You probably wouldn't know it from reading the Internet, but most U.S. airline passengers are generally happy with the service they receive, although shrinking seats and lengthening airport security lines could eventually cut into their contentment.

A new survey conducted for Airlines for America, the primary industry trade group, indicated that 80% of people who flew in 2015 were satisfied with the experience, and 35% were very satisfied. Just 5% were somewhat dissatisfied, and 1% were very dissatisfied.

The Ipsos research firm, contracted by A4A, conducted an online survey of 3,019 people, randomly drawn from its research panel. "The air travel experience has typically been defined by anecdotes," A4A economist John Heimlich told reporters on a conference call to discuss the survey, which was released Tuesday.

Two particularly satisfied groups were composed of people enrolled in the TSA PreCheck program -- 49% of them were very satisfied -- and people enrolled in Customs and Border Protection Global Entry Program, of which 67% of them were very satisfied.

Passengers must sign up and pay fees for the programs, which enable quicker screening in dedicated security lines. PreCheck costs $85 to $100 for five years, while Global Entry costs $100. Of the survey respondents, 19% were in PreCheck and 13% were in Global Entry.

The survey also measured what factors are important in making the decision to travel by airline, as opposed to other modes of travel or not traveling at all.

Regarding the flight experience, among respondents who flew in 2015, 89% said legroom and seat comfort were most important, 79% said space for carry-ons was most important and 51% said food and drink options were most important.

Unfortunately, airline seat size has generally been shrinking. Average legroom, or pitch, has shrunk from about 35 inches in the 1970s to nearly 31 inches today, U.S. Senator Chuck Schumer (D.-N.Y) said last month in a prepared statement.

Passengers expect coach seats to have 28-inch pitch in ultra-low-cost airlines such as Spirit (SAVE) - Get Report and Frontier, but recently major airlines have also been reducing pitch. For instance, American (AAL) - Get Report , Delta and United all have seats with 30-inch pitch in Airbus A319 aircraft, according to

American also has 31-inch pitch in economy class in its Boeing 787s, which are typically used for long international flights. United has 32-inch pitch in economy class in its Boeing 787s.

Heimlich noted that airlines offer "segmentation of cabin and more diversity in models," enabling passengers to purchase the type of seats they want.

Regarding the preflight experience, among respondents who flew in 2015, 76% said the speed of getting through security was most important, 73% said the efficiency of the boarding process was most important, and 40% said free airport Wi-Fi was most important.

With summer travel looming, security lines have been growing longer at major airports, a result of insufficient staffing by the Transportation Security Administration.

Not only has the number of travelers increased, while the amount of funding provided by Congress has not, but also TSA assumed that more passengers would sign up for PreCheck.  However, as of March 1, only 9.3 million people had signed up.

"There is nothing satisfying about chokepoints," Heimlich said. "We would love to see more enrollment in those expedited screening programs."

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.