Airline Consumer Protections: Information on Airlines' Denied Boarding Practices, Report to Congressional Committees [open pdf - 663KB]
From the Highlights: "Some airlines overbook their scheduled flights (intentionally sell more seats than are available) to compensate for passenger no-shows. It is not illegal for airlines to overbook their flights. However, it can result in an 'oversale' where airlines cannot accommodate all passengers on a particular flight. In response, airlines may have to deny boarding to some passengers. DOT [Department of Transportation] is responsible for ensuring airlines adhere to their denied boarding practices as part of its consumer protection enforcement responsibilities. The FAA [Federal Aviation Administration] Reauthorization Act of 2018 included a provision that GAO [Government Accountability Office] examine airlines' oversales practices. This report focuses on denied boardings--the result of an oversale--and describes (1) trends in denied boardings and (2) airlines' actions related to denied boardings and mitigating the effects on passengers."
Government Accountability Office: https://www.gao.gov/