We have added a possibility to define aircraft speed depending on the stage of the flight.
If you go to Admin > Fleet and edit an aircraft there is Aircraft Speeds section. Insert the 'Speed' of aircraft in knots at each stage of the flight defined in hours (column 'From') i.e. 00:00 240, 01:00 460 etc.
This data along with GCD is used to calculate STA when creating a flight (see example below).