How light aviation flies

The hours on this page are local solar time. On that clock 12:00 is the moment the sun stands highest, in Lisbon as in Bucharest, while the clocks on the wall in those two cities can be an hour or two away from it, by time zone and summer time. Solar time is UTC plus one hour for every 15 degrees of longitude east of Greenwich, the arc the sun crosses in an hour, and through the year it stays within about a quarter of an hour of the true sun. Thermals follow the sun, so on this clock the flying day of the Alps lines up with that of Portugal or the Carpathians, and the hours of different places can be added together. The rules are in section 1 of PATTERNS.md.

Powered aircraft fly many times the hours of hang gliders, so each line shows its own kind’s flying time spread over the day, as a share that adds up to 100%. Kinds of very different size can then be laid over one another and compared by shape; how much each kind flies in total is on the monitor’s main page. The lines are shown once seven days have been counted, since a single day says more about that day’s weather than about the habits of pilots.

The days of the week are counted the same way, by the local solar date. They need several weeks. One rainy Saturday and one sunny Tuesday shift the split more than any habit of pilots, so the weekdays are shown only once three of each have been counted, and even then the weather of those particular days weighs on them more than anything else.

The direction of a turn comes from the change of course between two successive positions of the same aircraft, so it needs no turn rate in the packet. Only positions at most 5 seconds apart are used, because in 15 seconds a paraglider circling once every 20 seconds turns through 270 degrees, which reads as 90 degrees the other way. A change of course between 5 and 45 degrees a second counts as turning, which is a full circle in anything from 72 seconds down to 8. Right means clockwise seen from above. The rules for this section and the next two are in section 2 of PATTERNS.md.

A thermal is at least two full turns to the same side in a row that gain at least 50 m. One turn can be a manoeuvre, and turns that gain no height are a pilot looking for the core or spiralling down to land. Its climb is measured from a low point of the turns to the highest point reached after it, so the search turns before the core and the weak turns at the top stay out of the climb rate. On 6 October, 275 of 1,020 circling episodes of gliders and 215 of 743 of paragliders gained less than 50 m. At some airfields nearly all of them did, where gliders circle down over the field before joining the circuit. Turns to the same side within 10 minutes and 3 km of each other count as one thermal, because a pilot who widens out to recentre, or whose packets drop out for a while, would otherwise split one climb into several. Many free-flight instruments send FLARM, FANET and ADS-L under one address, and each of them would report every thermal again, so each aircraft is followed on the one system that gave it the most thermals that day. Gliders, paragliders and hang gliders are measured, each kind on its own, since a preference for one side, if there is one, need not be the same for wings flown so differently.

Two shares are given for each kind. The share of thermals counts a thermal once whether the pilot stayed for two turns or for twenty, which is the side chosen on entry. The share of degrees weighs each thermal by how long the pilot stayed in it. A kind is shown once it has 200 thermals, enough for its share to be read within about seven points either way.

The 95% interval is the range in which the share would land in 95 counts out of 100 if every thermal were a separate toss of the same coin. Thermals are not separate tosses. In a gaggle the pilot who arrives turns the way the pilot already circling turns, so one decision is counted once for every aircraft in the thermal, and the real uncertainty is wider than the interval drawn. How much wider depends on how often pilots fly together, which is measured further down.

Suppose every pilot of a kind chose the side of each thermal by tossing the same weighted coin, one that lands right as often as all those pilots together turn right. Pilots would still differ from one another by luck alone: of pilots with a dozen thermals each, a few would come out at four thermals in five on one side. The chart sets the number of pilots in each band of right-hand share against the number that coin would give the same pilots. If pilots had a side of their own, the bars would pile up towards both edges and leave the middle thin.

The difference is summed up in one number, the ratio of spreads: how much more widely the pilots’ shares scatter than the coin makes them scatter. At 1 they scatter as chance does. At 4 the typical distance of a pilot from the average is twice what chance gives, since the ratio compares squared distances. A pilot needs many thermals before a share means anything, so the test takes only pilots with at least 10 thermals over the current month and the previous one, and a kind is shown once 20 of its pilots qualify. A pilot here is one aircraft address, which brings two errors in opposite directions. A club glider flown by several pilots mixes their habits and narrows the spread, while a pilot who flies the same site with the same group every weekend follows that group’s side and widens it. A wide spread is therefore a sign of personal preference and stops short of proving it.

