At busy, complex airports, pilots often receive changes to their planned departure runway after they have left the gate and begun to taxi. By this time, their performance data and flight plan have been loaded into the flight management system (FMS). These runway changes happen for any number of reasons, and they can be mandatory or optional. A mandatory change can happen if the wind shifts or other weather conditions make another runway more favorable, or if airport maintenance or an aircraft incident closes a runway. Pilots can be asked to accept an optional runway change to expedite departures or arrivals.
