The longest scheduled commercial flight is Singapore Airlines' service between Singapore and New York, at roughly 15,300 kilometres and around 18 to 19 hours depending on winds and direction. It has held or contested the title since 2018.
Why it is hard
An ultra-long-haul route is not just a longer version of a normal one. Fuel weight compounds against itself: carrying fuel requires fuel, and beyond a certain range the aircraft spends an increasing share of its payload capacity carrying its own fuel.
That is why ultra-long routes are typically flown with reduced passenger loads. The original Singapore–Newark service, operated with the Airbus A340-500 from 2004, was configured with about 100 seats in an aircraft that could carry over 300 — all business class, because that was the only way the economics worked.
Singapore Airlines suspended the route in 2013 when fuel prices made it unviable, and restored it in 2018 with the Airbus A350-900ULR, an aircraft specifically modified for the mission with extra fuel capacity and a lighter cabin.
The competitors
Several routes trade places near the top depending on schedules and seasonal routing: Singapore–New York, Singapore–Newark, Auckland–New York, Perth–London, Dallas–Sydney and Doha–Auckland have all held or approached the record.
Qantas has flown research flights on Sydney–London nonstop under Project Sunrise, and has ordered A350-1000ULRs to operate it commercially — roughly 17,000 kilometres and around 20 hours.
What ultra-long-haul does to people
Twenty hours in a pressurised tube raises real physiological questions, and airlines flying these routes have taken them seriously.
Cabin altitude and humidity matter more than on shorter flights, which is one reason composite aircraft — the 787 and A350 — dominate the segment: a composite fuselage tolerates higher humidity and lower cabin altitude than aluminium.
Lighting is programmed to manage circadian rhythm rather than to look pleasant. Meal timing is designed around destination time. Qantas's research flights carried scientists monitoring crew and passenger sleep, alertness and melatonin.
Crew regulations are the binding constraint on some routes: beyond certain durations an aircraft needs multiple full crews and onboard rest facilities, which costs both weight and revenue space.
The historical comparison
Pan Am's transpacific Clipper service in 1936 took roughly six days from San Francisco to Manila, with overnight stops at Honolulu, Midway, Wake and Guam. The aircraft could not do more.
Ninety years later a single aircraft flies further than that route in one sitting. The constraint has moved from the machine to the people inside it.