The applicable law for airline travel is Sec. 382 of the Airline Carrier Access Act. It provides as follows: § 382.117 Must carriers permit passengers with a disability to travel with service animals.
(a) As a carrier, you must permit a service animal to accompany a passenger with a disability
(1) You must not deny transportation to a service animal on the basis that its carriage may offend or annoy carrier personnel or persons traveling on the aircraft
(2) On a flight segment scheduled to take 8 hours or more, you may, as a condition of permitting a service animal to travel in the cabin, require the passenger using the service animal to provide documentation that the animal will not need to relieve itself on the flight or that the animal can relieve itself in a way that does not create a health or sanitation issue on the flight.
(b) You must permit the service animal to accompany the passenger with a disability at any seat in which the passenger sits, unless the animal obstructs an aisle or other area that must remain unobstructed to facilitate an emergency evacuation.
(c) If a service animal cannot be accommodated at the seat location of the passenger with a disability who is using the animal, you must offer the passenger the opportunity to move with the animal to another seat location, if present on the aircraft, where the animal can be accommodated.
(d) As evidence that an animal is a service animal, you must accept identification cards, other written documentation, presence of harnesses, tags, or the credible verbal assurances of a qualified individual with a disability using the animal.
(e) If a passenger seeks to travel with an animal that is used as an emotional support or psychiatric service animal, you are not required to accept the animal for transportation in the cabin unless the passenger provides you current documentation (i.e., no older than one year from the date of the passenger's scheduled initial flight) on the letterhead of a licensed mental health professional (e.g., psychiatrist, psychologist, licensed clinical social worker, including a medical doctor specifically treating the passenger's mental or emotional disability) stating the following:
(1) The passenger has a mental or emotional disability recognized in the Diagnostic and Statistical Manual of Mental Disorders—Fourth Edition (DSM V );
(2) The passenger needs the emotional support or psychiatric service animal as an accommodation for air travel and/or for activity at the passenger's destination;
(3) The individual providing the assessment is a licensed mental health professional, and the passenger is under his or her professional care; and
(4) The date and type of the mental health professional's license and the state or other jurisdiction in which it was issued.
(f) You are never required to accommodate certain unusual service animals (e.g., snakes, other reptiles, ferrets, rodents, and spiders) as service animals in the cabin. With respect to all other animals, including unusual or exotic animals that are presented as service animals (e.g., miniature horses, pigs, monkeys), as a carrier you must determine whether any factors preclude their traveling in the cabin as service animals (e.g., whether the animal is too large or heavy to be accommodated in the cabin, whether the animal would pose a direct threat to the health or safety of others, whether it would cause a significant disruption of cabin service, whether it would be prohibited from entering a foreign country that is the flight's destination). If no such factors preclude the animal from traveling in the cabin, you must permit it to do so. However, as a foreign carrier, you are not required to carry service animals other than dogs.
(g) Whenever you decide not to accept an animal as a service animal, you must explain the reason for your decision to the passenger and document it in writing. A copy of the explanation must be provided to the passenger either at the airport, or within 10 calendar days of the incident.