Autopilot — Page 373, Lesson 448

I want to walk you through the Autopilot Flight Director System — the AFDS — as it's laid out on the Boeing 737-400. This is the heart of how the aircraft flies itself, and I want you to understand it as a professional would, from the ground up. First, the architecture. The AFDS is a dual system. That means it's built with redundancy — two separate brains, not one. It consists of two individual Flight Control Computers, which we call FCCs, and a single Mode Control Panel, the MCP. So you have two computers doing the thinking, and one panel where you, the pilot, tell them what you want. The two FCCs are identified as A and B. That's a critical distinction, because each one is tied to its own set of hardware. For autopilot operation — A/P operation — each FCC sends control commands to its own pitch and roll hydraulic servos. Those servos are the muscles that actually move the flight controls. And here's the key point: they operate through two separate hydraulic systems. So FCC A drives servos on one hydraulic system, FCC B drives servos on the other. That's your redundancy — if one hydraulic system fails, the other FCC and its servos can still fly the aircraft. Now, for Flight Director operation — FD operation — each FCC positions the FD command bars on its respective Primary Flight Display, the PFD. So the Flight Director isn't moving the controls; it's just showing you, on the screen, where the aircraft should be going. The FCC computes the command, and the command bars on your PFD tell you what to do. Let me bring in the Mode Control Panel now, because that's where you interact with all of this. The MCP has mode selector switches. These are the buttons you press to select the desired command modes for the AFDS and for the autothrottle, the A/T. When you press a switch, it illuminates — that light tells you the mode is selected. And here's the logic: the switch illuminating indicates that the mode is selected, and that the mode can be deselected by pressing the switch again. So it's a toggle — press to engage, press again to disengage. But there's a subtlety. While a mode is active, de-selection can be automatically inhibited. And that inhibition is indicated by the switch light being extinguished. So if the system decides you can't deselect right now, the light goes out to tell you that. And if pressing a mode selector switch would conflict with the current AFS operation — the current Auto Flight System operation — then pressing it has no effect at all. The system simply ignores you. Now, how do you get out of a mode? All AFDS modes can be disengaged by selecting another command mode, or by disengaging the autopilot and turning the Flight Directors off. So you have two escape routes: pick a different mode, or kill the autopilot and the FDs entirely. Let me look at the MCP layout itself. You'll see the mode selector switches arranged across the panel. There's COURSE, with its ARM and OFF positions. There's V NAV — vertical navigation. VERT SPEED for vertical speed selection. ALTITUDE. L NAV — lateral navigation. Then the autopilot engage section: A/P ENGAGE with CMD A and CWS A, and on the other side CMD B and CWS B, plus DISENGAGE. CMD is command mode — the autopilot actively flying. CWS is Control Wheel Steering, which I'll get to. There's F/D ON and OFF for the Flight Directors. VS for vertical speed, ALT for altitude, APP for approach, HDG for heading, SPEED, N1, C/O, LVL CHG for level change. And you'll see the MA — that's the Master Annunciator, the MA lights that alert you to mode changes. There are the heading and IAS/MACH selectors, the VOR LOC switch, and the SPD INTV and ALT INTV — speed and altitude intervene functions. Now, the MCP parameter selection. The key point here is that parameter selections common to both FCCs — for speed, heading, altitude, and vertical speed — are all made from the MCP. So you set these values once, on the panel, and both computers get them. But the course selection is different — it's split. 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 HSI is the Horizontal Situation Indicator — that's your navigation display showing the course. The First Officer's course selector does the same for the B side: it provides selected course information to the B FCC, the No. 2 VHF Nav receiver, and the First Officer's HSI course pointer and deviation bar. So each pilot's course selector feeds their own computer, their own nav receiver, and their own display. That's the segregation — A side and B side stay separate. Now let me walk you through a concrete example of an outer loop input in roll: Heading Select and Hold. This is one of the most fundamental autopilot modes. The heading select mode sends roll commands to turn and maintain the heading shown in the MCP heading display. So you dial in a heading on the MCP, and the autopilot will roll the aircraft to turn toward it, then hold it. But here's a critical detail: 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 — to the right. If you rotate it counterclockwise, it turns left. The autopilot will not take a shortcut the other way; it turns the way you turned the knob. The bank angle limit is established by the Bank Angle Limit Selector on the MCP. So there's a separate selector that sets how steeply the aircraft is allowed to bank during this turn. That's your limit on the roll. And how do you engage it? Pressing the Heading Select Switch on the MCP engages the heading select mode. And when it's engaged, HDG SEL is annunciated for the AFDS — that's the Flight Mode Annunciation, the display that tells you what the autopilot is actually doing. You'll see the annunciations there on the PFD, telling you which modes are armed and active. Let me also bring in the sensor inputs, because the autopilot needs data to work with. The autoflight computer receives inputs from the aircraft's sensors — including manometric, or air, data. That's your air data — things like airspeed and altitude derived from pressure measurements. And shows the inputs to the MCP — how the panel receives its information. So let me tie this all together. You have two Flight Control Computers, A and B, each with its own hydraulic servos and its own Flight Director display. You have one Mode Control Panel where you select modes and set parameters. The mode selector switches illuminate to show selection, and can be inhibited or ignored if there's a conflict. Course selection is segregated — Captain feeds A, First Officer feeds B. And in heading select, the autopilot turns the way you turn the knob, banks to the limit you set, and annunciates HDG SEL so you always know what it's doing. That's the AFDS architecture and the heading select mode. Take a moment to let that sink in — the redundancy, the segregation, and the way the MCP gives you control over it all.