49 CFR 39.51 - What is the general requirement for PVOs' provision of auxiliary aids and services to passengers?
(a) If you are a PVO that is a public entity, you must furnish appropriate auxiliary aids and services where necessary to afford an individual with a disability an equal opportunity to participate in, and enjoy the benefits of, a service, program or activity. In determining what type of auxiliary aid or service is necessary, you must give primary consideration to the requests of individuals with disabilities.
(b) If you are a PVO that is a private entity, you must furnish appropriate auxiliary aids or services where necessary to ensure effective communication with individuals with disabilities.
(c) If a provision of a particular auxiliary aid or service would result in a fundamental alteration in the nature of the goods, services, facilities, privileges, advantages, or accommodations being offered or in an undue burden, you shall provide an alternative auxiliary aid or service, if one exists, that would not result in a fundamental alteration or undue burden but would nevertheless ensure that, to the maximum extent possible, individuals with disabilities receive the goods, services, facilities, privileges, advantages, or accommodations you offer.
(d) As a PVO, it is your responsibility, not that of a passenger with a disability, to provide needed auxiliary aids and services.