The other big factor, says VanAmburg, is the industry's steady stream of mergers, which often wreak havoc on customer satisfaction as airlines struggle to merge staffs, schedules and IT systems.
As an example, he cites the 2010 Delta/Northwest merger, which led to a "nosedive" in Delta's score (56) in 2011. Having worked out the kinks, the carrier has been on an upswing ever since, scoring 71 this year.
United maintains its last-place position with a score of 60 as the airline continues to struggle to absorb Continental. "There are a lot of disgruntled passengers flying United, particularly because [the merger] is taking so long," said VanAmburg. "They're three years into it and there are still so many hiccups."
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However, United spokesman Rahsaan Johnson pointed out that the airline has reduced complaints year-over-year and improved on-time performance and overall reliability. "United is investing significantly to provide employees the tools to deliver great service and improve our customers' experiences," he said.
As the American Airlines-US Airways merger moves forward, VanAmburg said, there also may be turbulence ahead
"We can't say that American will drop next year," he said, "but it's more likely than not that they're going to run into the same challenges that we've seen in every other airline merger to date."
—By Rob Lovitt of NBC News