Flight Director Systems — Page 348, Lesson 405

Let's get into the Flight Director Systems, specifically the navigation modes. This is where the flight director stops being just a display and starts actively steering you toward a chosen path. First, the big picture. The flight director system, or FDS, can take navigation information from a few different sources. The key idea here is that these sources can be linked to the flight director display, to the autopilot, or to both at the same time. So when you select a mode, you're deciding what information gets fed in and where it goes. Let's start with the simplest one: Heading Mode. This is exactly what it sounds like. You pick a direction, and the aircraft keeps flying that way. The pilot selects the desired heading using a knob on the HSI — that's the Horizontal Situation Indicator — or using the central FDS/autopilot panel, or on the MCP, the Mode Control Panel. Once you've set that heading, the FDS will indicate a "fly to" command. That means it shows you a steering cue, telling you which way to turn to bring the aircraft onto the desired heading. It's a direct command: turn left, turn right, to get where you want to go. Now, the next mode is LOC/VOR, which is also called LNAV. This is where it gets more interesting. This selection allows VOR, Localizer, or INS/IRS/FMS/GPS navigation information to be fed and displayed on the FDS. Let me unpack those acronyms. VOR is a ground-based radio beacon. Localizer is the lateral guidance part of an instrument landing system. INS is Inertial Navigation System, IRS is Inertial Reference System, FMS is Flight Management System, and GPS is Global Positioning System. So this mode lets you use either ground-based radio aids or the more modern internal navigation systems. Let's walk through how it works with a VOR. First, you tune the station and correctly identify it — that's a critical step, you must confirm you're on the right beacon. Then you select VOR/LOC on the mode selector. Next, you set the desired track to or from the beacon in the course selection window, using either the HSI, the MCP, or the CDU, which is the Control Display Unit. Once that's done, the FDS will give you appropriate steering commands to intercept and then maintain that track. So it's not just telling you to fly a heading; it's actively guiding you to capture a specific course line and stay on it. Intercepting the localizer is very similar, but we'll discuss that in more detail later. For now, just know it follows the same principle. Now, here's the modern part. With aircraft being fitted with more accurate navigation systems, it's only logical that this information should be made available to the pilot and autopilot. So INS/IRS/FMS/GPS information can be selected for display in very much the same way as the VOR information, and it's represented in the same way. The flight director treats them all the same — it just needs a desired track to fly. And here's the really elegant part, the key insight. Because the actual track of the aircraft is being compared with the desired track, and the flight director commands are given to fly the desired track, the flight director system will effectively compensate for drift. Think about wind. If a crosswind is pushing you off course, the flight director sees that your actual track doesn't match the desired track, and it commands a correction. There's no need for wind velocity information, even though the INS could provide it. It's purely an effect of the flight director giving commands to fly the desired track. The system doesn't need to know why you're drifting; it just sees the error and corrects it. So to summarize what we've covered: Heading Mode is a simple selected heading, and LOC/VOR or LNAV mode lets you track a course from a VOR, localizer, or modern navigation system, with the flight director automatically compensating for drift. That's the foundation.