Aviation Consumer Protection: Few U.S. Aircraft Have Lavatories Designed to Accommodate Passengers with Reduced Mobility, Report to Congressional Committees [open pdf - 1MB]
From the Highlights: "Flying can pose significant challenges for persons who rely on wheelchairs, including the lack of wheelchair accessible lavatories on most flights. In 1990, DOT [Department of Transportation] required wheelchair accessible lavatories on twin-aisle aircraft used mainly for long flights. It did not require them for single-aisle aircraft, although DOT continued to study the issue. Since 1990, technological advances have enabled single-aisle aircraft to fly longer distances, and these aircraft now make 99 percent of domestic flights. In 2016, a DOT advisory committee recommended that DOT require accessible lavatories in certain single-aisle aircraft in the future. The Federal Aviation Administration (FAA) Reauthorization Act of 2018 included a provision that GAO [Government Accountability Office] examine the availability and designs of lavatories on commercial aircraft and the ability of passengers with disabilities to access them. This report describes (1) what is known about lavatory designs and accessibility for persons with reduced mobility and (2) the challenges wheelchair-bound passengers and others face while traveling on single-aisle aircraft without accessible or functional lavatories."
Government Accountability Office: https://www.gao.gov/