Per kilometre, a one-hour flight is considerably worse than a ten-hour one. The reason is that the expensive phases of flight are fixed in duration and the cheap phase is what varies.
Climb is where the fuel goes
Getting an aircraft off the ground and up to cruising altitude requires engines at high thrust for a sustained period, and fuel flow during that phase is far above cruise levels.
The climb takes roughly the same time and fuel regardless of how far the aircraft is going afterwards. It is a fixed charge attached to every departure.
Cruise is efficient by comparison, because thin air reduces drag and the aircraft is steadily getting lighter as fuel burns off.
The fixed charge is spread across the distance
On a short sector the aircraft may barely reach cruise before beginning descent, so the fixed cost is divided among very few kilometres.
On a long sector the same charge is spread across thousands of kilometres of efficient cruising, which drives the per-kilometre figure down sharply.
Taxiing, holding and queuing add further fixed consumption, and these are worst at congested airports where short-haul services concentrate.
Aircraft are matched to sector length
Short routes are flown by smaller aircraft with fewer seats, so the fuel is divided among fewer passengers, which compounds the effect.
Long-haul aircraft carry more fuel weight, which costs energy to lift, but the larger cabin and the long cruise more than offset that penalty per kilometre.
This is why the per-kilometre comparison flatters long-haul while the total emissions of a long-haul trip remain far larger. The two measures answer different questions.
Connections multiply the penalty
An itinerary broken into two sectors incurs two climbs, and the routing is usually longer than the direct line as well.
A short connecting flight feeding a long-haul service can carry a per-kilometre figure several times that of the long sector it feeds.
Direct routing is therefore one of the few genuinely reliable levers a traveller has, and it usually costs nothing beyond a modest fare difference.
Where ground transport wins
Trains avoid the climb entirely and, where electrified, draw from a grid that is steadily decarbonising, which is a trajectory aviation does not yet have.
Over distances where rail journey times are competitive door to door, the emissions comparison is not close.
Beyond that range the time cost rises steeply, which is why short-haul aviation persists on routes where no fast rail alternative exists.