SAN FRANCISCO • Do not expect to get a boarding pass on cue even if you have paid full fare.

As two legging-wearing teenage girls found out recently when they were denied boarding a United Airlines plane, your choice of outfit can get you grounded.

Major carriers United, Delta and American all list bare feet as grounds for removal from flights in their official contract of carriage documents.

United's policy also covers passengers "not properly clothed" and American's warns that it may refuse to transport passengers "clothed in a manner that would cause discomfort or offence to other passengers".

Those extra words, according to Mr Brian Sumers, a reporter at the travel industry website Skift, mean that anyone who shows up in a T-shirt scrawled with profanity or a lewd message may be barred from a plane.

Those in bikini tops may also have a problem.

"You know, people would have told you two decades ago that it was a good idea to dress nicely on a plane because you never know if the airline might take care of you, like maybe bump you up to business or first class because you look nice," he noted.

"But that doesn't really happen now. Airlines know who their best customers are and they're the ones they will bring up if there's a seat available."

Mr George Hobica, founder of the travel fare advice site Airfare Watchdog, also shares the view that "everyone believes no one gets upgraded anymore based on how they look".

But he added: "It does happen."

He recounted wearing a navy-blue suit on a United flight a few years ago from New York to Los Angeles.

"I was in the lounge, where everybody was dressed in sandals and gym clothes, and I heard them call my name," he said.

He thought he was being bumped, but instead was upgraded to first class. "I don't have status on any airline," he said, "because I buy based only on price."

He concluded that the freshly pressed suit was the source of his luck.

Mr Hobica also knows of friends who had been upgraded while wearing clothes they considered nicer than what they might don to the gym or the grocery store.

He shared a conversation he once had with a gate agent friend at Lufthansa.

"She told me she would upgrade people based on how good-looking they are, how pregnant they are or how nicely they're dressed," he said.

"She said: 'Look, we oversell flights and, of course, we go down the status list first. Absolutely, we look at your miles.'"

But if no one on the flight warrants special privileges, the absence of ripped jeans or tattered sneakers can help, Mr Hobica said.

Airlines on their part trot out well-rehearsed lines that will not ruffle any feathers.

Ms Ashton Morrow, a Delta spokesman, said her airline, like most, adheres to set corporate practices that do not take appearances into account in selecting passengers for unexpected perks.

"There are a number of factors that go into upgrading, including loyalty and recent delays we might want to make up for," she said. "Upgrading based on wardrobe is never our policy."

Still, dressing well for flights may yield more subtle benefits.

"I will say that when I see someone come on the plane and he's dressed nicely and his children are dressed nicely, I do take notice," said Ms Kate Linder, a United flight attendant since 1978.

"When someone is a little dressed up and looking like he made an effort, it's almost like he's showing respect for himself and for everybody else on the plane," she added.

Ms Nancy Marquis, another long- time United flight attendant, retired in 2007 but is still a frequent flier.

She said she understood the inclination to dress "like a slob" on a plane. When she became a flight attendant in 1969, seats were bigger and security was not as much of a bother.

Now, she said, "you have less personal space and fewer amenities, so you want to be as comfortable as possible".

But you could be upgraded to the more comfortable business class - if you put in a bit more effort to dress up.

NYTIMES