US airlines, already trying to sell extra legroom and faster lines, are finding another innovation hard to push: second seats for overweight passengers.

It's a very hard sell, literally. On airlines such as Southwest and United, people who can't fit into 43cm seats with the armrests down and their seat belts fastened must buy a second seat or they don't fly.

US Airways and American Airlines are likely to offer free second seats, but on a full flight they make extra-large passengers pay for them.

Fliers who've been compressed by their neighbours love the idea.

"They should be required to pay for the extra space," Scott Land, 54, a thin man, said.

People like Washington bookseller Gary Lewis, who's 2m tall and weighs 180kg, hate being forced to buy second seats.

"There should be wider seats for everyone," Lewis said.

Many passengers, even svelte ones, would agree.

The problem is, Boeing set its 43cm standard for economy-class seats in 1954.

At that time, the average US adult male weighed about 75kg and the average female weighed 63kg.

Today, the average US male is almost 3kg heavier, according to the National Centre for Health Statistics, and the average female almost 11kg heavier.

Precise seat widths vary by airline, but as a rule of thumb, a human seat of up to 50cm can be shoehorned into a 43cm seat.

Even that is "very uncomfortable, because you're sitting there touching someone else", said Greg Eagerton, 47, a hospital administrator.

And touching puts it mildly. Eagerton believes - accurately - that the discomfort is "more common now than it was even five or 10 years ago".

Indeed, one-third of Americans today are obese, the federal health statistics centre reports.

"So why not change one-third of aircraft seats to accommodate them?" said Rebecca M. Puhl, director of research at the Rudd Centre for Food Policy and Obesity at Yale University in New Haven.

She sees possible bias against obese people in making them buy second seats.

Such bias was "increasing at disturbing rates" at the national level, Puhl said in a 2007 study that she co-authored, titled Changes in Perceived Weight Discrimination Among Americans.

It found that reports of weight bias in a surveyed group increased from 7 per cent in the mid 90s to 12 per cent a decade later.

"It's a sensitive subject, and it has to be handled with care," Southwest spokeswoman Ashley Rogers said of selling second seats to obese people.

"But if the arm rest cannot go down, they would be asked to buy a second seat. Most of the time, if they really need two seats, they know it beforehand."

But not always, said Peggy Howell, spokeswoman for the National Association to Advance Fat Acceptance, an advocacy group for overweight people based in Oakland, California.

"I have personal friends who have been called out of line and asked to buy a second seat," Howell said.

"If they see someone standing there who they decide is too big, they get called out, and it's embarrassing and humiliating."

American, JetBlue and US Airways continue to give oversize passengers a free second seat if one's available.

Some carriers want obese people to buy second seats in advance but will refund the price of the second seat if the flight's not full.

Air Canada, a pioneer in selling second seats to obese passengers, was forced last year by a Canadian Supreme Court decision to treat them as handicapped passengers.

The extra seat is now mandatory and free on flights within Canada but requires a doctor's note that the obesity has a medical cause.

WASHINGTON MCT