Autoland — Page 388, Lesson 477

We're starting a fresh topic now: Autoland, and specifically the behaviour of the autopilot system as you leave the go-around mode. Let's get into it. First, a key altitude to remember: 400 feet RA. RA is radio altitude, the height above the ground measured by the radio altimeter. Below 400 feet RA, the autopilots must be disengaged if you want to change either the pitch or the roll mode from GA. GA here is the go-around mode. Above 400 feet RA, you're free to select other pitch and roll modes. Now, the sequence of changing modes matters. If you change the roll mode first, that selected mode engages in single A/P roll operation. That means only one autopilot is controlling the roll, and it's the autopilot that was first engaged. Meanwhile, pitch control stays in the dual A/P GA mode—both autopilots still handling pitch in go-around. The pitch mode is trickier. You cannot change it from GA until sufficient nose-down trim has been input to allow single A/P operation. This 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 it built up while flying the approach. If the pitch mode is the first to be changed from GA, the selected pitch mode engages in single A/P 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. CWS is control wheel steering, and the R means roll. So you get control-wheel-steering in roll while the pitch is in your selected mode. Now, with pitch still engaged in GA, here's what happens as you climb. ALT ACQ engages when approaching the selected altitude. ALT ACQ is altitude acquire. Then ALT HOLD engages at the selected altitude, provided the stabilizer position is satisfactory for single A/P operation. ALT HOLD is altitude hold. The transition from GA to ALT ACQ is normally successful if the selected altitude is at least 1000 feet above the GA engagement altitude. That's the altitude where go-around engaged. A higher selected altitude may be required if full GA thrust is used—full go-around thrust gives you a steeper climb, so you need more room. Here's a failure condition to watch. If the stabilizer trim is not satisfactory for single A/P operation, ALT ACQ is inhibited. The A/P disengage lights illuminate steady red, and pitch remains in GA. To extinguish those disengage lights, you have two options: select a higher altitude, or disengage the autopilots entirely. Now let's move to the Approach (APP) Mode with a single autopilot. You can execute a single A/P ILS approach by engaging only one autopilot after pressing the APP switch. The operation is the same as dual, with three exceptions. First, A/P status of 1 CH is annunciated for the entire approach after localizer capture. CH is channel. So the system tells you it's running on one channel. Second, full automatic flare and touchdown capability is not available. FLARE is not annunciated, and stabilizer trim bias is not applied. That trim bias is what helps set up the flare in a dual approach. Third, an A/P GA is not available. You cannot get an autopilot go-around with a single autopilot. There's a diagram on the next page showing an automatic landing. It's an exercise where you fill in the pitch and roll armed and engaged modes in the annunciator boxes at each stage of the approach and landing. Point A is already started for you—Approach mode has been selected there.