Like thousands of American Airlines passengers last Dec. 29, Kate Hanni and her family were stuck aboard a jet for hours, out on the runway. They were hungry, bored, mad and, in the case of Flight 1348, sick of the smell wafting through the cabin from the lavatories.

When the ordeal finally ended, some passengers from the 67 separate American flights — which each spent at least three hours stranded — e-mailed or called in their complaints to the airline. Some vented on blogs. Most grumbled and went about their business.

And the airline industry thought it would, too.

Ms. Hanni (pronounced HAN-eye), who said she had never even written a letter of complaint in her life, decided she would get a law passed making lengthy confinement on an airplane illegal.

“I was fuming,” she said. “It was imprisonment.”

She thus became an unlikely and, thus far, powerful adversary to an industry accustomed to riding out its major service lapses with only the lightest of government scrutiny.

A successful real estate agent, occasional rock ’n’ roll singer and mother of two, Ms. Hanni, 47, essentially put her life on hold to take on the airlines...