Skip to content

ATIS & weather from the sim

Available now.

What it does

All weather comes from X-Plane itself -- there are no external weather calls -- so ATC always matches whatever you're actually flying in, real weather or custom weather. xatc.weather.service reads the sim's own weather datarefs, converts them into a WeatherSnapshot (wind, altimeter, ceiling, visibility, temperature, flight category), and generates a looping ATIS broadcast with an information letter that advances only when the weather actually changes.

Example output

Seattle-Tacoma International information Alpha. Wind one eight zero at one zero.
Visibility one zero. Sky clear. Temperature one five, dew point eight.
Altimeter two niner niner two. Landing and departing runway one six left.
Advise on initial contact you have information Alpha.

Wind is spoken as magnetic, converted from the sim's true-heading data using the aircraft's own true/magnetic heading gap (there's no separate magnetic-variation dataref this reads). The active runway comes straight from runway selection, so the ATIS and what Clearance/Ground actually assign never disagree.

How it decides when to advance the letter

A live KPDX flight found the original version of this rule too twitchy -- a cloud layer hovering right at an okta-coverage boundary flickered the reported ceiling by a couple hundred feet every few ticks, and each flicker read as a real flight-category change, advancing the letter every couple of minutes. AtisGenerator now smooths the ceiling (a ~2-minute rolling window: a majority vote on whether there's a ceiling at all, then the median height among the readings that have one, rounded to the nearest 100 ft) before comparing anything, and is_significant_change itself compares against thresholds a reading has crossed, not the raw values:

  • flight category (VFR/MVFR/IFR/LIFR) changes, including becoming or stopping being unreported
  • the smoothed ceiling crosses 500, 1,000 or 3,000 ft AGL (in either direction -- gaining or losing a ceiling reading entirely counts too)
  • visibility crosses 1 or 3 SM (when both readings report it)
  • the wind direction goes calm/variable, or comes back from being calm/variable
  • wind direction shifts 30° or more, but only while the wind is 10 kt or more -- a near-calm or variable direction is too noisy to mean anything operationally
  • wind speed changes 10 kt or more
  • a gust appears or disappears altogether, or changes by 10 kt or more once already present
  • altimeter changes 0.02 inHg or more
  • the active runway changes (fed by runway hysteresis below, so a real runway change -- not a momentary flicker -- is always a fresh letter)

Comparisons are always against the reading the letter last actually advanced on, not the immediately-previous call -- so a slow drift (altimeter creeping up 0.01 inHg at a time) still eventually crosses a threshold instead of resetting every step and never accumulating. Absent any of the above, the letter still advances once an hour, matching how a real ATIS keeps pace even when nothing operationally significant has happened.

This is a simplified approximation of the FAA's real significant-change rule set, not the full rule.

Range

ATIS is only heard within 60 nm of the airport broadcasting it -- tuning an ATIS frequency from farther out gets silence, matching a real D-ATIS-equivalent service volume, instead of pulling in a broadcast from an airport nowhere near the flight.

Where the numbers come from

  • Near the aircraft (on the ground, or near the destination once close): sim weather datarefs directly -- wind, visibility, and altimeter at the aircraft's own position, and cloud layers/wind by altitude from the sim's regional data.
  • A distant airport (the arrival ATIS while still enroute): X-Plane's own real-weather METAR file, if it's running in real-weather mode and has downloaded one -- read directly off disk. In custom-weather mode there's no METAR file, so this falls back to the regional datarefs once you're close enough, filling in only what a METAR would have been missing (visibility) rather than overriding a real METAR's own numbers with guessed regional data.

Runway hysteresis

A related fix from the same KPDX flight: the active runway itself flickered between 28R/28L on a wind reading of "variable at two," since runway selection is a pure, stateless function that answers fresh from whatever the sim reports each call, with nothing remembering what it picked last time. xatc.atc.runway_selector.RunwaySelector wraps it with hysteresis -- a calm or variable reading never causes a switch away from the current runway, and any other new answer has to keep coming back the same way for a few minutes before it's adopted, except when the current runway has drifted beyond its tailwind or crosswind limit, which switches at once rather than waiting out the sustain window (a stale runway shouldn't get a takeoff clearance).

This is live. A shared RunwaySelector backs every runway pick for the departure airport -- the ATIS broadcast and whatever Ground/Clearance actually assigns come from the exact same instance, so they can't disagree even mid-hysteresis -- and a second, separate instance does the same for the destination's arrival planning. The arrival-side selector resets the moment real destination weather (a METAR) first replaces the aircraft's own weather as its stand-in, so an early guess from the wrong airport's wind can't anchor the arrival runway for minutes once the real reading is in. For VFR pattern work (departure and destination are the same field), both sides share one selector, since it's the same runway question either way.

Configuration

xatc run --replay ... --weather-fixture {north-flow,south-flow} seeds a fixed weather snapshot for development without a live sim; --live mode always reads real sim weather continuously instead.

Limitations

  • Flight category and significant-change detection are simplified approximations, not the FAA's full published rule set.
  • No plugin-based weather API fallback (XPLMGetMETARForAirport) -- only datarefs and the METAR file.
  • ATIS is delivered as a looping voice broadcast on its own frequency; it isn't currently woven into what Clearance/Ground/Tower say beyond confirming the letter you state back.