Actually, the 400' is a guideline for hobby use drones, sorry, uav's, less than 7kg. Anything 7-20kg MUST stay below 400', anything over 20kg can only be flown in two areas of the uk, and neither of those places will allow civilian uav use.
The "dronecode" is guidance.
400' is GENERALLY accepted to be the limit of clear los vertically, and 500m horizontally, but, that all depends on size, shape, colour etc on what the limit actually is.
Cap722 is what you need to read.
"Operating within Visual Line of Sight means that the Remote Pilot is able to maintain direct, unaided (other than corrective lenses) visual contact with the UA which is sufficient to monitor its flight path in relation to other aircraft, persons, vessels, vehicles and structures for the purpose of avoiding collisions. Within the UK, VLOS operations are normally accepted out to a maximum distance of 500 m horizontally and 400 ft vertically from the Remote Pilot. Operations at a greater distance from the Remote Pilot may be permitted if an acceptable safety case is submitted. For example, if the aircraft is large it may be justifiable that its flight path can be monitored visually at a greater distance than 500 m. Conversely, for some small aircraft, operations out to a distance of 500m may mean it is not possible to assure or maintain adequate visual contact."
And
" The standard CAA permission for SUA/SUSA in the 7 kg or less category allows flight in congested areas to within 50 metres of persons, structures etc. (or within 30 metres if the persons are under the control of the person in charge of the aircraft). This category of SUA/SUSA cannot fly within 150 metres of open-air assemblies of 1,000 people or more where only a standard permission has been granted. "
Article 241 of the ANO, "A person must not recklessly or negligently cause or permit an aircraft to endanger
any person or property"
This is your 'get out of jail' clause for setting the rth to a height over 400', so as to clear surrounding structures. So if your chimney is 660', thats what? 200m give or take, then set your rth to 250m, that ensures your rth doesn't inadvertently break the ANO, which IS law, and ultimately is what any judge has to go by, not the 'dronecode' guidance.
Other aircraft will be giving the chimney a wide birth, as they have minimum separation distances to comply with too.
From the AIR NAVIGATION ORDERS.
This IS the law...
" unmanned aircraft
94.—(1) A person must not cause or permit any article or animal (whether or not attached to a parachute) to be dropped from a small unmanned aircraft so as to endanger persons or property.
(2) The person in charge of a small unmanned aircraft may only fly the aircraft if reasonably satisfied that the flight can safely be made.
(3) The person in charge of a small unmanned aircraft must maintain direct, unaided visual contact with the aircraft sufficient to monitor its flight path in relation to other aircraft, persons, vehicles, vessels and structures for the purpose of avoiding collisions.
(4) The person in charge of a small unmanned aircraft which has a mass of more than 7kg excluding its fuel but including any articles or equipment installed in or attached to the aircraft at the commencement of its flight, must not fly the aircraft—
(a)in Class A, C, D or E airspace unless the permission of the appropriate air traffic control unit has been obtained;
(b)within an aerodrome traffic zone during the notified hours of watch of the air traffic control unit (if any) at that aerodrome unless the permission of any such air traffic control unit has been obtained; or
(c)at a height of more than 400 feet above the surface unless it is flying in airspace described in sub-paragraph (a) or (b) and in accordance with the requirements for that airspace.
(5) The person in charge of a small unmanned aircraft must not fly the aircraft for the purposes of commercial operations except in accordance with a permission granted by the CAA."
Note paragraph (4), then subparagraph (c)
Also, from same...
"Small unmanned surveillance aircraft
95.—(1) The person in charge of a small unmanned surveillance aircraft must not fly the aircraft in any of the circumstances described in paragraph (2) except in accordance with a permission issued by the CAA.
(2) The circumstances referred to in paragraph (1) are—
(a)over or within 150 metres of any congested area;
(b)over or within 150 metres of an organised open-air assembly of more than 1,000 persons;
(c)within 50 metres of any vessel, vehicle or structure which is not under the control of the person in charge of the aircraft; or
(d)subject to paragraphs (3) and (4), within 50 metres of any person.
(3) Subject to paragraph (4), during take-off or landing, a small unmanned surveillance aircraft must not be flown within 30 metres of any person.
(4) Paragraphs (2)(d) and (3) do not apply to the person in charge of the small unmanned surveillance aircraft or a person under the control of the person in charge of the aircraft.
(5) In this article, “a small unmanned surveillance aircraft” means a small unmanned aircraft which is equipped to undertake any form of surveillance or data acquisition."
So, in fact, to stick to 'the law', when flying a phantom, you should set your rth to 50m higher than the highest structure. Then it avoids contravention of, ANO 95.(2)(c)
As, the ANO only restricts SUA over 7kg from exceeding 400' without CAA permission, then having rth set to 250m/800' with a phantom is legal, assumimg you can see it. So, if you want to fly high, put some strobes on to assist...
