Bigger Aircraft Are Not Always More Efficient
The intuition that bigger aircraft are more efficient per passenger is widespread and, on a fixed route, wrong often enough to be worth checking.
Three aircraft, one route
Take a 5,550 km transatlantic sector and run the fuel burn calculator across the three classes, each at a representative full load:
- Regional jet, 2.4 kg/km, 76 seats → 13,320 kg fuel, 175.3 kg per passenger
- Narrowbody, 3.1 kg/km, 180 seats → 17,205 kg fuel, 95.6 kg per passenger
- Widebody, 7.5 kg/km, 300 seats → 41,625 kg fuel, 138.8 kg per passenger
The narrowbody wins, and it is not close — 95.6 against 138.8 for the widebody, a difference of about 45%. In CO2 that is 302 kg per passenger against 438.5.
Seats per kilogram of fuel is the metric
The widebody burns more than twice the narrowbody's fuel per kilometre and carries only two-thirds more people. Efficiency per passenger is seats divided by burn, and on that measure the narrowbody's ratio is simply better.
This is why the industry's efficiency gains have come as much from densification and higher load factors as from engines. An aircraft flying two-thirds full is a third less efficient per passenger than the same aircraft flying full, and no engine improvement matches that.
Why widebodies exist anyway
Not because they are inefficient by design, but because they do things a narrowbody cannot: very long sectors beyond narrowbody range, routes with cargo in the belly that adds revenue for very little extra fuel, and slot-constrained airports where the number of movements is capped and the only way to carry more people is a bigger aircraft.
The comparison above is also a like-for-like on one route, which flatters the aircraft best matched to it. On a 12,000 km sector the narrowbody is not an option at all.
Short sectors are the inefficient ones
The same narrowbody over 500 km burns about 1,550 kg, or 8.6 kg per passenger — which looks excellent until you divide by distance. Per passenger-kilometre, short flights are markedly worse than long ones, because climb consumes a large fixed amount of fuel that a short cruise never amortises.
That is the real argument behind replacing short flights with rail where the option exists: not that the absolute figure is large, but that it is inefficient per kilometre travelled.
Cruise burn is not the whole flight
The presets are representative cruise averages, and a real flight includes taxi, take-off, climb, descent, holding and diversion reserves. On a long sector the cruise dominates and the estimate is reasonable; on a short one the fixed phases are a large share and the calculator understates.
Treat short-sector figures as a floor. The direction of the error is known, which is more useful than a false precision.
Compare per passenger-kilometre
The single most useful habit in this subject is to divide by distance as well as by passengers. Absolute per-flight figures make long flights look terrible and short ones look fine; per passenger-kilometre reverses much of that and is the number that actually compares modes of travel.