The next frontier in airline fees — those pesky payments for baggage checks and seat upgrades — may be higher charges during peak demand periods.

In other words, the $25 you now pay to check a bag may go to $30 if you insist on flying during busy travel periods such as Christmas or Labor Day.

The prediction that airlines will start to adjust fees based on demand comes from Jay Sorensen, a noted consultant for the industry on what air carriers like to call "ancillary fees."

He has also forecast that ancillary fees will generate nearly $50 billion for the airline industry worldwide in 2014. And now carriers are looking for ways to push that higher.

Sorensen said Spirit Airlines has already started the practice by raising checked bag prices by $2 during the holiday travel period. The higher fees kick in on flights from Dec. 18 to Jan. 5.

Over the next two years or so, Sorensen said, he expects airlines to raise fees during peak travel periods on extras such as early boarding, seat upgrades and checked bags.

But he warned that government regulators may clamp down if airlines fail to clearly post the fee changes with ample warning.

SEATS SLIM DOWN ON HAWAIIAN

One of the nation’s oldest airlines is jumping on a trend already embraced by its younger competitors: installing slimmer seats to cram more passengers into its planes.

Hawaiian Airlines, in business for 85 years, will replace its traditional seats with so-called slim-line seats on its Boeing 717s — the smaller jets that fly between the islands. By adding the seats with thinner cushions, Hawaiian can squeeze up to 10 extra seats per plane, fitting up to 128 passengers in each jet.

"That will allow us to maintain the amount of personal space that our guests have in the main cabin while at the same time doing that with a slightly denser configuration," Peter Ingram, chief commercial officer for the airline, said during an investor presentation.

United, Alaska, Southwest and Spirit have already added slim-line seats. An online survey taken this year found that 83 percent of passengers who have tried such seats say they are less comfortable than traditional seats.

The added seats may be one reason why Hawaiian executives are predicting higher profit margins next year.

Hugo Martin, Los Angeles Times