On the Boeing 777 aircraft, two autothrottle actuators are required, one for each throttle lever, both controlled by either one of two computers. The consequent potential need for four remote servo-amplifiers has been eliminated by using two ‘smart’ autothrottle servo motor units (ASMs) independently controlled by ARINC 429 data bus rate command signals from the active computer.
The development of the B777 ASM from a servo motor unit used on B727 is recounted and the ASM design and ARINC 429 data bus control architecture are described in detail.
