The cruise speed is the speed at which an aircraft travels during the cruise phase.
Overriding the cruise speed
When you schedule a flight, the standard optimum cruise speed of the given aircraft type is used. In some cases it can be useful to override this setting, though, for example to make sure the arrival time falls into a free slot. To do so, simply select an explicit arrival time for each day in the flight planning interface. The buttons below the arrival times allow you to quickly skip between slots, apply the arrival time to all days or to reset it do the default based on the optimum cruise speed of the aircraft.
Tip: If an arrival time set by skipping to the previous or next slot causes a "performance error" because the resulting cruise speed falls outside the capabilities of the given aircraft type, try adjusting it manually until the error goes away.
Scheduling without aircraft assignment
By setting a fixed arrival time for a flight day that has no aircraft assignment yet, you are essentially planning a hypothetical flight. This can be of use in the connection analyzer or to help build complex, future schedules in general.