United among carriers with most complaints in the US

UNITED STATES - Bigger isn't necessarily better when it comes to airlines. American and United, the two largest US carriers, are also tops in customer complaints, a study released by the US PIRG Education Fund shows.

The consumer advocacy group found that the two carriers ranked in the top four for complaints for 2011 through 2013, based on Department of Transportation data. The worst performer was Spirit Airlines, which had the most complaints among 13 US carriers each year in the study which spanned 2009 through 2013.

Southwest Airlines, the fourth largest US carrier by traffic, had the lowest customer complaint scores for each of those years. United Continental Holdings ranked second in complaints in 2011 and 2012, and dropped to third in 2013, the study shows. American Airlines was fourth in 2011, third in 2012, and fourth in 2013.

At United, complaints spiked in 2012 to 4.28 per 100,000 passengers, its highest in the five years studied, after technology changes during the merger with Continental caused outages and led to poorer service. United's complaint rate improved to 2.14 in 2013.

United spokeswoman Megan McCarthy said customer satisfaction has improved as the airline focused on enhancing service. Spokesman Matt Miller said American, under new management, has improved in on-time performance and mishandled bags.