How it is measured
- Status, timeline, cause and held centers: the national airspace status feed, checked every minute, and the air traffic control advisories it links to.
- Delays by airline and region: departures and arrivals at IAD 30 or more minutes late, by their estimated delay against the schedule, sampled every 10 minutes while the stop is active and once more after it ends.
- Totals and cancellations: arrival and departure boards for IAD, every 30 minutes while active and once more after the end. Delayed means 15 or more minutes late at IAD; cancelled is the airline's cancelled status; codeshares count once. Flights scheduled from the start until an hour after the end are counted.
- Held at origin: the same arrivals boards, which carry each flight's scheduled and actual departure from its origin. A flight is held when it is still scheduled, not departed, 15 or more minutes past its scheduled departure at a sample taken during the stop. In scope: its origin is in a held center or listed facility; flights from elsewhere are late for other reasons.
- Inbound in the air: the same arrivals boards. At each board taken during the stop, a flight to IAD is in the air when it has departed (en route, or with an actual departure time) and not landed, cancelled or diverted; one still shown in the air over an hour past its arrival estimate, or departed more than 18 hours earlier, is treated as stale and left out.
- Diversions: an inbound flight marked diverted during the stop or the wrap-up after it (not one marked before it could have departed), or one still shown in the air 30+ minutes past its arrival estimate that ADS-B (by its ICAO call sign) then found on the ground at another airport, not its origin. The diversion airport is the airport ADS-B found it at, else the updated destination. Cancellations never count. Regional flights flown under another call sign can't be found on ADS-B, so their diversion airport may be unknown.
- Aircraft on the ground and holding: ADS-B positions around IAD, every 5 minutes; the airline is read from the call sign. Holding counts airline flights only (passenger, regional and cargo), not private, military or medical flights.
- Weather: METARs, SIGMETs and weather alerts.
- News: headline links filtered to this airport and these days; we show titles and links only.
- This stop was recorded before detailed tracking began, so it has no airline numbers.
Data sources are listed on the credits page.