So it can replace the VG’s job of providing attitude — Page 340, Lesson 397

Right, so we’ve just been talking about systems that need attitude information — the autopilot being the classic example. Now, on more modern and larger aircraft, that attitude information doesn’t have to come from a Vertical Gyro, or VG, anymore. Instead, they can use data from their INS or IRS — that’s the Inertial Navigation System or Inertial Reference System. The reason is simple: the INS is just a more modern and much more sensitive version of a gyro. So it can replace the VG’s job of providing attitude. Now let’s look at how the pilot actually controls this whole setup. We have the Mode Controller, or on modern aircraft, the Mode Control Panel — the MCP. This is the pilot’s interface with the Flight Director System, the FDS. Through the mode controller, the pilot can do three main things: change the mode of the FDS, alter the pitch trim, and switch the FDS display on or off as required. Now, an important point — the modes that are actually available depend on the fit of the aircraft. Different aircraft have different equipment, so the mode options vary. On modern aircraft, the MCP is the panel where most of the information for the autopilot is programmed, and it replaces the older, separate FD mode controller. So instead of having a separate box for the flight director modes, it’s all integrated into the MCP. We’ll get into the typical modes and the pitch trim a bit later, and we’ll describe the MCP in more detail later too. Then we have the Mode Annunciators, also called Mode Indicators, or on the PFD — the Primary Flight Display — they’re called the Flight Mode Annunciators, the FMA. These are a series of simple lights, magnetic indicators, or a small panel of illuminated indicators. Their job is to show the pilot what mode, or what phase of a particular mode, the FDS or autopilot is currently in. So the pilot sets a mode on the MCP, and the annunciators tell them what’s actually happening. 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 a key detail: these panels are usually only powered when the FD switch is on. So if the flight director is switched off, the annunciators won’t be lit. Let me show you what a typical electromechanical flight director system looks like, so you can see how these components fit together. So to recap the flow: the attitude information comes from the VG, or on bigger modern aircraft, from the INS/IRS. The pilot programs the autopilot through the MCP, and the FMA tells them what mode is active. That’s the basic architecture we’re building on.