A soft sided bag fits more easily under the seat. In my experience, soft carriers are less likely to be measured because they look smaller. Be sure to make your pet reservation early as there's a limit on the number of pets allowed on a flight.
See below for Air Tran info
Traveling with a Pet
Baggage policies
Small, domesticated cats, dogs and birds that will fit in an approved carrier under the seat are accepted.
•The pet kennel is limited to maximum dimensions of 8.5" high x 17" long x 12" wide.
•The carrier must be leak proof and ventilated on at least two sides.
•There is no weight restriction on pets.
•The pet must remain in the carrier and under the seat for the duration of the flight.
•The pet carrier counts as either a passenger's one (1) carry-on bag or personal item.
Airfare for a pet is $69 one-way and is non-refundable. Six (6) pets are allowed per flight with a limit of one (1) pet per paying customer traveling on the same flight.
As limited space is available, advanced reservations must be made. Of course, service animals (e.g., Seeing Eye dogs) are always welcome onboard regardless of the number of pets already booked on the flight.
Customers traveling with birds to San Juan, Puerto Rico must present required documentation — a certificate and a permit.
•Certificate — The certificate must be issued by a veterinarian within 30 days of travel
•Permit — The permit is granted by the Department of Agriculture in Puerto Rico. Request a permit by telephone at 787-796-1650 or 787-796-1835.
For additional information about traveling with a bird, contact your veterinarian, the US Department of Agriculture or your destination state's Department of Agriculture.
For additional information about traveling with a pet or to book pet travel, please contact the AirTran Airways reservations desk at 1-800-AIR-TRAN (1-800-247-8726) or 678-254-7999 for international calls.
In many airports, AirTran Airways sells pet kennels for a nominal fee at the ticket counter. AirTran Airways does not transport pets as checked baggage.