Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 20:30 and arrives at Jeju City International Airport at 21:45.
The flighttime is about 1h 15m. The distance travelled ("as the bird flies"): 271 Km. So the average speed is roughly 217 Km.
The best day to arrive at Seoul is Saturday and it would be best to avoid Sunday, as prices are on average 91.26% higher than on Saturday. (Average prices, based on 819430 datapoints.)
We have a lot more statistics for flights KE 1227 and between Seoul and Jeju City.
