The primary flight-control system (PFCS) is powered by 28V direct current generated by two dedicated generators on each engine and can revert to main DC power.

There are two types of electronic computer in the PFCS: the actuator control electronics (ACE), primarily an analogue device, and the primary flight computer (PFC), which is digital. There are four ACEs and three PFCs.

The redundancy of the ACEs is in functional distribution; ACE designations match the left, centre and right hydraulic systems on the aircraft. Total failure of one ACE has the same effect as a hydraulic system failure.

ACEs decode signals, from multiple transducers on the pilot controls and those on the primary surface actuators. Both positions, converted to a digital value, are sent over triplex bi-directional ARINC 629 buses, to the PFC, to process further surface commands. These are returned over the same buses for the ACEs to convert into further analogue commands to each actuator.

PFCs are left, centre and right, as are the databuses. All three, called "channels", are of the same design.

PFC redundancy is in the calculating elements. Each contains three independent and different sets of microprocessors, ARINC 629 interfaces, and power supplies - "lanes". All lanes perform identical calculations; failure of one will cause only that lane to be shut down. A channel can be operated normally on two lanes; another lane failure will cause that channel to be shut down.

The 777 can be operated indefinitely with one lane of nine failed. The master minimum-equipment list will allow dispatch with two lanes out of nine failed (not within the same channel) for some days and with one PFC inoperative for a short period.

The PFCS receives data from other aircraft systems by two different methods. Firstly, the air-data inertial-reference unit, standby attitude and air-data reference unit, and the autopilot flight-director computers transmit directly onto the flight-controls data buses and to the PFC.

Other systems, such as the flap-slat electronics units, proximity-switch electronics unit, and engine-data interface unit, transmit data on a separate set of four systems databuses. The PFC gets data from them through the dual aircraft information-management systems. This gateway between the two main sets of ARINC 629 buses maintains safe separation between the critical flight controls and the essential systems, while allowing data to be interchanged.

Source: Flight International