Autoland — Page 388, Lesson 477

I want to walk you through the autoland sequence, and we're starting right where the previous lesson left off — leaving the go-around mode. This is the transition phase, and it's governed by a hard altitude rule: below 400 feet radio altitude, the autopilots must be disengaged before you can change either the pitch or the roll mode out of GA. Above 400 feet radio altitude, you're free to select other pitch and roll modes. Now, the order in which you change those modes matters, and the system has a specific logic for each case. If you change the roll mode first, the selected mode engages in single autopilot roll operation, and it's controlled by the autopilot that was first engaged. Meanwhile, pitch control stays in the dual autopilot GA mode. So you've got a split — roll is now single, pitch is still dual. The pitch mode is the tricky one. It cannot be changed from GA until sufficient nose-down trim has been input to allow single autopilot operation. That nose-down trim is added automatically, and its purpose is to reset the previous trim inputs that were applied during the auto-approach. So the system is undoing the trim that was set up for the approach before it lets you leave GA in pitch. If pitch is the first mode changed from GA, the selected pitch mode engages in single autopilot operation, again controlled by the first autopilot that was engaged for the approach. The second autopilot then disengages, and the roll mode changes to CWS R — that's Control Wheel Steering in the roll axis, meaning the roll channel reverts to manual steering through the control wheel. Now, with pitch still engaged in GA, here's what happens as you climb: ALT ACQ engages when you're approaching the selected altitude, and ALT HOLD engages at the selected altitude — but only if the stabilizer position is satisfactory for single autopilot operation. That's a key qualifier. The transition from GA to ALT ACQ is normally successful if the selected altitude is at least 1000 feet above the GA engagement altitude. And a higher selected altitude may be required if you're using full GA thrust — because full thrust gives you a steeper climb, so you need more room to capture. Here's the failure case: if stabilizer trim is not satisfactory for single autopilot operation, ALT ACQ is inhibited, the autopilot disengage lights illuminate steady red, and pitch remains in GA. To clear those disengage lights, you have two options — select a higher altitude, or disengage the autopilots entirely. Now let's move to the Approach mode with a single autopilot. You can execute a single autopilot ILS approach by engaging only one autopilot after pressing the APP switch. The operation is the same as dual, with three specific exceptions. First, an autopilot status of 1 CH is annunciated for the entire approach after localizer capture — that's telling you you're running on a single channel. Second, full automatic flare and touchdown capability is not available — FLARE is not annunciated, and stabilizer trim bias is not applied. Third, an autopilot go-around is not available. So with a single autopilot, you're landing manually — the system will guide you down the localizer and glideslope, but it won't flare or touch down for you, and you can't rely on the autopilot for a go-around. Let me show you the sequence visually. This is Figure 27.1, the autoland sequence. And here's the categories of low visibility landing capabilities. The diagram on the next page shows an automatic landing, and the exercise is to fill in the pitch and roll armed and engaged modes in the annunciator boxes at each stage. I've started you at point A, where Approach mode has been selected. So as you work through it, you're tracking which modes are armed and which are engaged in both pitch and roll at each phase of the approach and landing.