How should I address a woman who serves food and drinks to passengers on a plane?

In the US at least, both have fallen out of favor in recent years for a couple of reasons: because there are more men working in the field than there used to be, and because of a general trend towards less gender-specific occupation titles. The more widespread term now is flight attendant, for both men and women.

They are called flight attendants, and you must not assume it will be a woman.

Other terms, such as “air host/hostess” and “steward/stewardess” are quite dated. I would advise against using them. And if you were to use them to address a flight attendant, you will be lucky if you get any service at all, because of the offense they would cause.

Furthermore, since it is generally considered pretty rude to address a person by their occupation—many servers in restaurants, for example, hate being called “waiter” or “waitress”—I would advise addressing a flight attendant with “sir” or “ma'am” if you need to use an address at all. “Excuse me” should be sufficient to get a flight attendant’s attention when necessary.

I think the industry term is cabin crew (as opposed to flight crew)

...although in America they are probably 'in-flight refreshment facilitation operatives".

While I agree with the other answers, if I was writing a period piece, I would use stewardess or air hostess. I'm not sure what time period air hostess was from, but it feels more 60's to me.

I agree with phenry's answer (and those of the others) about both terms being offensive of late, and obsolete as well. But given the fact that the question was asked from China, I thought I might as well put forth my opinion.

I believe there was a major difference between a stewardess and an air hostess.

I remember being taught that a stewardess is the lady who is in charge of preparing and distributing food in the aeroplane.

An air hostess on the other hand is the lady who is in-charge of welcoming you onto the aircraft, providing you with your pillows and blankets, in charge of your safety (remember the original air hostess served as medical nurses abroad international flights) and any other enquiries you might have.

However, like everyone pointed out, this is pretty obsolete now (as has the system of unwed airhostess) and everyone is known as a flight attendant or cabin crew nowadays. And they usually end up taking care of both the activities mentioned above.