A map of permissions, not of ground.
An airspace is a volume: a closed shape on the map, a floor, a ceiling, and a class that states what is required of you inside it. That is the whole data model, and it is worth holding in mind, because pilots tend to picture airspace as lines on a chart and then get surprised by the third dimension. The line you are looking at is the shadow of a box — and the box has a lid you may be under, over or exactly level with.
The classes are ICAO’s, from A to G, with national additions layered on top. In practice a soaring pilot meets a small number of them repeatedly: a CTR wrapped around an airport from the surface upward, a TMA stacked above it in steps like an upside-down wedding cake, airways above that, and the enormous class E or G volume in between where the flying actually happens. Around them sit the special-purpose volumes: danger areas, restricted and prohibited zones, temporary reserved areas that exist only on certain days, and — the ones that pay your rent — gliding sectors negotiated locally so that a soaring club can climb where a light aircraft may not.
What the class actually demands.
National variation is real: the same letter can carry different visibility minima, different equipment rules and different national exemptions on either side of a ridge. The chart in the cockpit is a summary. The AIP of the state you are flying in is the authority — and it is the document your database was compiled from.
Three rulers in one sky.
Here is where good pilots get caught. A floor or a ceiling can be published in three different references, and they do not move together. AGL — above ground level — rides up and down with the terrain beneath you, so a 2500 ft AGL lid is at a different height above the sea over the valley than over the ridge. AMSL — above mean sea level — is fixed geography, the same number everywhere in the volume. And a flight level is neither: it is a pressure altitude, measured with the altimeter set to the standard 1013.25 hPa, which means its true height above the sea moves with the weather.
That last one is the trap. On a low-pressure day, FL65 sits lower over the ground than it did on the high-pressure day you flew last week — roughly 27 ft for every hectopascal below standard, so a 20 hPa drop lowers the floor of that TMA by about 165 metres. The airway did not move. Your altimeter did. This is why a computer that knows only your GPS altitude cannot draw the floor of a flight-level airspace correctly: it needs the QNH of the day, and it needs you to have entered it honestly.
The corridor between the lid and the floor.
Leave your altitude alone and drag only the QNH. The red lid does not move — an AGL limit does not care about pressure — but the blue floor slides down the screen as the pressure falls, and the corridor you were happily thermalling in gets thinner by a hundred metres or more. Then drag the terrain up a ridge: now the lid climbs toward you instead. Two limits, two different physics, one very narrow gap on a bad day.
Where the shapes in your instrument come from.
The chain is longer than most pilots assume. Each state publishes its structure in the AIP, in prose and coordinates. Volunteers and commercial providers compile that into machine-readable files — in gliding, most commonly the plain-text OpenAir format, a list of polygons with a class, a floor and a ceiling each, still readable in a text editor thirty years after it was invented. Your instrument loads that file and draws it.
Which means the structure in your panel is exactly as current as the last file you copied into it. Aeronautical information changes on the international AIRAC cycle — every 28 days, worldwide, on a published calendar — and a new danger area or a redrawn TMA arrives on one of those dates whether or not you updated. An out-of-date airspace database is not a small inconvenience: it is a warning system that is confidently wrong, which is worse than no warning system, because you have stopped checking. Update it at the same moment you update anything else — over WiFi from the panel takes a minute.
How a warning is actually computed.
Inside the instrument the test is simple and runs many times a second. For each nearby volume: is my projected position inside the polygon in two dimensions, and is my altitude between the floor and the ceiling, converted into the same reference the volume is written in? Both true means inside. The interesting engineering is in the word projected. A warning that fires the instant you cross the boundary is useless — you are already in it. So the computer takes your ground track and speed and looks ahead by a set time, and warns when the predicted path enters the volume.
That look-ahead is the setting worth understanding. At 160 km/h a 60-second horizon is 2.7 km of thinking distance: enough for a decision, short enough that the alarm is about you and not about the county. Set it too long in busy airspace and every alarm becomes background noise; set it too short and it becomes a report of what already happened. The same reasoning applies to the vertical: a small buffer under a floor, so that a strong thermal near a TMA base talks to you before it takes you through.
And there is the honest limit. The computer checks the volumes in its database against the position it believes it has. It does not know about this morning's NOTAM, the parachute drop that goes active on request, or the clearance you did or did not receive on the radio. Silence from the instrument means nothing was found in the file — not that the sky is yours.
The alarm you keep is the alarm that works.
Every experienced pilot has met the instrument that shouts about a class E ceiling forty kilometres away, and every experienced pilot has met the pilot who solved that by turning airspace warnings off entirely. Both are failures of configuration, not of the idea. A well-set-up system warns about the volumes that would actually cost you — the controlled airspace, the active danger area, the TMZ you are not equipped for — and stays quiet about the ones your club has flown inside legally for forty years.
Do that work once, on the ground, with the local chart and someone who flies the area. The reward is a panel that only speaks when it matters, and a habit of believing it when it does. That is the entire point of the exercise: not a screen full of coloured shapes, but one sentence arriving early enough to be useful.
Airspace support on every navigation page — the 57 and 80 mm vario that carries the whole picture, not just the climb.
3D terrain and airspace on the biggest display in an 80 mm cutout, updated over WiFi from the panel.
Airspace, terrain and the live traffic picture on one platform — with the database updated over the aircraft’s own connection.