Let’s pick this up right where the autopilot needs attitude information — Page 340, Lesson 397

Let’s pick this up right where the autopilot needs attitude information. I want you to picture the vertical gyro, or VG, as the source of that attitude data. On more modern and larger aircraft, though, we often replace the VG with data from the INS or the IRS — the Inertial Navigation System or the Inertial Reference System. The reason is simple: the INS is a much more modern and much more sensitive version of a gyro, so it can supply the same attitude information with far greater precision. Now, the pilot doesn’t just fly the flight director system by hand. He uses a device called the Mode Controller, or on modern aircraft, the Mode Control Panel — the MCP. The mode controller lets the pilot change the mode of the FDS, the Flight Director System. It also lets him alter the pitch trim, and switch the FDS display on or off as required. So think of it as the pilot’s command centre for the flight director. The modes that are available depend on the fit of the aircraft — meaning different aircraft have different equipment, so they offer different modes. On modern aircraft, the MCP is the panel on which most of the information for the autopilot is programmed, and it replaces the separate FD mode controller. So instead of having a dedicated little box just for the flight director modes, the MCP does that job and more. We’ll discuss the typical modes and the pitch trim a little later, and I’ll describe the MCP in more detail later as well. Now, how does the pilot know what the system is actually doing? That’s the job of the Mode Annunciators, or Indicators — on a PFD, the Primary Flight Display, they’re called the Flight Mode Annunciators, or FMA. These are a series of simple lights, magnetic indicators, or a small panel of illuminated indicators that show the pilot what mode, or what phase of a particular mode, the FDS or autopilot is in. So if the autopilot is capturing the localiser, or tracking the glideslope, the FMA tells you exactly which phase you’re in. In more basic systems, the most common indications you’ll see are during an ILS approach — that’s the Instrument Landing System. And here’s an important detail: these panels are usually only powered with the FD switch on. So if the flight director switch is off, the annunciators won’t be lit. That’s a good thing to remember, because it tells you the annunciators are tied to the flight director being active. Let me show you what a typical electromechanical flight director system looks like, so you can see how the mode controller, the annunciators, and the attitude information all fit together. So to tie it together: the VG or the INS/IRS gives you attitude, the MCP or mode controller lets you command the modes and pitch trim, and the FMA tells you what the system is doing — but only when the FD switch is on. That’s the core of how the flight director and autopilot interact with attitude information.