Oil prices have fallen by about half since June, making it much cheaper just in time for drivers to fill up their cars for Christmas travel. But the decline has had no perceptible difference on the cost of flying. Fares are up, and many airlines are still levying significant fuel surcharges on tickets.

There are many reasons airlines have not lowered fares to reflect the decline in oil prices. Some companies are still paying high prices for fuel because they have to abide by long-term contracts. But the biggest reason is that they don't have to.

Demand for air travel is strong, and a series of megamergers has significantly reduced competition. The four biggest U.S. airlines — Delta, Southwest, United and American — control about 80 percent of capacity, down from 11 companies as recently as 2005. For most travelers, that means higher prices and jam-packed planes.

After suffering through years of losses and bankruptcies, airlines are prospering. The International Air Transport Association says the industry's profits will grow by about 26 percent, to a record $25 billion, next year. The association says North American airlines now have profit margins higher than in the 1990s, before the 9/ 11 attacks devastated the industry.

As they have become more financially sound, airlines have increasingly flexed their political muscle. Earlier this year, airlines were pushing Congress to change how they are allowed to advertise fares. Currently, airlines are required to tell customers the total cost of flights, including taxes. But the industry wants to advertise pretax airfares so consumers would see the total cost of a ticket only at the time of purchase.

A measure, approved by the House in July, called the Transparent Airfares Act of 2014, would do exactly this, making it easier for airlines to mislead travelers about the true costs. The Senate has not voted on it, but the bill has a chance of becoming law next year when Republicans take control of both houses of Congress.