1. A method comprising: receiving, at a flight management system of an aircraft, a required time of arrival (RTA) of the aircraft at a first location; receiving, at the flight management system, an input indicating an RTA index value; determining an adjusted speed of the aircraft based on the RTA and the RTA index value, wherein the adjusted speed is different than a target speed associated with a flight plan of the aircraft, and wherein the RTA index value indicates a degree of preference between maintaining the target speed and achieving an RTA speed that enables the aircraft to meet the RTA at the first location; and setting the aircraft to fly at the adjusted speed.