Autoland — Page 381, Lesson 470

I want to walk you through the autoland sequence now — this is the heart of how a modern airliner lands itself in low visibility. Let's start with a critical safety rule that governs everything else. The Radio Altimeter Rule If a radio altimeter is inoperative, do not use the associated FCC — that's the Flight Control Computer — or the A/T, the autothrottle, if affected, for approach and landing. Here's the key consequence: failure of a single radio altimeter causes the autoland system to fail passive. Let me unpack that. "Fail passive" means the system doesn't just keep flying blindly — it detects the fault and hands control back to you, the pilot, in a safe, controllable state. So one bad radio altimeter, and you're not doing an autoland; you're flying the approach manually. The Approach (APP) Mode Now the sequence itself, using a Boeing aircraft as our example. It all starts with the Approach mode, which we call APP. The APP mode arms the AFDS — the Autopilot Flight Director System — to capture and track the localizer and the glide slope. You can engage it for dual or single autopilot operation, but here's the crucial distinction: autoland requires dual autopilots. Single autopilot operation won't give you an autoland. Let me explain dual A/P operation, because that's what we need for autoland. Approach mode allows both autopilots to be engaged at the same time. Dual A/P operation provides fail-passive control through landing flare and touchdown, or an automatic go-around. And here's a beautiful detail about how fail-passive works with two autopilots: during fail-passive operation, the flight controls respond to the A/P commanding the least control movement. Think about that — if one autopilot commands a hard left and the other commands a gentle left, the system follows the gentler command. That's how it stays safe and smooth. Before you can even select approach mode, one VHF Nav receiver must be tuned to an ILS frequency. And for a dual A/P approach, the second VHF Nav receiver must also be tuned to the ILS frequency, and the corresponding autopilot must be engaged prior to 800 feet radio altitude. That 800 feet RA is a hard gate — you must have that second autopilot in by then. Localizer and Glide Slope Armed Here's the step-by-step. After you set the localizer frequency and course, you press the APP switch. The APP switch illuminates, and you get an annunciation: VOR LOC and G/S armed. Now, the APP mode permits selecting the engagement of the second autopilot. This arms the second A/P for automatic engagement — and note the conditions: after LOC and G/S capture, and when descent below 1500 feet RA occurs. So the second autopilot doesn't just engage immediately; it waits for those two conditions. You can intercept the localizer in the HDG SEL mode — that's heading select — or in CWS R, which is Control Wheel Steering, or in L Nav, lateral navigation. And either the LOC or the G/S can be captured first, though it's most common to capture the localizer first, then the glide slope. Localizer Capture Now let's look at localizer capture. The capture point is variable — it depends on the intercept angle and the rate of closure. But here's the hard limit: it does not occur at less than half a dot deviation. That's your minimum — you won't capture the localizer until you're within half a dot of the centerline. Upon LOC capture, several things happen simultaneously: VOR LOC annunciates captured, "1 CH" is annunciated for the A/P status — that's your autopilot status indication — the previous roll mode disengages, and the aeroplane turns to track the localizer. Glide Slope Capture The G/S can be captured from above or below, though from below is generally preferred. Capture occurs at 2/5 dot deviation — note that's a different threshold than the localizer's half dot. On G/S capture: G/S annunciates captured, the previous pitch mode disengages, the APP switch light extinguishes if the localizer has also been captured, the aeroplane pitch tracks the glide slope, and the annunciated N1 thrust limit for the A/T — the autothrottle — is GA, which is go-around thrust. Exiting APP Mode Once both LOC and G/S are captured, the APP mode can be exited only three ways: pressing the TOGA switch — that's Takeoff/Go-Around — or disengaging the autopilot and turning off both FD, flight director, switches, or re-tuning a VHF Nav receiver. Those are your only exits. After Capture — The Failure Case Finally, here's what happens if the ILS signal becomes invalid after both are captured: the autopilots will disengage, and the FD command bars will retract to indicate an invalid ILS signal. That retraction is your visual cue — the flight director bars pull away, telling you the signal is no longer trustworthy and you're on your own. Let me tie this together. The whole sequence is a carefully choreographed handoff: arm the system, capture the localizer, capture the glide slope, engage the second autopilot below 1500 feet, and ride it down through flare and touchdown — all fail-passive, all protected by that radio altimeter rule we started with.