it's also possible that "small craft" originally referred to boats of little workmanship and that the sense shifted later.