Q. I've noticed that if I fly to Europe using a "free" frequent flier ticket, the taxes and fees vary depending on the airport I connect through. For example, if I connect through London on my way to Venice the taxes added to the "free" ticket are higher than if I connect through Madrid. What is going on here?

A. As you've discovered, when you cash in miles for a frequent flier ticket it's not exactly free. The airlines sometimes add fuel surcharges, which goes to their bottom line, but they also tack on taxes. Some of these taxes go into the coffers of the U.S. government, but the largest fees, often called "passenger service charges," or simply "airport taxes" are charged by foreign airports. And there are sometimes as many as three different taxes. Zurich Airport charges a  $59 "noise cancelation fee" and Amsterdam a $6 "noise isolation surcharge." A Paris airport might charge $90 in extra fees, whereas Madrid might charge $34. London has particularly high "duties" that must be paid, and they vary depending on the class (economy, first or business) that you're flying in. And that's each way, so it really adds up.

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