Air Carrier Access Act Technical Assistance Manual (2005)

You must allow service animals to accompany passengers with disabilities in the cabin consistent with FAA safety requirements. You must allow the service animal to sit in close proximity to its user, as long as the service animal does not block the aisle or other emergency evacuation route in violation of FAA safety regulations. Often this will mean that the service animal will sit under the seat in front of the disabled passenger to avoid obstructing an aisle or other space. Some service animals are held by their users in their arms as an adult would hold a human infant (limited to infants under two years of age) of roughly the same size. [Sec. 382.55]