With the world's airlines collectively losing around f11.4m a day, they are looking closely at the bottom line -- of their passengers.
In the battle to squeeze maximum earnings from its seats, Air France-KLM is introducing an explicit policy aimed at obese flyers.
From February 1, the airline and its Dutch subsidiary, KLM, will insist that "passengers with a high body mass" book a second seat at 75pc of the original fare.
The definition of overweight depends on the cabin: in economy class, the airline says its seats are designed for passengers with a waistline up to 135cm (53 inches). If the cabin turns out not to be full, Air France will refund the price of the extra seat.
Obese passengers are a growing problem for airlines on three counts. First, with average weight across the population rising, fuel consumption is increased.
Next, with more overweight flyers, more passengers experience discomfort from sitting next to them. And with airlines collectively filling a higher proportion of seats, there is literally less room for manoeuvre on board for the cabin crew to shuffle passengers around.
The first airline to address the problem was Southwest Airlines of the US, which now flies more passengers, of all shapes and sizes, than any other airline in the world.
The seats aboard its Boeing 737s, though, are one-size-only, and one passenger in 200 cannot fit into them.
The airline says: "Our responsibility is to provide safe and comfortable air transportation for each and every customer." So for the past three decades it has enforced a simple policy for "persons of size": if a passenger cannot sit in a single seat with both armrests down, they must buy a second ticket. The fare is refunded on any flight that has one or more seats available.
A rival airline, United, has recently declared that passengers in a fully booked economy cabin who cannot buckle the seat belt using a single extender must buy an upgrade on the same flight, or book an extra seat on the next available flight.
Other airlines can be expected to follow suit in order to boost earnings.
Ryanair caused controversy last year when it floated the idea of a "fat tax" on people who "invade the space" of fellow passengers.
Even though thousands of Ryanair customers voted online in favour of such a policy, it has not been implemented.