Revenue tons enplaned by U.S. air carriers 1990-2021

* The number of short tons transported on a flight by an air carrier. Includes freight and mail.

The source specifies:
Data are for all scheduled and nonscheduled service by large certificated U.S. air carriers at all airports served within the 50 states and the District of Columbia. U.S. territories are not included in the data. Not all scheduled service is actually performed. Moreover, for several years, total performed departures exceed total scheduled departures because nonscheduled departures are included in the totals. Prior to 1993, all scheduled and some nonscheduled enplanements for certificated air carriers were included; no enplanements were included for air carriers offering charter service only. Prior to 1990, freight includes both freight and express shipments, and mail includes priority and nonpriority U.S. mail and foreign mail; beginning in 1990, only aggregate numbers are reported.
Large certificated air carriers operate aircraft with seating capacity of more than 60 seats or a maximum payload capacity of more than 18,000 pounds and hold Certificates of Public Convenience and Necessity issued by the U.S. Department of Transportation authorizing the performance of air transportation. Data for commuter, intrastate, air taxi, small-certificated, and foreign-flag air carriers are not included.