Autopilot — Page 373, Lesson 448

Let's start with the architecture of the AFDS — the Autopilot Flight Director System. This is a dual system, meaning it has two independent channels. It consists of two individual Flight Control Computers, which we call FCCs, and a single Mode Control Panel, the MCP. The two FCCs are identified as A and B. For autopilot operation — that's the A/P — each FCC sends control commands to its own pitch and roll hydraulic servos. So FCC A drives its servos, FCC B drives its servos, and these servos operate the flight controls through two separate hydraulic systems. That's your redundancy: two computers, two sets of servos, two hydraulic systems. Now, for Flight Director operation — the FD — each FCC positions the FD command bars on its respective Primary Flight Display. So FCC A drives the Captain's command bars, FCC B drives the First Officer's. The command bars are the little symbols on the PFD that tell you where to pitch and roll the aircraft manually. Let's move to the MCP Mode Selector Switches. These are the buttons you press to select the desired command modes for the AFDS and for the A/T — that's the autothrottle. When you press a switch, it illuminates to indicate that the mode is selected. Pressing it again deselects the mode. But here's an important detail: while a mode is active, de-selection can be automatically inhibited, and this is indicated by the switch light being extinguished. So if the system doesn't want you to deselect, the light goes out to tell you that. There's also a safety feature: if engaging a mode would conflict with the current AFS operation — that's the Auto Flight System — pressing the mode selector switch has no effect. The system simply ignores you. And all AFDS modes can be disengaged by selecting another command mode, or by disengaging the A/P and turning the FDs off. Now let's look at the MCP parameter selection. Parameter selections common to both FCCs — for speed, heading, altitude, and vertical speed — are all made from the MCP. So the MCP is your single point of entry for those targets. There are two course selectors and two course displays on the MCP. The Captain's course selector provides selected course information to three places: the A FCC, the No. 1 VHF Nav receiver, and the Captain's HSI course pointer and course deviation bar. The First Officer's course selector does the same for the B FCC, the No. 2 VHF Nav receiver, and the First Officer's HSI course pointer and deviation bar. So each side has its own complete path. Finally, let's look at an example of an outer loop input in roll: Heading Select and Hold. The heading select mode sends roll commands to turn and maintain the heading shown in the MCP heading display. After mode engagement, roll commands are given to turn in the same direction as the rotation of the heading selector only. So if you rotate the heading knob clockwise, the aircraft turns clockwise — it only turns the way you rotate the selector. The bank angle limit is established by the Bank Angle Limit Selector on the MCP. Pressing the Heading Select Switch on the MCP engages the mode, and HDG SEL is annunciated for the AFDS — that's the flight mode annunciation telling you the mode is active. That's the core of the AFDS architecture and the MCP. Let me know if you want to dig into any of these components further.