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I saw a question here that inspired one of my own. If an larger, person carrying aircraft, say some sort of hovercraft, could not fly above 300 ft would it be allowed to fly? Here in Australia, 300ft is the minimum height for an aircraft to fly over a town, so if an aircraft simply could not go any higher would it be allowed to fly at all? Or only in certain circumstances?

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    $\begingroup$ There are two different questions here, "is it an aircraft" (in your title) and "is it allowed to fly". It would most certainly be considered an aircraft regardless of altitude (debatable I guess, but if it supports its own weight in the air, I would consider it an aircraft). Is it allowed to fly is different and more complicated, depends on where you want to fly I guess, as long as you can meet minimum altitude requirements, it should be a "yes", but those requirements change depending on location... $\endgroup$ – Ron Beyer Jun 7 '17 at 3:31
  • $\begingroup$ Plz check this answer for the FAA definition of an aircraft: it does not state a maximum altitude. So according to the FAA it would be an aircraft, most probably according to CASA as well. $\endgroup$ – Koyovis Jun 7 '17 at 4:25
  • $\begingroup$ @RonBeyer, If you chuck this in as an answer, I'll accept it! $\endgroup$ – Woodman Jun 7 '17 at 4:29
  • $\begingroup$ What sort of aircraft wouldn't be able to fly higher than 300 feet? I would think that any aircraft which can't go above 300 feet would also have a lot of trouble at 100 feet, since the difference in air pressure between 100 feet and 300 feet is pretty small. $\endgroup$ – Tanner Swett Jun 7 '17 at 11:53
  • $\begingroup$ related: en.wikipedia.org/wiki/Ground_effect_vehicle $\endgroup$ – Thomas Jun 7 '17 at 19:46
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There is actually a legal Australian definition for aircraft, in Part 1 section 3 of the Civil Aviation Act:

aircraft means any machine or craft that can derive support in the atmosphere from the reactions of the air, other than the reactions of the air against the earth’s surface.

So those drones you can buy from the store are aircraft (and if you knock one out of the sky because it annoys you, you are technically bringing down an aircraft and could be punished). However the latter half of that definition suggests hovercrafts are not aircraft.

Also, the minimum height over populated areas in Australia is 1000ft, and 500ft over non-populated areas (CAR157)

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    $\begingroup$ That is a funny definition because, strictly interpreted, it excludes all heavier than air aircraft. A hovercraft flies the same way as a helicopter or an aircraft, only that the pressure field on the ground is more limited and more visible. $\endgroup$ – Peter Kämpf Jun 7 '17 at 20:05
  • $\begingroup$ @PeterKämpf That seems not correct. A hovercraft relies on the increased air pressure between ground, hull, and skirt. Take the ground (or the skirt) away, and the hovercraft will not hover any more. $\endgroup$ – bogl Apr 30 at 7:15
  • $\begingroup$ @bogl: Every aircraft needs to accelerate air downwards in order to create lift. How is this acceleration stopped? By extra pressure on the ground. Only that you hardly notice that pressure increase because it is spread over such a wide area for a high flying aircraft. Now move it down to hovercraft altitude and the pressure increase is easily noticed - just as it is for a hovercraft. In principle, there is no defining difference. Take the ground away, and an aircraft will also not fly eventually. $\endgroup$ – Peter Kämpf Apr 30 at 19:14
  • $\begingroup$ @PeterKämpf Yeah, and without ground there wouldn't be plane factories and pilots anyway. But no - the notion of removing the ground is a theoretical one. Atmospheric pressure on a planet depends on the existence of a ground, yes, but aircraft lift does not immediately depend on a ground. Hovercraft and GEV directly rely on the proximity of ground. $\endgroup$ – bogl May 1 at 7:19

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