Autoland — Page 388, Lesson 475

We're starting a new topic now: Autoland. This is the heart of Cat 3 low-visibility operations, and I want to walk you through the sequence and the special modes that make it work. Let's begin with the very end of the landing sequence, because that's where the excerpt starts. When reverse thrust is applied, the autothrottle system—the A/T—is automatically disengaged. But here's the key point: even if you never deploy reverse thrust, the A/T automatically disengages approximately 2 seconds after touchdown. So that's a hard time-based cutoff, independent of what the crew does with the thrust levers. Now, the automatic flight control system itself—the AFCS—stays on until the flight crew manually disengages it. That manual disengagement is the moment the autoland sequence is considered complete. So the sequence isn't finished at touchdown; it's finished when the crew takes over and switches the system off. Now let's look at the other features of autoland, and the first one is runway alignment. You've heard me talk about the pitch channel and the roll channel, but the yaw channel hasn't come up yet. Any auto-flight system capable of an autoland must be capable of what's called "kicking off drift" prior to touchdown. This is known as runway alignment mode. It's typically armed at the same time as the flare mode, and it engages at less than 100 feet. Here's how it works. During the approach, from 1500 feet down, the yaw channel continuously computes the difference between heading and track. That's the difference between where the nose is pointing and where the aircraft is actually going—the drift angle. When align mode engages, the rudder deflects to align the aircraft with the runway centre line before touchdown. This manoeuvre is called de-crabbing, or drift "kick-off." So instead of landing with the nose cocked into the wind, the system straightens the aircraft out just before the wheels touch. The next function is roll-out. This is another function of Cat 3 autoland systems. Roll-out gives steering commands on the ground proportional to localizer deviation along the centre line. So as the aircraft rolls down the runway, the system measures how far off the localizer centre line you are, and it produces steering commands proportional to that deviation. These commands can show left or right steering guidance through a rotating "barbers pole" indicator, known as a para-visual display—the PVD. Alternatively, automatic steering can be achieved by applying the deviation signals directly to the rudder channel and nosewheel steering, keeping the aircraft on the centre line throughout the ground roll. Now let's talk about the A/P go-around mode, the GA mode. This is a critical safety feature. The A/P GA mode requires dual A/P operation—you need both autopilots engaged. It becomes armed when FLARE armed is annunciated. And there are two hard limits: the A/P GA mode cannot be engaged before flare arm is annunciated, and it cannot be engaged after the A/P senses touchdown. There's an important note here. If the GA mode is selected after touchdown but prior to A/T disengagement, the A/Ps will disengage and the A/Ts may command GA thrust, with the procedure being flown manually. So you'd be flying the go-around by hand, with the autothrottles possibly pushing the thrust up. Pressing either TOGA switch—that's Take-Off/Go-Around—engages the GA mode, and GA engaged is annunciated for the AFDS. When that happens, the MCP IAS/Mach display becomes blank, and the airspeed cursors are positioned at the AFDS commanded speed. That command airspeed is the flap manoeuvring speed. Now let's break down the pitch control and roll control in the GA mode. On GA engagement, the thrust levers advance toward the reduced GA N1. The A/P initially commands a 15 degree nose-up pitch attitude, and the airspeed cursors display manoeuvring speed for the flap setting. When a programmed rate of climb is established, the A/P switches its strategy—it controls pitch to hold airspeed based on the normal flap manoeuvring speed. For roll control, with the GA mode engaged, the A/Ps maintain the aeroplane ground track that existed at GA engagement. So whatever heading or track you were on when you pressed TOGA, the autopilots hold that ground track as you climb away. Let me tie this together for you. The autoland sequence has this structure: the approach brings you down, the yaw channel handles drift, flare arms and engages, runway alignment kicks off the drift below 100 feet, touchdown happens, the A/T disengages automatically about 2 seconds later, roll-out steers you down the centre line, and the crew manually disengages the AFCS to complete the sequence. If anything goes wrong and you need to go around, the GA mode is there—but only between flare arm annunciation and touchdown sensing, and only with both autopilots engaged. That's the full picture of autoland and its associated modes.