Autopilot — Page 363, Lesson 432

Let’s pick this up right where the autopilot’s engagement rules leave off. I want to walk you through the automatic disengagement conditions first, because these are the exact situations where the A/P—the autopilot—will drop out of control on its own, and you need to know every single one of them cold. Here’s the full list of what automatically disengages the A/P. Pressing either A/P disengage switch—that’s the pilot’s or the co-pilot’s dedicated disengage button. Pressing either TOGA switch—that’s Take-Off/Go-Around—with a single A/P engaged in CWS or CMD below 2000 feet RA, where RA is radio altitude. Pressing either TOGA switch after touchdown with both A/Ps engaged in CMD. Pressing a lighted A/P engage switch. Pushing the A/P disengage bar down. Activating either pilot’s control wheel trim switch. Moving the Stabilizer Trim Autopilot Cut-out Switch to CUT-OUT. Loss of respective hydraulic system pressure. Repositioning the EFI transfer switch—EFI being the Electronic Flight Instrument transfer. Either left or right IRS system failure or FAULT light illuminated—IRS is the Inertial Reference System. And finally, loss of electrical power or a sensor input which prevents proper operation of the engaged A/P and mode. Now, a critical operational rule: only one A/P can be engaged at a given time unless the approach (APP) mode is engaged. So if you press the engage switch for a second A/P while not in APP mode, that second autopilot engages as selected and the first A/P disengages. The second A/P then operates in CWS or CMD without interrupting CWS or command operation—so the handover is seamless. There’s also a specific behaviour when you engage an A/P with the CMD engage switch during FD-only operation—FD being Flight Director—while pitch or roll commands are more than ½ scale from centred. In that case, the A/P automatically engages in CWS for pitch and/or roll, and the FD command bars retract. So the autopilot won’t slam into command mode with a big command error; it goes into CWS and pulls the flight director bars back. Now let’s move to automatic synchronization, which is the heart of a smooth engagement. Beyond the pre-engage requirement that the autopilot circuits are electrically complete, you must also ensure that on engagement the ‘take-over’ is effected smoothly and without ‘snatching’ of the aircraft’s control system. In other words, the aircraft must be trimmed for the desired flight attitude before engagement, and the automatic control system must be synchronized to maintain that attitude on engagement. Here’s how it works in the majority of autopilot systems. Synchronization is effected by specifically designed synchronizing circuits which automatically sense any existing ‘standing signals’ in the pitch and roll channels and automatically reduce or ‘wash out’ these signals to zero. This stops the servo actuator in a position which is synchronized with the datum attitude detected by the sensing element. That position is indicated by the return of the trim indicator pointer to its central position. So let me tie that together. A standing signal is any residual error signal in the pitch or roll channel before engagement. The synchronizing circuit senses it and washes it out to zero, so the servo actuator—the motor that moves the control surfaces—stops at a position matched to the datum attitude. And you, the pilot, see this as the trim indicator pointer returning to centre. That’s your cue that the autopilot is properly synchronized and ready for a smooth, snatch-free takeover. That figure shows the A/P actuator in parallel, which is the physical arrangement that lets this synchronization work without fighting the pilot’s inputs. So the key takeaway: engagement isn’t just about electrical completeness—it’s about trimming the aircraft and letting the synchronizing circuits wash out standing signals so the servo stops at the right datum position, confirmed by the trim indicator centred. That’s what prevents the aircraft from snatching when the autopilot takes over.