The American Society for the Prevention of Cruelty to Animals actually cautions against flying pets too large to take with you in an under-seat carrier. But if air travel with a larger pet is unavoidable, there are a number of precautionary steps you can take to ensure that your animal companion arrives safe and sound.
Your veterinarian can make certain your pet is healthy enough for air travel and that vaccinations are current, as well as providing a health certificate dated within 10 days of departure. If you're traveling outside the continental United States, there may be additional requirements, so make sure you leave enough time to plan and obtain needed documentation. If your pet doesn't have a microchip, your vet can take care of that, too. Make sure your pet's collar ID tags are up-to-date, and add a collar tag with information on your destination in case your pet escapes.
Your pet-travel checklist should include getting an appropriately sized USDA-approved shipping crate. The ASPCA recommends writing a variety of information directly on the crate, including destination details, and attaching a photo of your pet for identification purposes in case he escapes, or gets lost or misdirected. Book a direct flight, if possible, to lessen the chances your pet will be mishandled or not end up at the right destination.
Every step of your journey, both on the ground and on the plane, tell airline personnel that you're traveling with a pet in the cargo hold. Once they know, you'll be better able to ask about your pet's status during your trip. Your pet is relying on you to be vigilant about his safety!
People often wonder if it's better to tranquilize a pet for air travel. We generally don't recommend it, since it can make it harder for them to breathe.
For much more information on this topic, visit the ASPCA's site at aspca.org/pet-care/air-travel-tips.