Major U.S. airlines’ “going to see Grandma” surcharges are piling up this holiday season, and my, what big teeth they have.

Continental Airlines and American Airlines are among the carriers to expand surcharges for flights on peak travel dates from now through May, according to airline representatives.

“Basically, it’s a way of the airlines making more money off of travel days that they know are going to be in high demand,” said Graeme Wallace, chief technology officer of FareCompare.com.

The major carriers started adding surcharges in September, after American Airlines designated three peak holiday travel dates. Other major airlines and American soon extended the surcharges to more dates.

Recently, most of the major airlines have added a new wave of designated dates and upped the charges.

United Airlines, Delta Air Lines and its Northwest Airlines subsidiary are charging $10, $20 or even $30 above the published base fare on designated dates, said Ryan Berryman, senior vice president of Oversee.net, parent of travel search portal LowFares.com. Continental has similarly expanded its surcharge dates, spokeswoman Mary Clark said.