The autothrottle adjusts the throttles to obtain and maintain a safe flying
speed as calculated by the speed command system.
The No. 2 speed command system must be in approach mode to permit autothrottle engagement. Based on the speed command signal, the servo drive moves the throttles to achieve computed speed. At touchdown the autothrottle is automatically disconnected by the spoiler actuator.
The servo drive is connected via a clutch which means that an autothrottle command can be overridden by a manual input. There are no maximum thrust limit switches, and autothrottle must be monitored to prevent over boosting the engines.
Autothrottle operation is restricted to the approach phase. When engaged, it will maintain the minimum speeds as given in the speed booklet, providing the calculated aircraft weight is correct.
This system is rather "clumsy" and since speed control in a DC9 is not a problem,
the system was almost never used.