The current record for longest commercial flight is 21,602.22km by Boeing 777-200LR.
However the designed maximum range for B777-200LR is 15,800 km. So how is this possible?
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The design range is fully loaded. This flight had 35 passengers, no cargo, full fuel and was run as a project and flown by a test pilot. It was not a commercial flight in the meaning that most people would recognise.
They had to wait for weeks until the winds aloft, especially the jet stream, were at their most advantageous.