The golden age of air travel has long since passed, and it’s not coming back any time soon, if consumer complaint data released this week by the U.S. Department of Transportation are any guide.

More than 20,000 air travelers filed complaints in 2015, an increase of 30 percent over the previous year. And at a time when low fuel prices are helping U.S.-based airlines record fat profits, complaints about fares nearly doubled to more than 1,800.

American Airlines — the nation’s largest carrier — received the most complaints with 3,983 passengers complaining about its service. United — the most-complained-about airline in 2014 — was in second place with 2,721 complaints.

But low-cost carrier Spirit Airlines had, by far, the most complaints per passenger, recording nearly 12 complaints for every 100,000 passengers who got on one of its flights. By contrast, Southwest Airlines had just half a complaint per 100,000 passengers, a feat bettered only by Alaska Airlines.

On average, domestic carriers received just under two complaints per 100,000 passengers.

While the price of an average airfare fell by 6.2 percent to $372 between the third quarter of 2014 and the third quarter of 2015, the most recent period for which Department of Transportation data are available, the “big four” U.S. carriers (American, Delta, Southwest and United) made profits of $22 billion last year, an increase of more than $15 billion over 2014.

Melanie Hinton, a spokeswoman for the airline industry group Airlines for America, said that despite the increase in complaints, the total number of complaints remains low relative to the number of air passengers.