Autopilot — Page 373, Lesson 446

Let’s start with the big picture. The autopilot is not a single box—it’s a whole system, and on the Boeing 737-400 that system is officially called the Automatic Flight System, or AFS. The AFS is made up of two main parts: the Autopilot Flight Director System, which we call the AFDS, and the Auto throttle, which we call the A/T. Those two work together, and they’re the basis for the JAR objectives and exams, so this is exactly what you need to know. Now, where does the AFS get its information? Two big sources. First, radio navigation. To let the auto-flight system capture and track a radio beam, data signals are sent to the relevant auto-flight computer from the VOR and ILS receivers. So when you’re flying an approach, the VOR and ILS receivers feed the beam information into the auto-flight computer so it can steer the aircraft onto and along that beam. Second, computer-generated data. Modern aircraft can follow a computer-generated flight profile in both roll—that’s lateral—and pitch—that’s vertical—from a Flight Management System, the FMS. The steering signals come from the Flight Management Computer, the FMC, and they’re connected to the auto-flight computer to control the attitude of the aircraft. So the FMC is the brain that plans the profile, and it sends steering commands to the auto-flight computer to actually move the aircraft. Now, let’s talk about what the FMC provides. It provides N1 limits and target N1 for the auto throttle, and it provides command airspeeds for both the auto throttle and the AFDS. N1 is the fan speed of the engine—the low-pressure compressor speed—and the FMC tells the auto throttle what N1 limits to respect and what target N1 to aim for, plus what airspeed to command. How do you operate all this? The AFDS and the A/T are operated from the AFDS Mode Control Panel, the MCP. The FMC, on the other hand, is operated from the Control and Display Unit, the CDU. So you’ve got two control interfaces: the MCP for the AFDS and auto throttle, and the CDU for the flight management computer. What does the MCP actually do? The AFDS MCP provides co-ordinated control of the autopilot—the A/P—the Flight Director—the FD—the auto throttle, and the altitude alert functions. So from one panel you can coordinate all of those. And how do you know what mode the system is in? The AFS mode status is displayed on the Flight Mode Annunciators, the FMA, on each pilot’s Primary Flight Display, the PFD. So the FMA tells you what the autopilot is actually doing right now. Normally, the AFDS and the A/T are used to maintain airspeed and thrust settings that are calculated by the FMC. So in normal operation, the FMC does the thinking, and the AFDS and auto throttle do the flying to hold those airspeed and thrust values. Now, let me walk you through the inputs to the MCP, because that’s where it all comes together. Look at Figure 26.12, "Inputs to the MCP." You’ll see the MCP sits at the center, and around it are all the systems feeding it. There’s the pitch inner loop, the roll inner loop, and the yaw inner loop—those are the inner loops of the autopilot, the fast, stabilizing loops. Then there’s the magnetic heading reference system, the attitude reference system—which could be a vertical gyro, an INS, or an IRS—and the aircraft reference data. There’s also the manometric reference system, which is the air data computer, and the VOR/ILS receivers, and the flight management system. All of those feed into the MCP, and the MCP handles the autopilot engage and outer loop input selection. So the MCP is the hub. It takes all those inputs—attitude, heading, air data, radio navigation, flight management—and lets you select what the autopilot should do. That’s the outer loop input selection. The inner loops are the fast, stabilizing loops that keep the aircraft stable; the outer loop is the higher-level guidance, like "capture this heading" or "track this VOR radial." Now, let me show you the actual MCP panel itself. Look at Figure 26.13, "Mode control panel." You’ll see the buttons and knobs you’d actually use. There’s the A/P ENGAGE with CMD A and CMD B—those are the two autopilot command channels, A and B. There’s CWS A and CWS B, which is Control Wheel Steering. There’s the DISENGAGE button. There’s the Flight Director on/off, the F/D ON and OFF. There are mode buttons: V NAV for vertical navigation, L NAV for lateral navigation, VOR LOC for VOR localizer capture, APP for approach, HDG for heading, ALT for altitude, VS for vertical speed, LVL CHG for level change, SPEED, N1, and C/O for changeover. There’s the altitude selector with the digits, the heading selector, the IAS/MACH selector, and the vertical speed wheel with UP and DN. And there’s the MA—that’s the master caution—and the SPD INTV and ALT INTV for speed and altitude intervention. And finally, look at Figure 26.11, "Boeing 737-400 Flight Mode Annunciations." That shows you what the FMA displays on the PFD—the annunciations that tell you which modes are armed and which are active. So to tie it all together: the FMC plans the flight, the MCP lets you select the modes, the auto-flight computer takes the inputs from the VOR/ILS receivers, the FMC, the air data computer, and the attitude and heading systems, and the AFDS and auto throttle execute. The FMA on the PFD tells you what’s happening. That’s the whole AFS architecture on the 737-400.