WASHINGTON (AP) -- Airlines generally arrived later, lost more luggage and caused more consumer complaints in 2004 than they did the year before, an annual study of aviation quality finds.
Only four of the 14 major airlines rated in both 2003 and 2004 improved, according to the report released Monday. They were 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 federal Bureau of Transportation Statistics.
Headley said the aviation system is also being taxed because more planes and more people are flying than they did in the two years after the Sept. 11, 2001, attacks. However, 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 terrorists hijacked planes and flew them into buildings.
On-time performance worsened last year, with 78.3 percent of flights arriving on time, down from 82 percent in 2003. Skywest was on time the most, American Eagle the least.
The problem of deteriorating airline service came to a head over the Christmas holidays, when delays and cancellations inconvenienced more than 500,000 passengers. Regional carrier Comair canceled all its flights during the holiday weekend, and US Airways' baggage system failed.
A report by the Transportation Department's inspector general blamed bad weather and computer problems at Comair, and faulted poor planning and poor labor relations at US Airways.
''In a simpler system they seem to be able to do this better,'' Headley said. ''When it's more taxed, the quality goes down.''
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, and Southwest the least.
Last year, 4.83 bags were lost, stolen or damaged for every 1,000 passengers. Atlantic Southeast had the highest rate of mishandled bags, while AirTran had the lowest.
There was a small increase in the number of passengers ''bumped,'' or denied boarding for space _ 0.87 passengers per 10,000 boardings in 2004, up from 0.86 per 10,000 in 2003.
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 was co-authored by Brent Bowen, director of the University of Nebraska's aviation institute, and was based on Transportation Department statistics.
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The 18th annual Airline Quality Rating showed that domestic carriers' performance dropped in on-time arrival, involuntary bumping, mishandled luggage and customer complaints, compared with 2006.
Complaints increased 17 percent last year over 2004 and the rate of mishandled baggage jumped from 4.83 per 1,000 passengers to 6.06 in 2005.