CHICAGO -- Passenger dissatisfaction with airlines' customer service has sunk to its lowest level in years at a time when carriers are charging more and more for tickets and services.

An annual survey being released Tuesday by the University of Michigan found customers giving airlines the worst grades since 2001, with the industry's overall scores dropping for the third straight year.

United Airlines and US Airways Group Inc., which are in talks to potentially combine into a single carrier, finished next-to-last and last, respectively, in the university's American Customer Satisfaction Index.

Continental Airlines Inc. and US Airways Group Inc. registered the biggest declines from 2007, both experiencing double-digit percentage drops.

A familiar bright spot in the results was Southwest Airlines Co., which led the industry in passenger satisfaction for the 15th consecutive year.

While unhappiness with airlines is nothing new, this year's survey produced "really dismal numbers," said Claes Fornell, a University of Michigan business professor and director of the research center that compiled the data.

"There's no other industry anywhere that has so many basic mishaps in terms of not delivering the basics," he said.

"They're supposed to deliver passengers with their luggage to a particular destination within a certain time frame, and they frequently fail to do that."

He said airlines' management has to be blamed despite some factors beyond their control, such as higher jet-fuel costs and congested airports.