Flight Director Systems — Page 348, Lesson 405

Right, let's pick up with the Flight Director Systems and get into the navigation modes. We've already covered the basics of how the flight director works, so now we're looking at the different ways you can command it to fly. The first mode I want to walk you through is the Heading Mode. This is the simplest navigation mode. In this mode, the aircraft will keep flying in the direction you select. You, as the pilot, select a desired heading using a knob on the HSI, 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 to bring the aircraft onto that desired heading. So it's a direct command: you pick a heading, and the flight director tells you how to get there. Now, the next selection is the LOC/VOR mode, which is also referred to as LNAV. This is where things get more interesting. This selection allows VOR, Localizer, or INS/IRS/FMS/GPS navigation information to be fed into and displayed on the FDS. Let's break down the VOR case first. After you've tuned and correctly identified the station you want to use, you select VOR/LOC on the mode selector. Then you set the desired track to or from the beacon in the course selection window, using either the HSI, the MCP, or the CDU. Once that's done, the FDS will give you the appropriate steering command to intercept and then maintain that track. So it's not just flying a heading — it's flying a specific track relative to a beacon. Intercepting the Localizer is very similar, but we'll discuss that in more detail later. Now, here's the key point about modern aircraft. 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 on the display. And here's the really important part — the reason this mode is so powerful. 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. There's no need for wind velocity information, although the INS could provide it. It's purely an effect of the flight director giving commands to fly the desired track. Let me make sure that's clear, because it's a fundamental concept. If you're in Heading Mode and there's a crosswind, you'll drift off your intended ground track. But in LNAV mode, the system is comparing where you actually are relative to the desired track, and it commands you to fly back onto that track. So it automatically corrects for wind drift without you having to calculate any wind correction angle. That's the beauty of track-based navigation versus heading-based navigation. So to summarise what we've covered: Heading Mode is simple — you select a heading and the FDS commands you to fly it. LNAV mode is track-based — you select a course or a navigation source, and the FDS commands you to intercept and maintain that track, automatically compensating for drift. Both are fundamental modes you'll use constantly in the aircraft.