Autoland — Page 388, Lesson 472

We're now into the autoland sequence itself. I want to walk you through exactly what happens, in order, as the aircraft descends through the approach. This is the heart of the autoland chapter, so let's take it step by step, starting at 1500 feet radio altitude. First, a quick definition. Radio altitude, which we abbreviate as RA, is the height of the aircraft above the ground, measured by the radio altimeter. It's different from barometric altitude, which is height above sea level. For autoland, everything is referenced to radio altitude, because that's the true height above the runway. So, at 1500 feet RA, shortly after the aircraft has captured both the localizer and the glideslope, and is descending below that 1500-foot mark, the second autopilot couples with the flight controls. Let me unpack that. In a dual autopilot approach, the first autopilot is already flying. At this point, the second autopilot engages and takes control of the flight controls as well. At the same moment, the FLARE mode arms — that's annunciated, meaning it's shown to the pilots. The autopilot go-around mode also arms, but interestingly, it is not annunciated. And if the aircraft has ROLL OUT mode available, that will also arm now. Also at this point, the autoland status is annunciated. It will show either "LAND 2" or "LAND 3". LAND 2 is for fail-passive aircraft, and LAND 3 is for fail-operational aircraft. I'll come back to what those terms mean, but for now, just know that's the annunciation you'd see. Now, an important limitation. The pitch and roll axes cannot be manually overridden into CWS — that's Control Wheel Steering. If you try to do that, the result is autopilot disengagement. So once you're in the autoland sequence, you don't grab the controls and try to steer it manually through CWS; that will kick the autopilots off. Next milestone, 800 feet RA. The second autopilot must have been engaged by 800 feet RA to execute a dual autopilot approach. If it hasn't been engaged by then, engagement of the second autopilot is inhibited on descending through 800 feet. So there's a hard gate there — you either have both autopilots in by 800 feet, or you don't get a dual autopilot approach at all. Now we move to 400 to 330 feet RA. This is where the stabilizer gets automatically trimmed an additional amount nose-up. The elevators neutralize and hold that pitching-up moment. Let me explain why. This biasing aids the flare — it pre-positions the aircraft for the flare manoeuvre. And here's the safety logic: if the autopilots subsequently disengage, the aircraft will tend to pitch nose-up. That prevents a hard contact with the ground, and it aids the initiation of a go-around. But there's a caveat for the pilots: if the autopilots do disengage, you may need to apply forward control column force to hold the desired pitch attitude, because the aircraft is biased nose-up. There's also a failure condition here. If FLARE is not armed by approximately 350 feet RA, both autopilots automatically disengage. So that's a hard limit — no flare armed by 350 feet, and the system gives up. Now we get to the flare itself, at 45 feet gear altitude, which corresponds to 50 feet radio altitude. Let me define gear altitude, or GA. It's a calculated altitude, pre-programmed into the computer, based on radio altitude, pitch attitude, and the known distance between the landing gear, the fuselage, and the radio altimeter antenna. So it's the height of the landing gear above the ground, computed from those inputs. The autopilot flare manoeuvre starts at approximately 50 feet RA and is completed at touchdown. When it starts, FLARE engaged is annunciated, and the flight director command bars retract. Also, the stabilizer trim is again automatically trimmed an additional amount nose-up. Now, the FLARE mode itself. It automatically engages, replacing the glideslope mode. Its job is to reduce the vertical speed from around 10 to 12 feet per second at 50 feet, down to 1 to 2 feet per second at the point of touchdown. And here's the controlling signal: it uses the rate of reduction of radio height. So it's monitoring how quickly the radio altitude is decreasing, and using that to control the flare. At the same time, the autothrottle begins retarding thrust at approximately 27 feet RA, so as to reach idle at touchdown. So the throttles start coming back at 27 feet, and they're at idle by the time you touch down. Next, at about 5 feet gear altitude, the flare mode is disengaged, and there's a transition to touchdown. The localizer disengages at this point, and the roll-out mode, if available, will engage. At about 1 foot gear altitude, the pitch attitude of the aircraft is decreased to 2 degrees. So the nose is being lowered slightly. And finally, at touchdown, a command signal is supplied to the elevators to lower the aircraft's nose, bringing the nose landing gear wheels in contact with the runway, and holding them there during the roll-out. So let me tie the whole sequence together. From 1500 feet, you get the second autopilot in, flare armed, go-around armed, and the LAND 2 or LAND 3 annunciation. By 800 feet, the second autopilot must be in. Between 400 and 330 feet, the stabilizer biases nose-up to prepare for flare and protect against hard contact. At 50 feet RA, the flare manoeuvre starts, reducing vertical speed from 10-12 feet per second to 1-2 feet per second. The autothrottle retards from 27 feet to idle at touchdown. At 5 feet, flare disengages and roll-out engages. At 1 foot, pitch is reduced to 2 degrees. And at touchdown, the elevators lower the nose gear to the runway and hold it there. One thing I want to make sure you understand is the difference between LAND 2 and LAND 3, because it's tied to fail-passive versus fail-operational. A fail-passive system, which gives you LAND 2, is one where, if a failure occurs, the system simply disengages and hands control back to the pilot in a passive way — it doesn't actively continue the landing. A fail-operational system, which gives you LAND 3, is one where, after a failure, the remaining system can still complete the autoland. That's the distinction you'll see in the annunciation. Now, I want to show you the full sequence visually, because it really helps to see the order of events laid out against the altitude scale. Let me bring that up for you.