Delta uniforms from the late 1960s. (Source: Delta Air Lines)
United Airlines requires separate heels of at least an inch but no more than three inches, or flats with pants only. In-flight shoes must have a heel of at least one-quarter of an inch, according to the Association of Flight Attendants, the labor union representing United's flight attendants. On Alaska Airlines, flight attendants' heels can also be no higher than three inches, and flats are acceptable once the cabin door is closed, the airline said.
Correction: An earlier version of this story said Alaska Airlines' maximum heel height is four inches. It is three inches. It also referred to the color of Delta's uniforms as "passport purple" instead of "passport plum" and the labor union representing United's flight attendants is the Association of Flight Attendants, not the Association of Professional Flight Attendants.