[Elora]

> It's that light from the aircraft travels faster than sound.

For all practical purposes we can say the light reaches the observer instantly, whereas the sound takes some (significant by comparison) amount of time. Over such short distances, and when comparing it to something that is so much slower (299,792,458 m/s vs 343 m/s, 874 thousand times faster), there is no point in measuring the infinitesimal time it takes light to travel the distance from the plane to the observer.

For the general case presented in the article, at the average cruising altitude and speed mentioned within it, the conclusion is that it takes sound from the plane so much longer to reach the observer than the instantaneous light from the plane, that the actual plane itself has traveled another 2.1km in that time. You are (instantly) seeing the current position of the plane but hearing the sound it emitted 2.1km ago.