Four-engine commercial aircraft like the legendary Boeing 747 and the superjumbo Airbus A380 have largely disappeared from passenger service, replaced by more efficient twin-engine widebodies. On the surface, it seems the era of the quadjet is over. But growing congestion at the world’s busiest airports and limited takeoff and landing slots may change that. When airlines can’t add flights, larger aircraft move more passengers in a single slot.

Could four-engine aircraft return? And where would they make sense today?