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18 U.S. Code § 31(a)(1) says,

The term “aircraft” means a civil, military, or public contrivance invented, used, or designed to navigate, fly, or travel in the air.

What exactly is an aircraft? Airplanes, helicopters, sailplanes, drones, etc are obviously aircraft, and I am quite certain that bullets and arrows are not aircraft (although I guess they do technically meet that definition?). What about something like a paper airplane, that does glide using aerodynamic effects but is not controlled or propelled after launch? Or what about an RC glider launched with a bungee (technically not very different from an arrow, except it's controllable)?

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  • $\begingroup$ And CFR 1.1 defines it as: "Aircraft means a device that is used or intended to be used for flight in the air." How many other definitions would you like to see? And for what purpose or context? $\endgroup$
    – Michael Hall
    May 14 at 16:09
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    $\begingroup$ @MichaelHall maybe legal precedent or FAA interpretations covering borderline cases? $\endgroup$
    – Someone
    May 14 at 16:14
  • $\begingroup$ The line is blurred, because every new invention may fall in one or the other way. Common sense will rule, until authorities will explicit tag stuff. So, why a paper plane is an "aircraft" (designed to fly), but not your lost umbrella (because of high winds)? (Exhausts are designed to travel in the air). Do no try to interpret laws like an engineer document (but also the contrary: there are a lot of interpretation and implicit conventions). $\endgroup$
    – Giacomo Catenazzi
    May 15 at 9:46
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    $\begingroup$ Oh yay, an argument about definitions. $\endgroup$
    – Organic Marble
    May 15 at 18:38
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    $\begingroup$ I'm reminded of a case where a UK motorist was charged with having lights on his car that weren't working and he claimed they weren't lights, just ornaments, because he only attached them for decoration and never wired them up. $\endgroup$
    – Michael Kay
    May 16 at 7:37

2 Answers 2

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Federal criminal code aside, the definition of an aircraft under Title 14 CFR §1.1 defines an aircraft as “a device that is used or intended to be used for flight in the air”. Flight, in this context, refers to sustained flight, either by means of air moving around lifting surfaces for heavier than air aircraft, or suspended from an envelope containing a gas, which displaces a volume of air weighing more than the gas contained within the envelope. Projectiles, on the other hand, do not make use of sustained flight. They are only capable of making limited air travel along a ballistic trajectory under the influence of gravity e.g. flight of an arrow or javelin.

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  • $\begingroup$ { I would have thought a cruise missile is not a projectile and is specifically not referred to as such? } $\endgroup$
    – Fattie
    May 15 at 18:12
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There are cases of overlap here. By definition, an aircraft is an object that uses air for support against gravity. This would include lifting mechanisms such as wings or buoyancy, and generally infers sustained flight.

A projectile is an object that has force applied to it to get airborne. A typical example is a thrown stone or a cannon ball.

Gliders and rockets are interesting as they can fit both definitions.

Gliders are launched with external force yet have wings and are capable of sustained flight. Rockets may or may not use wings. While under power they can be considered aircraft (or spacecraft), once fuel is expended they are projectiles.

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    $\begingroup$ And then you have things like submarine launched en.wikipedia.org/wiki/Tomahawk_(missile) It is initially launched by a bust of compressed gas (projectile), then it launches its rocket engine which powers it, as it transitions into starting the jet engine and unfolding its wings, (aircraft). $\endgroup$
    – user1937198
    May 14 at 23:27
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    $\begingroup$ "once fuel is expended they are projectiles" - the same can be said about most non-glider aircraft... $\endgroup$
    – Torben Gundtofte-Bruun
    May 15 at 14:08
  • $\begingroup$ Well there you go. And what about controllable projectiles? Seems there are 2 schools of thought: powered and/or controllable. And lift can be borderline too, as anything with an angle of attack will be lifting. $\endgroup$
    – Robert DiGiovanni
    May 15 at 19:02
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    $\begingroup$ So how would you classify a bird in this definition? (Or a bat, flying insect, etc.) It does not have any force applied to it to get airborne, a bird flies under its own power. It does operate on fuel (whatever food it eats). Obviously they are animals rather than manmade objects, but let's say someone made a mechanical bird that flies in the same way by flapping wings (I've seen toys that do this using stored power in a spring or rubber band for example.) The lack of control may be a defining factor in the case of toys... $\endgroup$
    – Darrel Hoffman
    May 15 at 20:20
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    $\begingroup$ Aircraft implies man made, controllable. @Criggie we can see how a glider can be considered both. $\endgroup$
    – Robert DiGiovanni
    May 16 at 4:18

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