This was a pig that truly could not fly.

The pig was ordered off a US Airways plane at Bradley International Airport in Connecticut after crewmembers decided the animal had become disruptive.

Laura Masvidal, a spokeswoman for US Airways parent American Airlines, said yesterday the pig was brought aboard by a passenger as an emotional support animal.

She said both the pig and its owner left the aircraft before it took off.

Jonathan Skolnik, a University of Massachusetts professor who was also a passenger, told ABC News that he initially thought the female passenger was carrying a duffel bag.

"But it turns out it wasn't a duffel bag. We could smell it and it was a pig on a leash," he said.

The woman sat in an empty seat next to him and tethered the pig - which he estimated weighed 50-70 pounds - to the arm rest, but the animal began walking back and forth, Prof Skolnik said.

Ms Masdival says under US department of transportation rules emotional support animals are allowed on commercial flights as long as they are not disruptive.