U.S. airlines are earning billions, and they're collecting more in fees on checked bags and reservation changes.

Whether airlines are making more or less money than before depends on which figures you use.

The Department of Transportation said Monday that airlines collected $3.5 billion in bag fees last year, a 5 percent increase over 2013, and $3 billion in reservation-change fees, a 6 percent hike.

Fees began escalating in 2008, when airlines were losing money and facing a sharp rise in fuel prices. Fees are still around, and they make up a growing share of airline revenue.

At Spirit Airlines, which touts low fares and adds lots of fees, only 63 percent of its revenue comes from fares. Southwest still lets customers check two bags or change a reservation for free; it gets 95 percent of revenue from the ticket price.

Net income at the 27 airlines counted by the government fell to $7.5 billion last year from $12.2 billion in 2013. However, net income can include one-time gains or losses, and analysts usually prefer to look at operating profit.

On that basis, the airlines did even better in 2014 than 2013 - pre-tax operating profit rose to $14.6 billion from $11.3 billion.

One carrier, Delta Air Lines, accounted for more than the entire industry's decline in net income because it scored a one-time tax gain of $8 billion in 2013. That caused net income to plunge from $10.54 billion to $649 million in 2014.

But take away the 2013 tax gain and 2014 losses on fuel-hedging contracts, and Delta saw just a modest decline in pretax operating profit - $2.93 billion last year, compared with $3.84 billion in 2013.

Other than Delta, both net income and operating profit rose at all the other leading airlines - American, US Airways, which is now part of American, United and Southwest - according to government figures. Those carriers control more than 80 percent of the U.S. air-travel market.