Why center autopilot in 757/767

[Jetdoc]

• During a category III autoland, the main ac buses are prevented from being supplied by a common source. The autoland condition is started by the flight control computers. The computers provide a ground to the isolation request relay . The isolation relay energizes, allowing a 28-volt dc signal to be sent to the bus power control unit (BPCU).
• Multichannel engage occurs when all A/Ps have detected LOC and G/S capture and 1500 feet from a valid radio altimeter (RA). The first channel in command must receive acceptable CMDENG criteria from the other two FCCs. All A/Ps engage pitch, roll, and yaw channels. The ASA displays LAND 3 (Fail Operational) or LAND 2 (Fail Passive). All phases of autoland are possible with LAND 3 or LAND 2 displayed.
• Autoland configurations are not active until each FCC recognizes LOC and G/S capture; the airplane is below 1500 feet radio altitude; and other autoland logic requirements are met. App can be anytime below 1500 feet but i can'tremeber the lower limit for engagement.
• The dot-bar matrix on the lower half lights when the FCC has accepted the associated mode request only. It does not indicate engagement.
• If the right bus were to fail, the static inverter and battery still power the C FCC. The battery will not discharge in the few remaining minutes of flight.

I have not worked on a B757 or B767 since 1992 and last taught a course on a B757 in 2004 but it sure feels good to talk about these aircraft again.