Let me set the scene first — Page 348, Lesson 406

I want to walk you through the Flight Director Approach, or FDA, procedure — this is the heart of how the flight director system flies an ILS approach for you. Let me set the scene first. We're coming off the previous point: the flight director does not give you wind velocity information, even though the inertial navigation system could provide it. That's purely an effect of the flight director giving commands to fly the desired course or track you've selected. So keep that in mind — the flight director is a command generator, not a weather computer. Now, the FDA. The first thing we do is tune the ILS frequency into the VHF nav radios, and we identify the station — that's standard practice, you always identify the beacon before you trust it. Then we set up the QDM for the approach in the course window. QDM is the magnetic bearing to the station, the heading you'd fly to get there. So we dial that into the course window. Next, the mode selector goes to AUTO/APP. That's the automatic approach mode. Once we do that, the annunciator will show that the flight director system, the FDS, is looking to capture the localizer — the LOC. That's the lateral beam that lines you up with the runway centerline extended. Now here's the sequence. After beam capture, in a similar manner to VOR tracking, the FDS will — given enough distance — arrange the intercept to establish on the localizer with a turn demand. And the annunciator panel will indicate LOC capture. So you get a turn command, you follow it, and the system tells you when you've captured the localizer. Once we're established on the localizer, the FDS is now looking for the glide slope signal. The annunciator GS light, or a magnetic indicator, will indicate that. The glide slope is the vertical beam that gives you the correct descent path to the runway. As soon as the glide slope is captured, the annunciator changes again, and the FDS will indicate a pitch nose-down demand to fly the ILS glide slope. So you get a pitch command — nose down — to follow the descent path. Now, here's a real-world trap. If the interception of the localizer has been misjudged, it's possible to end up established on the localizer past the point of GS intercept — that means you're above the glide slope. You've crossed the point where you should have intercepted the glide slope, but you're too high. Modern systems may automatically adjust and capture from above by increasing the rate of descent. So the newer computers will just steepen your descent to catch the glide slope from above. But on older aircraft, it was sometimes possible to temporarily switch the FDS mode to MAN/GS. That's manual glide slope mode. This forces the FDS into accepting a capture from above. So you're telling the computer, "I know I'm high, capture the glide slope anyway." And there are two more uses for MAN/GS. It can also be used to establish a fixed intercept angle of the LOC beam — so you control the angle at which you join the localizer. And it can force a LOC or GS capture condition if it is known that the beam-sensing circuits of the computer are inoperative. So if the computer's beam-sensing circuits are dead, MAN/GS lets you force the capture manually. Let me tie the whole sequence together for you. Tune and identify the ILS, set QDM in the course window, select AUTO/APP. The annunciator shows the FDS is looking for the LOC. On beam capture, you get a turn demand and the annunciator shows LOC capture. Then the FDS looks for the glide slope — the GS light or magnetic indicator shows that. On glide slope capture, the annunciator changes and you get a pitch nose-down demand. And if you're high, modern systems capture from above by increasing rate of descent; older systems use MAN/GS to force the capture. One thing I want you to notice: the annunciator is your eyes into what the flight director is doing at every stage. It tells you what the FDS is looking for, and when it has captured. That's your situational awareness during the approach.