An aircraft is 'imported' when it arrives, not after it stays for a period of time - The above quote is factually incorrect.
With all due respect, but the above is utter nonsense. An aircraft is not "imported" every time it lands at an airport in a country different than that of its registration. There are ICAO treaties preventing duties and taxes being levied on "transiting" aircraft and on their equipment and the fuel they are carrying.
Importing implies a permanent rather than a temporary situation, how you discriminate permanent (hence "imported") from temporary (just transiting) depends on local laws and might vary from country to country, so it needs to be checked on a case by case basis. I know for a fact that both Germany and Switzerland define "permanent" as "more than six months in a calendar year", much like the criterion that is used for revenue tax purposes. I also know for a fact that German customs has recently raided several German airfields in the Bodensee area, demanding copies of all landing and departure logs exactly for the purpose of determining which Swiss-registered aircraft hadn't left the country for over six months, making them de-facto imported (from a non EU country) and hence subject to German VAT.
Incidentally, if your aircraft is deemed to be "imported" by the authorities whether you paid VAT previously and in what country is immaterial, you will still have to pay VAT and possibly import duties (but only if not strictly within the EU area!) on the current estimated value of the aircraft (and that's a whole other can of worms in itself).
If strictly within the EU area it all becomes irrelevant as it's perfectly legal for a private person who is resident within the EU to own an aircraft registered in a second EU country and fly it in any third EU country without having to "import" anything. Which is what I do, actually (Italian resident, D- register, flown in Austria, Italy and France on a regular basis).
Any customs issues you might have flying in from a non-Schengen country such has the UK have strictly to do with the passengers and any goods they might be carrying, not with the aircraft itself.
Ciao,
Dg800
EDIT: another case were an aircraft is deemed to be "imported" is if it is transferred to the local register (coming from a non-EU country of registration). This makes f.e. used aircraft for sale in Switzerland extremely unattractive for EU buyers, as you can neither keep it on the Swiss register nor transfer it to the EU register of your choice without being automatically hit with a rather stiff VAT bill.
