Bart Jansen

USA TODAY

WASHINGTON – Cheap fuel prices juiced record profits for U.S. airlines last year even as customer complaints soared over cramped flights and mounting fees.

The 25 U.S. passenger airlines logged a record $25.6 billion in profits in 2015, more than three times the industry's after-tax earnings of $7.5 billion reported in 2014, the Transportation Department said Monday. Fuel prices averaged 35% lower in 2015 than the previous year.

The average fare of $377 in 2015 is down 3.8% from 2014 and down 19.2% from the inflation-adjusted average of $467 in 2000, Transportation Department records show.

But baggage fees added $3.8 billion to the bottom line, and fees from reservation changes chalked up another $3 billion, the department said. Airlines do not report other fees to the department.

While airlines' financial books look robust, passengers grumble about lost bags, deceptive ticket prices, poor customer service and shrinking seats and legroom. Travelers filed 20,170 formal complaints last year, up from 15,539 in 2014, according to the department’s Air Travel Consumer Report.

Airline complaints spike even as service improves

Airlines have invested $1.4 billion a month in new aircraft and equipment while paying down $8 billion in debt last year and returning $10.5 billion to shareholders last year, according to Melanie Hinton, spokeswoman for Airlines for America, an industry group representing most of the largest carriers.

"In short, this industry is working as well as it ever has before – to the benefit of the 2.2. million passengers who fly on U.S. airlines every day," Hinton said.

The investments haven't helped consumers, said Charles Leocha, a founder of the consumer-advocacy group Travelers United. The new planes have more seats and less legroom to eke out more money from each flight, he said.

“Everybody keeps telling us that we’re seeing all these improvements, but nobody’s seeing them," Leocha said. “New airplanes don’t help us when the planes are bigger with more seats on board and they’re squeezing more people into them."

Trey Bohn, executive director of the group Travelers’ Voice, said the $7 billion in baggage and reservation fees draw the most ire from travelers.

“Depending too much on revenue from these fees is not only an operating weakness, it also suggests to travelers that the nickel and the dime are more important than improving their product,” Bohn said.

Passengers with their knees pressed into the seat in front of them won't get any help from Congress this year. A House committee and the full Senate each rejected proposals to force Federal Aviation Administration to set minimum standards for the distance between seats.

Airlines to report 'blowout' record profits amid low gas prices, higher fees

The sixth consecutive year of profits came after a decade of economic struggles for airlines following the terrorist attacks in New York and Washington on Sept. 11, 2001, and the economic downturn in 2007. The industry lost $28.7 billion in 2005 and $24.5 billion in 2008.

The earnings last year were the highest at least since Congress deregulated the industry in 1978. The industry's previous peak earnings came in 2006 at $16.5 billion.

The earnings came on a combined $168.9 billion in revenue. The combined expenses totaled $140.9 billion, which included $45.4 billion for labor and $27 billion for fuel, the department said.