General initial guidance
With regard to the operation of Unmanned Aircraft at the smaller end of the market, it is the aircraft's mass which is the deciding factor with regard to the permissions/approvals that are required, and 20 kg is the significant number.
The overriding/all encompassing Article within the legislation is Article 138, which covers the subject of endangerment and applies to all aviation activity at all times: “A person must not recklessly or negligently cause or permit an aircraft to endanger any person or property.”
‘Small unmanned aircraft’ means any unmanned aircraft, other than a balloon or a kite, having a mass of not more than 20 kg without its fuel but including any articles or equipment installed in or attached to the aircraft at the commencement of its flight;
‘Congested Area’ means, in relation to a city, town or settlement, any area which is substantially used for residential, commercial, industrial or recreational purposes.
In addition, some additional limitations have been published within CAP 722, which is the CAA document that deals with the operation of unmanned aircraft within UK airspace.
Take note also that for electrically propelled aircraft, the battery itself is considered to be a part of the aircraft - it is the battery's charge that is the fuel. The logic for this is that the battery is basically the 'fuel tank' or, in other words, when the battery has run out of fuel, it still weighs the same.