I capitalized "Airlines" for a reason, but I would guess it's pretty representative for the country. The reason your number and my number are different is not because AA is an unusual airline, but because the two numbers are about completely different things. There are three groups of Americans: (A) those that don't fly in a given year, (B) those that fly once, and (C) those that fly multiple times. Roughly, your number is A/(A+B+C) while mine is B/(B+C).