Flying schools teach the pilot joining a thermal to turn the way those already in it are turning. The monitor counts how often that happens. Two aircraft share a thermal when, while both are circling, they stay within 500 m of each other horizontally and 300 m vertically for at least a minute in all. If each of them chose a side on its own, with the share p of right-hand thermals seen across the kind, they would agree p² + (1 − p)² of the time: with p at 56%, 0.56² + 0.44², which is 51%, barely more than a coin. The measured share says how far pilots in a thermal follow the first one in, and so how much of the side preference above may come from a few pilots who chose and many who followed. A pair of kinds is shown once 30 such pairs have been counted.

Flying from a field in the lowlands and flying among mountains are different trades, so the sections from here on can be read for either. The class describes the ground within 5 km of the aircraft: where the highest and the lowest ground in that circle differ by less than 600 m it is plain and hills, from 600 m it is mountains. A thermal takes the class of the point where it ended, a flight or a launch that of where it began, and time in the air that of where each stretch of it began. The rules are in section 12 of PATTERNS.md.

The class says nothing about which lift was flown. A glider climbing in a thermal over flat ground 4 km from a 700 m slope is filed under mountains, and a pilot soaring a 400 m hill under plain and hills. Nor does it follow regions: an Alpine valley floor a few kilometres wide is mountains, a high plateau plain and hills. The 600 m line was drawn on 7 October 2026 after looking at the relief around every flight start and every thermal of 6 October, in steps of 100 m. Every kind showed a dip between 500 and 700 m, and 89% of glider starts fell below 600 m against 21% of paraglider starts. One October day is a narrow base, so the line will be checked against a summer month, when the Alps fly differently and lowland thermals are stronger.

How wide a thermal is circled and how fast it lifts decide who can share it. The thermals are those of the circling sections above, at least two full turns and 50 m of climb each, for gliders, paragliders and hang gliders, each kind on its own since paragliders and hang gliders circle at different radii and speeds. The rules are in section 3 of PATTERNS.md.

The radius comes from the thermal as a whole: the distance flown along the circles divided by the angle turned. Over whole turns the wind makes the aircraft faster over the ground on one side of each circle and slower on the other by the same amount, so it cancels to within a few per cent, about 3% with a wind of a third of the airspeed. The speed and turn rate of a single position would carry the wind in full, which is why the method uses whole thermals only. A paraglider usually circles at 30 to 50 m of radius and a glider at 60 to 120 m. A kind is shown once 100 of its thermals have a radius.

The climb rate of a thermal is its average over the time circled, from the first position sampled to the last, given in metres a second, the unit of a variometer. One metre a second is about 200 feet a minute, and at 2 m/s a pilot gains a thousand metres in a little over eight minutes. An altitude offset in a device cancels in a difference, so the climb rate does not depend on the altitude reference the device uses. A kind is shown once 100 of its thermals have a climb rate, and an hour of the day once it holds 20 of them.

Two aircraft of different kinds share a thermal by the rule used for gaggles above: within 500 m horizontally and 300 m vertically for at least a minute while both circle. A glider and a paraglider in one thermal circle at different speeds and radii, and the vertical separation they keep while together, taken as its middle value, is what decides whether they meet. A pair of kinds is described once it has shared 30 thermals.

A cross-country flight alternates climbing in thermals with gliding between them, and the share of time spent circling says how the day divides between the two. It also says for how much of a flight the turn rate matters most to anyone predicting where the aircraft will be (question 1 of the monitor). The rules are in section 5 of PATTERNS.md.

The share is indicative. A thermal is seen only in positions at most 5 seconds apart, so an aircraft heard on a system that sends less often, FANET or most phone apps, never circles in the data however much it climbs, and all its time reads as gliding. Over all airborne time, free flight circled 1.6% of the time on 6 October 2026, a figure that describes its systems more than its flying. The share is therefore taken over the aircraft seen circling at least once that day, and one of them heard for part of the day on a slower system still pulls it down. A kind is shown once those aircraft have flown 10 hours, and an hour of the day once they have flown an hour in it.

Where in the sky each kind of aircraft is at each hour, in bands of height above the terrain. The 120 m edge is the ceiling of the open category of drones, the one anyone may fly without an authorisation, so the time a crewed aircraft spends below it is time spent in the drones’ airspace. Drones are shown on their own, split into confirmed and uncertain by the evidence rules of the monitor’s method; drones that broadcast only Remote ID are not received at all. The rules are in section 4 of PATTERNS.md.

A crewed kind is shown once it has 5 hours of flight and a kind of drone once it has 1 hour, since drones fly far less; an hour of the day gets a bar once the kind has flown 15 minutes in it. These figures describe where aircraft fly. A low-flying aircraft may be taking off, landing, soaring a ridge, towing a banner or on a rescue, and the feed cannot tell these apart, so the method deliberately makes no count of flying below minimum heights.

