Source
Every five minutes a collector reads the public flight board of Soekarno-Hatta airport (the same data shown on the arrivals and departures pages here) and records the latest scheduled time, estimated time, actual time and status for every flight. The board drops a flight about half an hour after it departs or lands, so the collector keeps the last state it saw.
Nothing is sourced from airlines or from third-party trackers. The figures describe what the airport itself published.
On time
A flight is on time when its actual time is no more than 15 minutes after its originally scheduled time. This is the standard OTP15 definition used across the industry. Delay is measured against the published schedule, never the estimate, so a flight that is "re-timed" during the day still shows the full delay.
For departures the actual time is the take-off time the board reports. For arrivals it is the landed or at-gate time. Early flights count as zero delay.
What is excluded
Today is never included, because its flights are still in progress. Flights that left the board without ever reaching a final status (departed, landed, cancelled) are counted as unresolved and left out of both the on-time rate and the average delay. Their share is shown on the page as a coverage figure.
Airlines with fewer than 30 completed flights in the selected period (10 for the single-day view) are listed but not ranked, because a handful of flights cannot support a percentage.
Averages
Average delay is the mean positive delay across all completed flights, so an airline with many small delays and one with a few very long ones can show the same figure. The "> 1 h late" column and the longest-delays list on each airline page exist to separate those two cases.
Cancellation rate is cancelled flights divided by every flight the board listed for the airline.
Codeshares and operators
Flights are attributed to the marketing carrier shown on the board, which is the airline whose code is on the ticket. A flight sold by one airline and flown by another is credited to the seller. The operating carrier is recorded and may be used for a separate view later.
Limitations
The board occasionally lists a flight with a schedule that has been moved, which can make a very late flight appear to leave early. Such rows are rare and are treated as zero delay rather than negative.
The board does not report an outcome for every flight, and the share it does report differs between airlines and terminals. Each airline's "Coverage" figure shows that share. A flight with no reported outcome may have been delayed past the point where the board stopped listing it, or its departure may simply never have been published, and the data cannot tell the two apart. Rankings between airlines with very different coverage should be read with that in mind.
If the collector misses a run (for example during an upstream outage), flights that departed in that gap may be recorded as unresolved. The coverage figure on each page shows how complete the period is.
Figures are recomputed on every page load from the stored history and may differ slightly from official airline or regulator statistics, which use different sources and definitions.