In terms of international air law, the most correct term is "Sub-Icao", that is to say, aeroplanes which for some reason or another are ineligible for issue of an ICAO compliant Certificate of Airworthiness.
In practice that term isn't used in any nation's domestic regulations, and every country has its own system of labelling and permissions.
In the USA, pretty much everything sub-ICAO, from a kitplane to a new Boeing that hasn't for a CofA yet, is labelled "Experimental". Their approach is built somewhat upon the amount of empty space they have there, and basically lets you build and fly whatever you like, so long as you don't fly it over any habitation. Part of their rules require you to paint the word "experimental" in clear view on the aeroplane. It is a particular approach that hasn't found favour in many other countries. Some of them still use the term "experimental" but every country which uses the word, means something different by it.
In Britain, sub-ICAO aircraft fall into several categories.
(1) True flight test. These are operated under "B-conditions", possibly supported by an EASA "Permit to Fly", probably not. they will either have a standard G-ABCD registration, or have a B-conditions number, which will look something like G-12-34. You don't see those used much nowadays. Permissions to fly a B-conditions aeroplane in any country but Britain are almost impossible to obtain.
(2) Permit to Fly. This is the lower UK-only equivalent to a CofA. They all look about the same on the paperwork, but have numerous flavours, including...
- Microlight / type approved (e.g. a CTSW)
- Microlight / amateur built (e.g. an X'Air)
- Microlight / type accepted. (e.g. a Tiger Cub)
- Amateur built (most of the LAA fleet)
- Vintage or ex military (any civilian owned Spitfire, most Austers, Condors....)
- CAA issued (could be anything, but most likely some temporary issue on something stuggling with the paperwork to help a company out. I have seen paperwork showing a VC10 on a permit, but the biggest in my logbook was an Islander for a ferry before fitting out).
(3) Deregulated
- This is SSDR microlights, hang-gliders, powered paragliders... For all reasonable purposes, there is no paperwork.
I think that the original poster, in referring (wrongly but understandably) to "experimental", meant "amateur built" (either light or microlight), for which my original answer is still correct.
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Last edited by Genghis the Engineer; 26th Jan 2015 at 07:30.