Climate ghoul John Gibbons is mad that food is cheap
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That only works if you only ever think about a single shipment on a single plane, and also ignore the fact that fuel consumption is somewhat proportionate to cargo weight. Sure there's probably enough other freight to justify any particular flight if you suddenly removed one shipment from the equation, but on a system-wide level the less freight there is waiting the more it will be consolidated and the fewer flights will be arranged overall, they don't just fly for the fun of it.
For the US trucking fleet you'd be correct.
But not for the air freight fleet.
For air freight, the critical constraint is time and gate capacity at the hubs, not the mass and volume of the individual aircraft. Oh, and how many planes they can land in a given span of time. My understanding is both FedEx and UPS are using Continuous Descent Operation to land one plane every 45 seconds on average, and they do that more or less continuously for about three hours a night. Basically the next plane crosses the threshold as the last plane taxis off, which is as close as the FAA will allow two planes to get.
UPS and FedEx have decided (rightly) that standardizing the fleet to a smaller number of airframe types is superior to right-sizing the planes for each sector, since most of their fleet only operates two sectors every 24 hours. Most of their planes are significantly under capacity on at least one of their two sectors, because a place is either receiving more than it sends or sending more than it receives.
To put it another way.. since FedEx and UPS have resolved to be able to get a parcel almost anywhere in 24 hours (give or take), and because they've decided to largely standardize their fleets to only a few airframe types, they actually have a lot of surplus capacity in the air every night. Some routes (looking at the midwest) literally do not EVER ship enough to make up for the fuel of the route, but they still have to make the route every night.
They have to make the route because the only way their overnight delivery works is by getting ALL the planes on the ground for (briefly) the same time, at the same place, every day. The night sort takes a few hours as planes start to filter into the hub, and then wait until all the things they're waiting for arrive. Since they know by computer everything that's expected, they're able to release planes to leave the hub as soon as everything they were waiting for gets through the sort.
As big as FedEx and UPS are individually, they still only account for ~20% of global air freight. The remaining 80% don't all follow their model, although some might, and that portion of the market can still freely contract if demand were to lower. It takes more than a few mandatory daily flights to keep current level of global perishable food import/exports moving.