The latest case against Spirit Airlines could provide a window onto the way airlines advertise -- and obscure -- their fares and fees, says experts not directly involved in the litigation.

The case hinges on the question of whether Spirit's so-called passenger usage fee is a deceptive violation of Department of Transportation (DOT) rules on airfare advertising -- or a clever way to make money in the difficult business of running an airline. In a challenge to Spirit's practices, Podhurst Orseck has filed a class-action suit that alleges the fees are deceiving consumers.

"I don't know anybody else that's charging this kind of fee," attorney Donald B. Mitchell with the Atlanta-based firm Smith, Gambrell & Russell told The Huffington Post. "What the plaintiffs are saying is, 'They're making [the PUF] look like a mandatory fee, and it's not a mandatory fee.'"

The issue in this case, he says, hinges on, "whether there is some deception to the consumer public."

There are two questions in play, says Anthony Marchetta, a partner at Day Pitney who handles class action lawsuits.

"Do people, one, have the opportunity to understand [the fee]?" Marchetta says, "and, two, make a decision about whether to pay it?"

Aviation consultant Robert Mann tells HuffPost Travel that, as of now, that's not the case.

"Spirit refers to the PUF [in] two places on its website, 'Our Optional Fees' and 'Taxes and Fees,'" he writes in an email. "Nowhere do they state how it is 'optional' or avoidable."

Proving that all members of the class action were similarly duped out of the $9-$17 per flight charge is the biggest hurdle the plaintiffs face, Mitchell says.

A case of timing
Mitchell points out that the new, "significant" case against Spirit comes on the heels of a July 24 decision in the U.S. Court of Appeals for the D.C. Circuit that upheld the DOT's right to regulate the way airfares are advertised, requiring airlines to differentiate between "government-imposed taxes" and "carrier-imposed fees," as they're referred to in DOT documents.

In other words, the door was just opened for a legal challenge against any airline that isn't plainly posting the "total, final price" of a ticket, Mitchell writes in an email to HuffPost Travel. "The hurdle issue in my opinion is whether the suit can go forward given that advertising of airfares is really in the DOT's domain."

"In Spirit's defense," Mitchell adds, "I think they change their display to meet the requirements [as they change]. But then they may push the envelope and try something different."

"By failing to force consistent display and reporting," Mann writes in an email, "DOT comes off looking flummoxed and rather toothless in this."

Discovery in the case will likely reveal who knew what and when they knew it. Marchetta says memos and other internal documents will be central to the suit, particularly if they describe advertising practices or internal strategies about decisions on how to apply the fee. "That's what discovery will focus on the case: how much disclosure was there to the public?" Marchetta says. "It all depends on how it was revealed."

As to whether passengers will get any money back, that's still an open question. A settlement could end with simply giving all members of the class vouchers for future Spirit flights. Either way, a big per-capita payout -- despite all the money Spirit has made off the fees -- is unlikely.

"It's designed to stop behavior," Marchetta says of a class action suit. "It's not designed to get money to individuals."

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