This is really a type-specific question, and the reader should refer to his or her own aircraft type; however, the following answer, which is based on the B737-300 autothrottle system, is given as an example. The autothrottle and APFDS work together to maintain the aircraft's airspeed and vertical path in different variations for the phase of flight and/or the mode of operation selected.

1. Takeoff. The AT sets takeoff thrust, and the APFDS adjusts pitch to maintain the airspeed, V2 + 10-20 knots.

2. Climb. Either

a. The APFDS controls airspeed with pitch commands, and the AT controls engine thrust to a specific NL climb thrust value (e.g., level change or V NAV climb).

b. The APFDS controls the vertical path attitude, and the AT maintains airspeed through the engine thrust control (e.g., vertical speed).

3. Cruise. The AT controls engine thrust to maintain airspeed, and the APFDS controls attitude to maintain altitude.

4. Descent. Either

a. The throttles retard to idle to gain the attitude that gives the maximum descent rate, and the APFDS controls the pitch to maintain airspeed (e.g.. level change/V NAV descent).

b. The AT controls airspeed, and the APFDS controls the vertical flight path (e.g., vertical speed' V NAV path descent).

5. Approach. The AT controls airspeed, and the APFDS controls the attitude to maintain a vertical path profile (e.g., autoland approach mode).