Flight Incheon International Airport departs from Seoul Incheon International Airport at 21:10 and arrives at Tokyo Narita International Airport at 23:30.
The flighttime is about 2h 20m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 666 Km.
The best day to arrive at Seoul is Sunday and it would be best to avoid Tuesday, as prices are on average 63.92% higher than on Sunday. (Average prices, based on 6959067 datapoints.)
We have a lot more statistics for flights NH 6896 and between Seoul and Tokyo.
