More people are expected to take to the skies during the Christmas holiday period, continuing the surge in travel demand that is projected to generate strong profits for U.S.-based airlines, according to industry forecasts.

During the 21-day Christmas travel season, 51 million passengers are expected to fly on U.S.-based airlines, representing a 3.5 percent increase over the same period last year, according to a forecast by Airlines for America, the trade group for the country’s carriers.

Travel demand has been growing steadily for the last four years, partly because of an improving economy and competition from low-cost carriers that has kept airfares relatively low, said John Heimlich, the chief economist for the trade group.

“Intense competition within the airline industry is enabling passengers to choose the flights that most closely match their preferences and budget,” he said in a statement.

The busiest days for travel are expected to be Dec. 21, Dec. 22 and Dec. 26, the trade group said.

Carriers in North America are expected to pocket $16.4 billion in profit next year, up 5 percent from 2017, according to a forecast issued Tuesday by the International Air Transport Assn., the trade group for the world’s airlines.

North American airlines have been responsible for more than half the profit for the world’s airline industry over the last three years, the forecast said.

Despite the healthy profit, the trade group warned, rising fuel and labor costs will put pressure on airlines in North America. The trade group also warned that airlines can face trouble if governments fail to adopt global security standards, reasonable levels of taxation and improvements to airports to accommodate the growing demand.