A computer-implemented method of controlling an aircraft including, determining whether a change in altitude at a predetermined thrust-setting can be performed, the determination including: determining a climb-requirement including initial and final altitudes and a required range of airspeeds at the final altitude; determining aircraft performance data including a thrust at the final altitude at the thrust-setting, an initial airspeed at the initial altitude and an aircraft total mass at the initial altitude; using the aircraft performance data to determine a final airspeed and drag at the final altitude if a change in altitude at the predetermined setting were performed; determining whether thrust at the final altitude at the thrust-settings and airspeed is greater than drag; and providing a signal indicating whether the calculated final airspeed falls within the required range of airspeeds, and whether the thrust at the final altitude and airspeed at the cruise thrust-setting is greater than drag.