The FlightStats Flex APIs are optimized for regularly scheduled commercial passenger service. Cargo flight schedules published in advance are available in the Schedules API or the Connections API. However, many cargo flights are unscheduled and depart on short notice based on variable demand. If a flight plan is filed with a civil aviation authority such as the Federal Aviation Administration that does not match a published flight schedule, our system creates an ad hoc flight schedule to track. In this case, it will be available via the Flight Status API near the time of departure.
Operational flight number suffixes (example: 353R or 928W) usually indicate that it's an unscheduled, ad hoc flight from an amended flight plan. We currently do not track the reason for why it was created by an operational data source.
The Connections API has some notable features for cargo forwarders and air freight handlers:
Connections Response (flightstats.com)