If Congress cuts airline fees, who would suffer?

A few senators must have finally had enough on one of their many flights to and from their home districts.

Last week, a bill was introduced that would "forbid airlines from imposing ridiculous fees" — specifically, the baggage and changed reservation fees that have become a staple for many large airlines.

If such a bill actually made it into law, what would it mean for airline companies? As much as 15 percent of some carriers' total operating revenue comes from just those two fees (not counting all the other fees that travelers may end up paying), according to Bureau of Transportation Statistics data. Low-cost airlines like Spirit Airlines and Allegiant Air are especially reliant on those revenue sources.

That wasn't always the case. Before 2009, those fees were a negligible part of most carriers' revenue. Today, airline profits would suffer if those fees disappeared.

From 2007 to 2014, "ancillary fees" in total (including other services like priority boarding and Wi-Fi) increased 1,400 percent, according to a report released last year by the Senate Committee on Commerce, Science and Transportation.

The industry trade group Airlines for America said the bill "seeks to reregulate airline industry pricing and service provisions, which were successfully deregulated in 1978."

The shift in how customers are charged for baggage started at low-cost carriers like Spirit in the late 2000s. It didn't take long for the larger carriers to also charge for checked bags and for reservation cancellation fees. Today, ancillary fees are worth tens of billions in revenue.

While airlines often argue that the unbundling of services offers customers more control over their flight costs, the new revenue sources also have been profit drivers for the low-margin airline industry.

The proposed legislation would require that fees be proportional to the related costs. But it's hard to estimate how overblown those fees currently are without detailed data from the companies. Here's how each airline's operating profit compares to its fee revenue over the last two years (Q4 2013 to Q3 2015):

Even a modest decrease in fees would have a substantial impact on the bottom line for low-cost companies like Spirit Airlines, Allegiant Air, Frontier and Virgin America. Sun Country Airlines could stop being profitable if its fees were reduced.

Spirit uses a sliding scale ranging from about $30 for the first bag during online booking to $100 at the gate. Southwest is the only airline that still offers free checked bags and free cancellation. Jet Blue started charging fees for travelers' first bags last summer.

It's probably unlikely that the bill's backers will be able to push airline regulation of this sort through Congress, and if they did carriers would likely raise base fares to make up the losses. The average inflation-adjusted airfare is up 14 percent since 2009, as high consumer demand continues to outweigh huge savings on jet fuel, according to Bureau of Transportation Statistics domestic flight data.