Autothrottle and pitch stability
Airplanes with engines mounted under low wings tend to experience significant pitching moment from thrust. As thrust is increased, the engine location with respect to the cg (i.e., below) causes a nose up pitching moment. This arrangement results in unstable speed stability characteristics when the autothrottle is engaged. If airspeed drops the autothrottle commands increased thrust which pitches the nose up, further reducing airspeed. The opposite happens if airspeed is too high leading to reduced thrust and the coupled nose down response.
When flying manually (neither autopilot nor autothrottle), pilots learn very quickly that throttle advances require nose down pitch commands while nose up command must be added when the throttles are retarded. While the response is inherently unstable, an experienced pilot has no difficulty keeping up as the unstable frequency is quite low.
When flying with the autopilot engaged, the required pitch control inputs to balance thrust pitching moment changes are generated automatically. Care is taken during autothrottle and autopilot design to ensure that the combined system with both autopilot and autothrottle active results in positive speed stability.
It is important to note that for airplanes that include pitch augmentation during manual control (no autopilot) the control system itself tends to compensate for pitch disturbances from thrust changes. For this reason, pilots have found that manual path control with the autothrottles engaged to control speed works well with the 777 and 787. Boeing training supports use of autothrottles during manual path control for these models.