Why do private jets fly higher?

Drag on an aircraft falls with air density, so for a given true airspeed a jet burns less fuel higher up. The limit is that thin air also reduces engine thrust and the lift the wing produces, so an aircraft can only climb until the speed at which it stalls and the speed at which it meets compressibility problems converge. Business jets reach that point later than airliners because they are comparatively light for their wing area, particularly after a few hours of fuel burn.

Cabin structure matters just as much. A larger difference between cabin and outside pressure means higher hoop stress on the fuselage and more fatigue cycles, so aircraft designed for 15,500 m (51,000 ft) carry a heavier pressure vessel than one designed for 12,500 m. That penalty is easier to accept on a 15-tonne payload aircraft than on an airliner built around seat-mile cost.

Above roughly 12,000 m most convective weather is below the aircraft, so flights are smoother and rarely need deviations. There is an operational benefit too: airline traffic concentrates in a band between about 9,000 and 12,000 m, and flight levels above 12,500 m are far less crowded. A business jet cleared direct at that altitude often flies a shorter track than the airway network would give it.

The generalisation only holds at the top of the market. Light jets have service ceilings around 12,500 m (41,000 ft), the same as an A320 or a 737, and older types sit lower still. And no aircraft climbs to its ceiling immediately: a fully fuelled jet has to burn weight off before the wing will hold it there, which is why long flights step-climb as they go.