In general, we show schedules as filed by the carriers. Our schedules go 11 months into the future, however, but not all carriers file their schedules that far in advance.
For these carriers, instead of just showing that these carriers stop operating at the end date of their schedule filings, we work with our schedule provider team to extend these carriers' schedules to the end of the 11-month period. Most carriers get extended based on the discontinue date of the schedules they file with us. However, in some cases carriers come back to us and inform us not to extend their schedules, so they are added to a do not extend list. It is at the carrier's discretion to advise if they do not want their data extended beyond the end date they provide us.
In cases where the carrier's filing has a flat end date that's less than 11 months into the future, we usually just extend the last week of the schedule until the end of the eleven months. It is true that seasonal flights sometimes get extended in these cases, but it's hard for us to predict if the carrier will upgrade the route to year-round or continue it seasonally.
In cases where some flights are extended farther out than others, we have to make an educated guess as to which flights to extend. Unfortunately, some seasonal flights in this case get extended when they shouldn't. This is especially true in the autumn transition from the Summer to Winter slot seasons.
While our algorithms are by no means perfect in predicting what the carriers will file, we plan to continue to extend the schedules as best we can. We feel that being off by 10% or so is better than being off by 100%. We continually work to improve these file extension algorithms.