JetBlue was once again ranked as offering the best service in an annual survey that also found five of the top six airlines are low-fare carriers.

AirTran Airways, Southwest Airlines and United rounded out the top of the national Airline Quality Rating study, released Monday.

"JetBlue probably has the right mix of services and management to maintain and secure this top position," said Brent Bowen, a co-author of the report and director of the University of Nebraska's aviation institute. The study is based on Transportation Department statistics.

With Alaska Airlines and America West in the five and six spots, respectively, United was the survey's only top-ranked airline that is not a low-fair carrier.

SkyWest, Comair and Atlantic Southeast Airlines appeared at the bottom of the list of 16 U.S. airlines.

Airlines generally arrived later, lost more luggage and caused more consumer complaints in 2004 than they did the year before, the report found.

Only four of the 14 major airlines rated in both 2003 and 2004 were found to have improved -- AirTran, Atlantic Southeast, JetBlue and United.

Airline service is getting worse because more people are flying at a time when carriers have slashed their work forces, said Dean Headley, a co-author of the study and associate professor at Wichita State University.

"Morale's going to be down and they're not going to care if they get the bags to the loading dock in five minutes, 10 minutes or 15 minutes," Headley said.

The seven largest carriers, for example, employed 12 percent fewer people in January 2004 than they did the year before, according to the Bureau of Transportation Statistics (search), which is part of the Transportation Department.

Headley said the aviation system also is under stress because more planes and more people are flying than they did in the two years after the hijackings on Sept. 11 , 2001, But the aviation infrastructure -- runways, airport slots and the air traffic control system -- is essentially the same as it was in the delay-plagued era just before the terrorist attacks.

Overall rankings by airlines were not being released until the study's complete findings were made public at a news conference Monday in Washington.

On-time performance worsened last year, with 78.3 percent of flights arriving on time, compared with 82 percent in 2003. Skywest (search) was on time the most, while American Eagle (search) was on time the least.

Complaints about airline service rose 27 percent last year, a much higher increase than the 3.3 percent growth in passengers. US Airways generated the most complaints, Southwest the fewest.

Last year, 4.83 bags were lost, stolen or damaged for every 1,000 passengers. Atlantic Southeast had the highest rate of mishandled bags, AirTran the lowest.

The report rated the 16 U.S. airlines that carried at least 1 percent of the 630 million passengers who flew domestically last year. Two carriers, Comair and SkyWest, met that threshold for the first time in 2004.

The report, compiled annually since 1991, is a summary of monthly quality rating for U.S. airlines that have at least 1 percent of domestic passenger volume during 2003. Categories include on-time arrivals, involuntary denied boardings, mishandled baggage and a combination of 12 customer complaint categories.

Headley assembled the report with Brent Bowen, director of the University of Nebraska's aviation institute.