A flight runs from the moment an aircraft is first heard airborne to the moment it lands or is lost for good. Coverage has holes, so a flight goes on across a silence of up to 2 hours when the aircraft was airborne and more than 150 m above the ground on both sides of it, and the distance between the two sides could be flown in that time at the fastest its kind flies: 70 km/h for a paraglider, 120 for a hang glider, 280 for a glider, 350 for a powered aircraft. A glider out of coverage for half an hour behind a ridge is therefore one flight. A silence that begins or ends near the ground, a silence longer than 2 hours, or a distance too long for the time ends it. So does a position of the aircraft standing on the ground, at 3 knots or less within 30 m of the terrain, for gliders, free flight and fixed-wing powered aircraft; a helicopter or a drone hovering low looks the same as one that has landed, so its flights end only on a silence. Slow flight low over a ridge does not end a flight, since a paraglider soaring into the wind can hang almost still over the ground. Across a silence the track is counted as a straight line, so the path lengths are a lower bound. A flight counts only if it lasts at least 2 minutes: on 6 October 2026, 493 glider flights whose start was not seen lasted a median of 6 seconds, an aircraft heard for a fix or two at the edge of coverage.

Each flight is described by how long it lasted, how far it got from where it began, how long its track was, and the solar hour of its first airborne position; an aircraft first heard already in the air gives a flight whose start was not seen, and its figures are those of the part seen. Many glider flights are short. At a winch site a launch takes a glider to 300 or 400 m, and on a day without lift that is a circuit of five or six minutes back to the field, flown again and again by pupils and by pilots checking the air; a glider relaunched within 20 minutes counts as a new flight each time, since a position standing on the ground ends the one before. Glider flights also carry the launch they began with, which the second chart uses: a winch launch to 400 m on a day without lift is a circuit of a few minutes, while a tug or an engine can take the glider to where the lift is. A kind is shown once it has 50 flights, a launch method once it has 30. The rules are in section 6 of PATTERNS.md.

Flights are also sorted by how they ended. A landing is seen when a position of the aircraft standing on the ground ends the flight. When a flight ends in silence, it is taken as landed at its last position if that position was less than 100 m above the ground and lower than the aircraft had been half a minute to a minute before: a glider, a hang glider or a paraglider low and coming down just before the silence has almost always landed, and the pilot switched off or the last receiver lost it behind the ground. A powered aircraft also needs an aerodrome within 3 km of that last position, since one low and descending elsewhere may simply have flown out of coverage. These are inferred landings, and they are counted apart from the seen ones. A flight with neither is counted as end not seen.

For gliders, hang gliders and paragliders the yardstick is the glide range: how far the aircraft could glide from its take-off without climbing at all. It is the height of the take-off above the landing times a typical glide ratio, 8 for a paraglider, 12 for a hang glider and 35 for a glider, plus a quarter for the lift met on the way. A paraglider taking off 1,000 m above its landing field has a glide range of 10 km, and a glider released 500 m above its airfield has 22 km. A flight that stayed within its glide range is local. One that went beyond it and came back is out and return, and one that landed beyond it is cross-country.

The take-off height of a glider is its release, or the top of its winch launch, when the tow or the winch was seen. Every other flight that starts on the ground, every paraglider and hang glider included, takes the top of its first climb, the highest point it reached before coming down 50 m. A flight from a flat field, or from a hill it lands back on, would otherwise start and end at the same height, with a glide range of zero, and a landing a few hundred metres away would count as cross-country. When a thermal follows the launch without a break, its top counts as well, which lengthens the glide range and makes the call of cross-country more cautious.

Two more rules keep short flights at home. A flight that lands within 1 km of its take-off is local or out and return, whatever its glide range. A paraglider counts as cross-country only beyond 5 km: from a hill a first climb of 200 to 300 m gives a glide range of 2 to 3 km, and a paraglider landing a few kilometres away has usually drifted down the valley. The glide range sorts flights and makes no claim about how each was flown.

For powered aeroplanes the take-off and the landing are matched to the nearest aerodrome within 3 km. A flight back to its own aerodrome is local within 25 km of it and out and return beyond that. One that landed at another aerodrome is one way, and one with no aerodrome at either end is counted apart. A kind is shown once it has 50 flights.

