Autoland — Page 388, Lesson 475

I want to walk you through the autoland sequence and its supporting features. Let's start with the very end of the sequence, because that's where the autothrottle and the automatic flight control system hand control back to you. When reverse thrust is applied, the autothrottle system is automatically disengaged. But here's the key point: even if you never deploy reverse thrust, the autothrottle, or A/T, automatically disengages approximately 2 seconds after touchdown. So that's a hard time limit — roughly two seconds after the wheels touch, the autothrottle is out of the loop regardless. 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 you, the pilot, switch the system off. Let's move to the other features of autoland. First, runway alignment. Throughout the approach, the yaw channel hasn't been mentioned much, but 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 altitude. During the approach from 1500 feet, the yaw channel computes the difference between heading and track. 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 the aircraft is crabbed into the wind during the approach, and just before landing, the rudder kicks the nose around to align with the centre line. Next, 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. The localizer is the lateral guidance signal that keeps you on the runway centre line. These commands can show left or right steering guidance through a rotating 'barbers pole' indicator, known as a para-visual display, or PVD. Alternatively, automatic steering can be achieved by applying deviation signals to the rudder channel and nosewheel steering, to keep the aircraft on the centre line throughout the ground roll. So you have two options: a visual indicator for the pilot to follow, or fully automatic steering through the rudder and nosewheel. Now the A/P go-around mode. The A/P GA mode requires dual autopilot operation. It becomes armed when FLARE armed is annunciated — that's the moment the flare mode is armed and shown on the annunciator. The A/P GA mode cannot be engaged before flare arm is annunciated, and it cannot be engaged after the A/P senses touchdown. So there's a window: only between flare arm annunciation and touchdown sensing. Here's an important note. If the GA mode is selected after touchdown but prior to A/T disengagement, the autopilots will disengage, and the autothrottles may command go-around thrust, with the procedure being flown manually. So you'd be hand-flying the go-around. Pressing either TOGA switch engages the GA mode. TOGA stands for Take-Off/Go-Around. When you press it, GA engaged is annunciated for the AFDS — the Automatic Flight Director System. The MCP IAS/Mach display becomes blank. MCP is the Mode Control Panel, IAS is Indicated Airspeed. The airspeed cursors are positioned at the AFDS commanded speed. And that command airspeed is the flap manoeuvring speed — the speed appropriate for the current flap setting. Let's look at A/P GA pitch control. Upon GA engagement, the thrust levers advance toward the reduced GA N1. N1 is the fan speed of the engine, and here it's a reduced go-around thrust setting. 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 controls pitch to hold airspeed based on the normal flap manoeuvring speed. So initially it's a fixed 15 degrees nose-up, then it transitions to holding the manoeuvring speed. Finally, A/P GA roll control. With the GA mode engaged, the autopilots 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 during the go-around. Let me show you the autoland sequence visually. That's the full autoland sequence from approach through flare, touchdown, and roll-out. And here's the categories of low visibility landing capabilities. So to tie it together: the autothrottle disengages at reverse thrust or 2 seconds after touchdown, the AFCS stays on until you manually disengage it, runway alignment kicks off the drift below 100 feet, roll-out keeps you on the centre line, and the go-around mode has strict arming and engagement windows with specific pitch and roll behaviour.