In 2000, the American Journal of Preventive Medicine estimated that the increasing weight of passengers was costing US airlines $US343 million for the additional 1.36 billion litres of fuel required to lift the extra kilograms.

Six years later, with fuel up 145 per cent, that cost has ballooned to $US840 million. The weight of passengers has also ballooned, and not just in the US.

The World Heath Organisation says more people - 1.7 billion - are overfed than malnourished and the number is increasing at an alarming rate, as are complaints from travellers forced to sit next to obese passengers.

In fact, investigation into a January 2003 fatal crash of a 20-seat turboprop flight revealed that passenger weight figures estimated by the Federal Aviation Authority, the US air safety regulator, were on the light side.

In August 2005 the agency increased the male passenger with carry-on weight from 84kg to 91kg for summer and 86kg to 93kg for winter.

The averages for women, who also are weighing in heavier, were increased by similar margins, with a top weight of 83.6kg.

This is not just a US issue. In 2002, Virgin Atlantic Airways paid $25,000 to an English passenger who claimed to have suffered a blood clot in her chest, torn leg muscles and acute sciatica after sitting next to a grossly overweight passenger from London to Los Angeles.

Texas-based Southwest Airlines has a clear policy on its website about carrying passengers judged to be overweight.

The policy has been in place since 1980, but only in the past few years has it been enforced after the airline noticed that nine out of 10 complaint letters were about passengers being seated next to a "customer of size".

Southwest "requires a customer to purchase the number of seats he/she occupies" a representative said.

"Asking a person to pay upfront for the product being used guarantees that everyone has a safe and comfortable experience. Plus, we will refund the additional purchase as long as the flight does not oversell."

The carrier notes that it ends up "refunding the majority of the second-seat tickets and most airlines enforce the same policy".

Southwest defends its policy against claims of discrimination by stating that its "mission and responsibility per our Contract of Carriage is to provide safe and comfortable air transportation for each and every customer".

"This policy has been upheld in court and is supported by the US Department of Transportation's stance that the purchase of a single ticket offers the use of a single seat."

The airline's gauge of an overweight passenger is the armrest, "as it serves as the boundary between seats".

Interestingly, the Oakland-based National Association to Advance Fat Acceptance (NAAFA) on its website encourages its members to get around that problem by raising the armrest upon boarding.

It states: "When you get to your seat during pre-boarding, raise the armrest between seats. This may give you the inch or two of extra space you need. The chances are that the passenger who will be seated next to you won't say anything. If he does, smile pleasantly and say that you'll both be more comfortable if the armrest is up."

On a positive note, NAAFA and similar organisations encourage members to select off-peak flights, book direct flights, allow extra time for connections and arrive early to avoid embarrassing confrontations.

Not surprisingly, these organisations call for wider seats - a move with which few would disagree.

It is impossible to justify such an action economically with existing aircraft types, but the widening of the population will be a major factor in designing successor aircraft, particularly for the A320 and 737 ranges.

Many suggest that a twin-aisle, six-across design will win the day and key carriers such as Southwest are warming to the concept. Current economy seats are about 43-45cm wide, but Airbus has upped the ante with 48cm wide seats for its A380.

Back to the present, what is clear, at least at Southwest, is that "customers of size" who cannot fit into a 43cm seat need to buy two seats.

Perhaps because of the sensitivity of the issue, other airlines detail the overweight policy in the contract of carriage, which few read.

Inconsistencies in airlines' approach to the problem disturb many overweight passengers.

Qantas, which has some of the longest routes in the world with an economy seat pitch of just 78cm, does not have a policy but states: "We do everything possible to meet any special needs our customers may have. Where a customer requests some extra space, we will consider their request at check-in and try to seat them next to an empty seat where possible." But a full flight means major problems.

Japan's All Nippon Airways has a policy for giant sumo wrestlers of an extra seat at half the economy fare.

An airline executive who requested anonymity said: "It is unfair that we charge passengers for excess baggage but not excess weight. What about the poor passenger who weighs just 68kg but has 30kg of baggage and gets slugged hundreds of dollars for excess baggage but the guy who weighs in at 100kg with a 15kg bag is fine?"

Most airlines put this issue in the too-hard basket.

As the relentless squeeze is applied to both carriers and passengers, perhaps the industry should adopt the suggestion of Dave Grotto from the American Dietetic Association, who told the Chicago Tribune: "Maybe instead of just using those boxes at the gates to limit carry-on bags to certain sizes, the airlines need to have a people-sizer with a sign asking, 'Do you fit into this?'"