Flight Director Systems — Page 348, Lesson 398

We're starting the Flight Director Systems chapter, and I want to pick up right where the text leaves off — with the mode annunciators, because that's the pilot's window into what the flight director is actually doing. Let me set the scene first. You're on an approach, and the flight director system, the FDS, is computing steering commands for you. But before you trust those commands, you need to know what mode the system is in — is it tracking the localizer, capturing the glide slope, or holding an altitude? That's exactly what the annunciator lights tell you. Here's the key sequence for an ILS approach. When the aircraft is awaiting capture of the localizer, the LOC light — that's the localizer light — will typically be amber. Amber means "armed," the system is ready and waiting. When the localizer has actually been captured, that light changes to green. Green means "engaged," the system has locked on. Then, approaching the glide slope, the GS light will be amber while it's armed, and again it will change to green to indicate that the FDS has locked on to the GS — the glide slope. Now, more capable systems have more functions and consequently more lights — for example, lights indicating the state of the autothrottle, and the flare or go-around modes. Here's where the display gets interesting. On EFIS-equipped aircraft — that's Electronic Flight Instrument System — the appropriate information is displayed on the PFD or EADI in an area called the Flight Mode Annunciator, the FMA. Because the same space can be used for all the different messages, it can be kept small. And this is a crucial point: all the modes for the FD, the autothrottle, and the autopilot are displayed here in one place. There's a color and position code you need to memorize. Newly changed information is often emphasized on the FMA by surrounding it in a box — that draws your eye to what just changed. Engaged modes are shown on the top line in green, while armed modes are on the line below in white. So green on top means "this is what's active right now," white below means "this is what's standing by." Now let's move into the Flight Director Modes themselves. As I said, the modes vary for each system. The modes I'm about to describe are the most common, but not all may be available depending on the aircraft fit — so don't assume every aircraft has every mode. Before we get into the modes, you have to appreciate a fundamental distinction in how data is displayed to the pilot. There are two kinds of information: raw and processed. The glide slope indicator to the side of the ADI — the Attitude Director Indicator — and the CDI bar, the Course Deviation Indicator, are what we call raw information. Raw means the information is not processed in any way. It simply indicates that the aircraft is left or right of track or localizer, or above or below the glide slope. And knowledge of the system will tell you by how many degrees — the raw display gives you the deviation, but it doesn't tell you what to do about it. Here's the crucial point: in order to anticipate turns, climbs, and descents for smooth flying, the FDS can take that raw data, compute it, and signal commands — either to the pilot via the flight director, or to the autopilot. That's the whole purpose of the system. Raw data tells you where you are; the flight director tells you where to go. So when you look at the FMA, you're seeing which of those modes is doing the computing right now — green on top for engaged, white below for armed, and a box around anything that just changed. That's your situational awareness for the entire autoflight system.