I have no flown with a pet in cabin before; however, I am considering doing so on DL this fall with a 22# Boston Terrier. This is my experience with Delta but, I was told the size of the carrier depends on the aircraft equipment we're flying so I called DL directly and they gave me the dimensions. Now I just hope they don't change the equipment! (I am using a "sturdi Products" size large for my doggie as research from Amazon indicated it was a better quality than the Sherpa brand.) From all my travels, I do know that the underneath portion of each seat is not the same. Typically the under seat storage for the middle seat (in a 3 across in coach section) is wider than the aisle or window seat. I'm concerned that the pet carrier may not easily fit under the aisle seat but I'm hoping it will and, if not, I'm hoping that the person stuck in the middle will be glad to trade seats with me! We shall see. (My biggest concern now is getting a chance to have the dog relief herself during our short connection in ATlanta!)

I strongly recommend you call AC and tell them the equipment you are flying on and ask them what is the largest size pet carrier you can use.

Good luck! I hope my experience helps.