Probably no higher than 3or 4" allowed, but many, if not most airlines, US and foreign have similar policies. They're called "concourse" shoes and must be worn through the terminal and on plane during boarding. One of my airlines wanted us to wear them until we got up to do service, which was total bs, as I agree it's dangerous during an evacuation. My current airline encourages us to remove heels and wear flat as soon as we board.

I love the way heels look, but my feet have issues and I have a doctor's note to wear flats at all times. I do, however, wear them during boarding and my demo as I like being taller and love the way they look.

BTW, isn't this Airline PILOT Central? Why are you talking about FAs?? (at least that's what you're going to get for comments from the boys.)