Flight Incheon International Airport departs from Seoul Incheon International Airport at 21:35 and arrives at Da Nang Da Nang International Airport at 00:25.
The flighttime is about 4h 50m. The distance travelled ("as the bird flies"): 2275 Km. So the average speed is roughly 471 Km.
There is an 2 hour time difference between Seoul and Da Nang
