Autopilot — Page 373, Lesson 446

Right, let's get into the autopilot chapter. I want to start by looking at the big picture of how the auto-flight system gets its information, and then we'll use the Boeing 737-400 as our example aircraft. First, the data sources. To capture and track a radio beam, the auto-flight computer needs signals from the VOR and ILS receivers. So when you're flying an approach, the VOR and ILS receivers feed that navigation data into the relevant auto-flight computer. Then we have computer-generated data. Modern aircraft can follow a computer-generated flight profile in both roll, which is lateral, and pitch, which is 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. Now, let me walk you through the inputs to the Mode Control Panel, the MCP. This is the panel you'll actually reach for in the cockpit. Look at Figure 26.12, which shows the inputs to the MCP. We have the Magnetic Heading Reference System, the Attitude Reference System, which could be a vertical gyro, an INS, or an IRS, and the Manometric Reference System, which is the Air Data Computer. These all feed into the MCP. The MCP handles the autopilot engage and outer loop input selection. Now, here's the key structure. The autopilot has inner loops and outer loops. The inner loops are the pitch inner loop, the roll inner loop, and the yaw inner loop. These are the fast, stabilising loops that control the aircraft's attitude directly. The outer loop is the higher-level guidance, like capturing a VOR radial or following the FMS flight plan. The outer loop generates the commands that the inner loops then execute. So the VOR/ILS receivers and the Flight Management System feed into the MCP, and the MCP selects which outer loop input you want. The autopilot engage function then connects that outer loop command to the inner loops. The inner loops—pitch, roll, and yaw—then drive the aircraft's control surfaces. Now, let's move to the example aircraft. For these notes, the Boeing 737-400 is used as the example. It's officially the AFDS that the JAR objectives and exams are currently based around, so this is the system you need to know well. The Automatic Flight System, the AFS, for the 737-400 consists of two main parts: the Autopilot Flight Director System, the AFDS, and the Auto throttle, the A/T. The Flight Management Computer, the FMC, provides N1 limits and target N1 for the A/T. N1 is the fan speed of the engine, so the FMC tells the auto throttle what fan speed to aim for. The FMC also provides command airspeeds for both the A/T and the AFDS. Now, how do you operate all this? The AFDS and A/T are operated from the AFDS Mode Control Panel, the MCP. The FMC is operated from the Control and Display Unit, the CDU. The AFDS MCP provides co-ordinated control of four things: the autopilot, the A/P; the Flight Director, the FD; the A/T; and the altitude alert functions. Finally, the status of the AFS modes is displayed on the Flight Mode Annunciators, the FMA, on each pilot's Primary Flight Display, the PFD. That's where you see what mode the autopilot is actually in. Normally, the AFDS and A/T are used to maintain the airspeed and thrust settings calculated by the FMC. So the FMC does the thinking, and the AFDS and A/T do the flying to match those targets. Let me just make sure you've got the key relationships. The FMC provides the targets—N1 limits, target N1, and command airspeeds. The MCP is where you select the modes and engage the autopilot. The FMA shows you what's actually happening. And the inner loops—pitch, roll, yaw—are what physically control the aircraft's attitude. That's the foundation. Now you understand how the auto-flight system is structured and where each piece of data comes from.