Source: Flickr user Mary-Lynn. 

Trying to decipher which airline is Americans' least favorite is like asking what color sky people prefer. Generally speaking, flying is a necessary evil -- if you want to travel anywhere beyond 300 miles in a relatively quick fashion, you don't have many other choices. But, the airport and airplane experience itself leave a lot to be desired for consumers. Between the lines, the security checkpoints, cramped airplane seats, and a growing list of optional fees, the experience of flying is downright unpleasant for many.

According to the latest survey from ACSI, which measures consumer satisfaction of 44 separate industries, the airline sector finished third from the bottom with a satisfaction rating of 69. The amazing thing is that the industry's satisfaction rating, which is unchanged from the previous year, is still tied for its highest mark since 1996.

Why airline customer satisfaction is tied for an 18-year high
What's caused airline satisfaction to "improve" since hitting a low of 61 in 2001? I'd suggest a number of factors have been at work.

Source: Andrew Pilloud via Flickr.

Obviously, the tragic hijacking events in 2001 that resulted in a dramatic increase in airport and in-flight security were a shock to travelers and likely resulted in a major dip in satisfaction. As time has passed and the screening technology in airports has improved, I'd suggest consumers have been more accepting of security measures, resulting in higher overall satisfaction.

Another big improvement that has taken off in recent years is the introduction of online reservations. Between kiosk check-ins at the airport to buying and printing your ticket at home, the hassle of waiting in line at the airport to drop off a bag or print a boarding pass has been reduced, resulting in happier customers.

Finally, consumers have also had a few years to adjust to baggage fees, which were first introduced by American Airlines, now part of American Airlines Group, in 2008. Similar to heightened security measures, baggage fees aren't necessarily welcome, but I'd opine that their shock and awe has begun to wear off for the consumer. 

America's least favorite airline
Let's face it: Not every airline is performing up to the expectations of consumers (as you probably presumed from an industrywide satisfaction rating of 69).

Of the airlines studied by ACSI, there was a discernible difference between national carriers like JetBlue (NASDAQ:JBLU) and Southwest Airlines (NYSE:LUV) and the major airlines. Per ACSI, JetBlue and Southwest finished with satisfaction scores of 79 and 78, respectively, which was actually down 5% and 4% from the prior year. Not surprisingly, both JetBlue and Southwest are the only two airlines that'll let you check your first bag for free (Southwest actually allows your first two bags to fly for free). These national airlines, despite growing, continue to retain their "smaller" feel, which I believe fliers appreciate.

On the other side of the column were the nation's major airlines, which were hurt by a number of factors ranging from baggage fees to on-time arrivals. The highest ranking major was Delta Air Lines (NYSE:DAL), with a score of 71. However, bringing up the caboose as the nation's most hated airline with a score of just 60, down 3% from the previous year, was United Airlines, part of United Continental Holdings (NASDAQ:UAL).

Why United is bringing up the rear
Why United? According to ACSI, the primary problem with United has been the slow integration of Continental Airlines. Although the merger occurred three years ago, both airlines are still struggling with reservations and refunds, resulting in unhappy customers.

As the ACSI notes, there's a precedence to this weakness that became apparent in its survey results in 2011 with Delta, when it bought Northwest Airlines. In that year, Delta saw its satisfaction score dip to just 56 from 62 in the prior year as integration issues hampered consumers' views of the company. As a consolation, however, Delta's satisfaction score has rebounded in a big way since 2011. United has witnessed no merger-related bounce whatsoever. In other words, mergers can create a dip in satisfaction due to integration issues, but not all mergers are long-term drivers of unhappy customers, as has been the case with United. 

Source: Elder Chaves via Flickr.

Optional costs are another sticking point for United. No consumer wants to pay optional fees if they don't have to, and certain optional fees of United's are quite steep. A ticket change, for example, could run a passenger as much as $200 with United, with an onboard pet in the cabin running an additional $125. With the exception of freebies like carry-on bags and seat selection, United tends to charge its passengers for most everything else.

Finally, it's worth pointing out that United Airlines has also had the highest number of boarding denials of all the airlines that ACSI examined. United, on average, denies 246 out of every 1 million passengers, compared to JetBlue, whose passenger boarding denial rate was less than four out of every 1 million passengers. Though United may very well have its reasons for this high boarding denial rate, it's also an easy way to tick off your consumer base.

It remains to be seen if United will ever realize significant benefits from its merger with Continental. But for the time being, it's evident that the company could struggle to gain or retain market share as long as consumers continue to view United as the nation's most hated airline.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.