An economics professor says his flight was delayed because a fellow passenger thought the equations he was writing might be a sign he was a terrorist.

American Airlines confirmed on Saturday that a woman expressed suspicions about a University of Pennsylvania economics professor, Guido Menzio. She said she was too ill to take the Air Wisconsin-operated flight.

Menzio was flying from Philadelphia to Syracuse on Thursday to give a talk at Queen's University in Ontario, Canada. He was solving a differential equation, but said he was told the woman thought he might be a terrorist because of what he was writing.

American spokesman Casey Norton said the crew followed protocol to take care of an ill passenger and then to investigate her allegations. They determined them to be non-credible, he said.

On Facebook, Menzio recounted the "unbelievable" experience in the present tense. "The passenger sitting next to me calls the stewardess, passes her a note."

The plane, ready to take off, then returned to the gate and the passenger left. Menzio was then asked to disembark the plane and "met by some FBI looking man-in-black".

"They ask me about my neighbor," he wrote. "I tell them I noticed nothing strange. They tell me she thought I was a terrorist because I was writing strange things on a pad of paper. I laugh. I bring them back to the plane. I showed them my math."

Menzio told the Washington Post that he was "treated respectfully throughout" the process but remains perturbed by a system that "relies on the input of people who may be completely clueless".