The four things that stop a UK drone flight, and how to check them in five minutes

Most guides to flying a drone in the UK tell you the rules. Fewer tell you what to do about them on the morning of a job, standing in a field, with a client waiting. This is the second thing.

There are four questions that decide whether a flight happens. Can you legally be in this airspace. Will the weather let your aircraft fly. Is there enough daylight. And is there anything about this specific place, this specific day, that no map will tell you. They fail in that order, and the first two are where almost every cancelled job comes from.

The airspace question is not one question

British airspace has more restrictions on it than most pilots expect, and they are not all where you would guess. The UK publishes a dataset of flight restriction zones for unmanned aircraft, updated every 28 days, and the current edition holds 1,073 separate zones.

Aerodromes are the obvious ones. Every airport and most airfields sit inside a Flight Restriction Zone, and inside one you may not fly at all without permission from the air traffic unit or the aerodrome operator. The zone is usually a circle of about 2 nautical miles, with rectangles extending from each runway end.

Then there are the ones people miss. Nearly a third of the zones typed as Flight Restriction Zones in that dataset are prisons. Fly over one without permission from His Majesty's Prison and Probation Service and you have committed an offence, and it is not a paperwork offence. There are barracks, nuclear sites, government buildings and royal parks in there too. A pilot who checks only for airports has checked for a minority of what is around them.

Central London is the clearest example. Within 30 kilometres of Trafalgar Square there are more than forty restriction zones. Some are airports you would expect, Heathrow and London City among them. Several are prisons. One is Hyde Park. One is a corridor for remotely piloted aircraft that sits between 50,000 and 66,000 feet, which no drone will ever reach and which still appears on the map because it is genuinely there.

That last one matters more than it looks, and it is worth saying plainly: a restriction that starts at 50,000 feet is not a restriction on you. A tool that shouts about it is training you to ignore the ones that count.

Where the rules actually live

The Civil Aviation Authority publishes the rules, and its Drone and Model Aircraft Code is the document to read once, properly, rather than skim. The airspace data itself comes from NATS, the UK's air navigation service provider, which publishes the restriction zones on a fixed 28 day cycle. Those dates are set years in advance, which means any map you use is either current or it is not, and you should be able to tell which.

The two limits that decide most flights are simple. You may not go more than 400 feet above the closest point of the surface. And you must keep the aircraft in sight at all times, which in practice binds harder than the height limit does.

The weather question is about your aircraft, not the forecast

A general forecast tells you the wind. It does not tell you whether your aircraft can fly in it, and the gap between those two things is where wasted journeys live.

Manufacturers publish a maximum wind resistance figure for each model. A DJI Mini 2 is rated to a Level 5 wind, which begins at 8.5 metres per second. A Mini 4 Pro is rated to 10.7. A larger commercial aircraft will take considerably more. So a 10 metre per second day is a cancelled flight for one pilot and an ordinary morning for another, and the forecast page cannot tell you which you are, because it does not know what is in your case.

Three other things matter as much as wind speed and get less attention.

Gusts, not average wind. An aircraft rated to 8.5 metres per second in steady air is not necessarily safe in a 6 metre per second wind gusting to 12. Gusts are what put drones into trees. Worth knowing: many free weather feeds do not publish forecast gusts outside northern Europe at all, and simply return nothing rather than saying so.

Rain, at all. Most consumer drones carry no ingress protection rating whatsoever. Not a low one. None. For those aircraft any measurable precipitation is a no, and the manufacturer will not honour a warranty claim that begins with the word drizzle.

Cold. Battery performance falls off sharply near freezing, and several popular models are rated only to 0 degrees. A cold clear morning looks like the best flying day of the week and can cost you a third of your endurance.

Daylight, and the part people forget

Sunrise and sunset are the obvious numbers. The one that catches people out is that the usable light for filming starts later and ends earlier than the official times, and that the difference grows the further north you are and the closer to midwinter. If the shot needs light rather than merely legality, plan against the golden hour rather than against sunset.

The five minute check

Here is the sequence that catches nearly everything, in the order that fails fastest.

First, the airspace where you will actually stand, not the postcode of the job. A field on the edge of a town can be inside a zone when the town centre is not.

Second, the wind and gusts for the hours you will be flying, against your own aircraft's published figures rather than against a general sense of whether it seems windy.

Third, precipitation and temperature across the same hours.

Fourth, daylight, with the shot in mind and not just the law.

Fifth, and this is the one no tool can do for you: the things that are not aviation data at all. Local byelaws about taking off and landing in parks and on council land. The landowner's permission, which is a separate matter from airspace entirely. Privacy, if you will be over housing. And short notice restrictions, which travel by NOTAM and are not in any 28 day dataset.

What we built, and what it does not do

Terasor's drone flight planner does the first four of those in one page, free, with no account. Pick a city, or open the map and click the spot you are actually going to. It shows the wind, gusts, rain, temperature and daylight hour by hour for the week ahead, judged against a cautious default aircraft, and it lists the flight restriction zones near that point with the distance to each boundary rather than to its centre, so it can tell you when you are standing inside one.

Sign in as a Terasor pilot and it judges the same forecast against the aircraft you actually fly, using manufacturer figures, at any location you choose.

Two things it deliberately does not do. It does not tell you that you may fly, because that is not a thing a website can know. And it holds no data on temporary restrictions, local byelaws or landowner permission, which is why it says so on the page rather than leaving you to assume the silence means something.

The honest summary is that a planning tool can rule a flight out in seconds and can never rule one in. Used that way it saves a wasted morning most weeks.

Weather data from MET Norway. Data supplied by the Met Office. Aerodrome reports from the NOAA Aviation Weather Center. Airspace restriction zones from the UK AIP.