Sean-bumble-bee
 Core User of DJI
Flight distance : 15997 ft
United Kingdom
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FP33 Posted at 5-24 01:03
Interesting and thanks for comment Labroides ... so if thinking practically and not theoretically, what distance from a cliff edge would be considered 'reasonable' to ascend to flight-level eg 600 ft ASL... 100m / 500m / or any distance when over water from the cliff and still be reasonably within the 'rules' because flying requires visibility of drone at all times, so, so long as you can see it, that would be considered reasonable. And because I would see / hear any search & rescue helicopters who sometimes (maybe twice a month) seem to pass en-route, near to cliffs in sufficient time to descend quickly to a safe level ?
If the applicable rule is 400ft AGL e.g. USA for recreational flyers, then, from a legal stand point, I doubt that there is any acceptable distance from a truly vertical cliff within which the drone could legally break that 400ft AGL ceiling.
However in some places the rule is that the 400ft long piece of string must be able to touch the surface of the earth somewhere. In such places you could climb to any AGL providing the drone was within 400ft of some point of the cliff's face. I think the rationale is that a manned aircraft should not be within 'x'ft or the cliff's face etc.etc..
To be honest I can never remember if the UK is strict AGL or the latter. Europe is, I think, the latter. BUT the CAA page https://register-drones.caa.co.uk/drone-code/where-you-can-fly
states
"Your drone or model aircraft must never be more than 120m (400ft) from the closest point of the earth’s surface.If you fly where the ground falls or rises, such as over hills, mountains or cliffs, you may need to adjust your flight path so that your drone or model aircraft is never more than 120m (400ft) from the closest point of the earth’s surface.[size=1.1875]"
In reality though, if the intention is to fly off the top of a cliff and out beyond the drop off, would it not be better to set the RTH height to the drone's minimum RTH height? There is unlikely to be any obstacle that is taller than that between the home point and the cliff edge. If connection was lost whilst out over the sea and providing it was beyond any threshold distances, then the drone is going to climb to launch-point + RTH-height height. I would suggest that as soon as the control connection was re-established the drone be lowered, if necessary, to be in accordance with the rules but note that if it was lowered whilst flying home in an RTH (check whether that would cancel the RTH, I don't think so but check) you might have to manually command a climb as it approached the cliff (again check blah blah blah).
All the above said, if the 'cliff' is neither vertical nor overhanging then the strictly AGL ceiling follows the slope, how many places in the UK have truly vertical or overhanging cliff faces that are taller than 400ft?
If the S&R helicopters fly through the area you are thinking of flying in then I would
a) keep in mind that you may need to ditch the drone i.e. CSC stop the motors to get out of the helicopter's way in time and that would require you switching the "emergency motor stop" option to "Anytime". It's not safe to fly the drone with the option permanently set to "Anytime". One of the posters on the Mavic Pilots used to fly at RC club site that flew at an S&R areodrome, the rule was that if the alarm sounded any RC model flying was to be flown into the ground i.e. crashed. En-route to a search or rescue I would suspect an S&R helicopter is going to be travelling at a fair lick.
b) question the wisdom of flying in the area.
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