A story problem: Mary weighs 120 pounds and is traveling with a 12-pound baby, a 30-pound diaper bag and two suitcases with a combined weight of 80 pounds. Dick weighs 155 and hauls a 10-pound duffel. Bob weighs 280 and is dragging a 40-pound wheel-aboard suitcase. How much does each passenger pay for a 1,440-mile flight from Chicago to Phoenix?

Trick question. Everyone knows it depends on how and when they booked their travel, where they’re seated on the plane, whether they’re entitled to check bags for free and all sorts of mysterious variables. But play along: Wouldn’t it make more sense to pay by the pound?

That’s how it works on Samoa Air, a tiny commuter carrier that serves the famously corpulent citizens of the Pacific Islands. On the world’s first pay-as-you-weigh airline, you (and your luggage) just step on the scale before you board and pay accordingly.

Samoa Air flies three- and nine-passenger prop planes. If you’ve flown on one of those, you’ve likely had the disarming experience of watching the pilot rearrange the passengers before takeoff to distribute the load evenly. Can’t have all the big people over one wing!

Major airlines don’t have that problem. But fuel is their biggest expense, and the heavier the load, the more fuel the plane uses.

A Norwegian economist, writing in last month’s Journal of Revenue and Pricing Management, makes a good case that a weight-based model is both economically sound and fair. Cargo is priced by weight, after all. Why not people? The economist isn’t suggesting everyone would be weighed at check-in at O’Hare. Tickets could be sold for specific weight ranges, for example, or passengers could self-report their weight when they book their flight. Those numbers could be enforced with random weigh-ins at the gate.

Yeah, we’re cringing too.

U.S. airlines are wary of offending the customers they delicately call “passengers of size.” To stay profitable, they’re packing more passengers per flight. That has led to some uncomfortable encounters with customers who are too big to fit between the armrests of the ever-smaller seats.

Some airlines now require plus-size customers to buy a second seat — or risk being turned away at the gate if the flight is full. Angry consumers deride it as a “fat tax.” It’s discrimination, they say.

Things are more straightforward on Samoa Air. Heavier passengers pay more to ride, but they also get more room. Problem solved.

Charging customers by weight might be awkward, but it’s not unreasonable. Airlines have gone to great lengths lately to reduce the weight of their flying metal tubes: lighter seats, lighter beverage carts, lighter carpet, even lighter paint on the outside of the plane. Yet they still charge the same fare to move 242 pounds of Mary, 165 pounds of Dick or 320 pounds of Bob. One trip, one fare.

Why? Because to do otherwise seems somehow, well, undemocratic. Charging by the pound would be perfectly reasonable. But it won’t fly here.

Editorial by the Chicago Tribune