One-way flights of powered aircraft are also counted by the pair of aerodromes they joined. A route is named only once 5 different aircraft have flown it in the same month, because a route flown by one or two aircraft would tell where a particular person went.

How gliders and hang gliders get into the air, and for aerotows at what height the tug lets them go, says how much of the airspace around a gliding site a launch occupies. The rules are in section 7 of PATTERNS.md.

An aerotow is a glider or hang glider flying together with a tow plane, within 300 m and at less than 20 km/h of difference in speed for at least a minute, from less than 100 m above the ground, since two aircraft that meet in formation aloft are no launch. The release is where they stop flying together, at the glider’s height above the ground then; a typical release is between 400 and 600 m. Tugs are counted once for each day they fly, by how many tows they made, and are never named.

On a winch the glider climbs at 10 m/s or more, 600 m a minute, and reaches 300 to 500 m in well under a minute. The winch is the commonest launch at many clubs in Germany, the Netherlands and Belgium, so the split between methods depends on which countries are flying. A winch launch is counted when a glider goes from under 50 m to at least 200 m above the ground within a minute, gaining at least 150 m of altitude at 4 m/s or more, at 70 to 150 km/h, with no aircraft beside it. The altitude gain is there because height above the ground also grows when a glider flies off a ridge or out over a valley: the first count, without it, found 1,016 winch launches on 6 October 2026 against 308 aerotows. The climb rate is there because an aerotow whose tug is not heard, and a self-launching glider, climb 150 m in about 90 seconds too, at 2 to 3 m/s.

A launch is counted only when the aircraft’s first airborne position, after more than 20 minutes unheard, is within 150 m of the ground: a winch launch’s first seconds, an aerotow’s first minute, a hang glider just off its hill. One first heard higher up joined the feed in the air, coming back into coverage after a gap, and is counted apart as a start not seen, outside the split. The first count, without this rule, gave 2,153 glider launches on 6 October 2026 against 1,745 glider flights, because reappearances were taken for launches.

A glider launch with neither is read by its climb over the first 150 m it gains. At 6 m/s or more it is a winch launch the winch rule missed, usually because the glider came into coverage partway up the cable, and it is counted with the winch launches: an aerotow climbs at 2 to 4 m/s, and nothing else a glider does climbs that fast that low. A straight climb at 1 to 5 m/s and 90 to 150 km/h is an aerotow or a self-launch that cannot be told apart. The tug may carry nothing the network hears, or fly just out of coverage, and a self-launching motorglider climbs at the same rates and speeds. Such a climb counts as a self-launch only when the OGN device database names the aircraft as a self-launching motorglider, from a short list of models (Stemme, Arcus M, DG-808, Antares and a few more) kept short so that no pure glider sharing a model name is taken in; an aircraft whose owner chose not to be identified stays in the mixed class. Anything else, including a launch that has not gained 150 m within 10 minutes, is other. Hang gliders keep the single class “no tow seen”, which mixes foot launches from a slope, aerotows whose tug is not heard and launches flown outside coverage. A start counts only when the flight it begins lasts 2 minutes. The split is shown once 100 glider launches have been seen from the ground up, the release height and the length of the tow once there are 50 aerotows, and tows per tug once 20 tugs have been counted.

Mountain wave lets a glider climb in smooth air high above the ridges, often flying straight or in figure eights along the wave. A climb counts as probably wave when it gains at least 300 m at 1 m/s or more for at least 3 minutes, starting above 2,500 m, while turning on average at most 3 degrees a second, a full turn in two minutes or slower, and less than one full turn net. A thermal turns at 5 degrees a second or more, so the turning rule keeps thermals out, and the net turn lets figure eights in. A climb of the same glider counts once in half an hour. Two more conditions keep out climbs that only look like wave. The ground within 5 km must rise and fall by at least 1,000 m, since a wave needs a ridge to set it off. And the wind at the climb’s height and hour, in the Open-Meteo weather model, must reach 20 km/h, since a wave forms only where the wind crosses the ridge. On 6 October 2026 no climb had that wind anywhere. On 7 October a south föhn blew over the Alps, and only Alpine climbs passed. The rules are in section 8 of PATTERNS.md.

Ridge lift and convergence lines can look the same. A glider running along a ridge or a convergence also climbs in straight lines, and above high ground 2,500 m is not much. That is why these climbs are labelled probable, and why only their count is published, in cells a quarter of a degree on each side. A cell is named once it holds 3 of them, after the nearest town of 5,000 people or more: “near” the town when it lies within 10 km of the cell’s centre, otherwise its distance and direction from it, as in “30 km S of Kerns”.