Autopilot — Page 377, Lesson 461

Let's pick this up right where we left off — we were just talking about the autopilot's manual control and the TCS button, the Touch Control Steering. When you press and hold that TCS button, the autopilot disengages and hands control back to you, the pilot, for manual flying. The moment you release the TCS button, the autopilot re-engages and once again holds the aircraft in the attitude in which you left it. So it's a temporary hand-off, not a full disconnect. Now I want to move into the autopilot's limitations and operational restrictions — the rules and warnings that govern when the autopilot can and cannot be used, and how it tells you something's wrong. First, let's talk about the disengage warnings. When an autopilot has disengaged, the "A/P" light flashes red and a tone sounds. That's your immediate alert that the autopilot is no longer in command. You can reset that warning by pressing either disengage light or either A/P disengage switch — so there are two lights and two switches, and pressing any one of them clears the warning. Now, the A/P warning light stays at a steady red under four specific conditions. Let me walk you through each one. First, when the stabilizer is out of trim below 800 feet RA on a dual channel approach. RA is radio altitude — the height above the ground measured by the radio altimeter. So if you're on a dual channel approach — meaning two autopilots working together for an autoland — and the stabilizer isn't trimmed properly below 800 feet, you get a steady red light. Second, when Altitude Acquire mode is inhibited during an A/P go-around. Altitude Acquire is the mode that captures and holds a selected altitude. During a go-around with the autopilot engaged, if that mode is inhibited — meaning it can't engage — the stabilizer isn't trimmed for single A/P operation, and you get the steady red light. There's a cross-reference there to the A/P Go-around notes. Third, when the disengage light test switch is held in position 2 — that's the red filament test. This is a test function that checks the red filament of the light bulb itself. Fourth, when there's an automatic ground system test fail. That's a self-test failure detected on the ground. Now, the light can also show steady amber — that happens when the disengage light test switch is held in position 1, the amber filament test. So position 1 tests the amber filament, position 2 tests the red filament. And there's one more condition: the light will flash amber if the autopilot automatically reverts to CWS pitch or roll while in CMD. CWS is Control Wheel Steering — that's the mode where you manually steer the aircraft and the autopilot holds the attitude you leave it in. CMD is Command mode, the normal fully-automatic mode. So if the autopilot is in CMD and it automatically drops back to CWS in pitch or roll, you get a flashing amber light. That light resets when either light is pressed or another mode is engaged. Now let's talk about the maximum pitch and bank angles. During normal autopilot operation, the maximum pitch angle is ±10 degrees, and the maximum roll angle is ±30 degrees. But here's the important caveat — these limits are not stipulated legally. They're not regulatory requirements. They will vary from aircraft to aircraft. So these are typical values, not universal law. Next, I want to cover gain adaptation. This is a crucial concept. Variations in flight parameters — things like altitude, speed, aircraft load, configuration, and rate of manoeuvre — all affect the handling characteristics of an aircraft. Handling characteristics are how the aircraft responds to control inputs. So as these parameters change, the aircraft behaves differently. To counter that, flight control systems incorporate what are called 'gearing' elements. These adapt the parameters to the aircraft so that their effect on handling characteristics is reduced. In automatic systems, the response is altered by changing the 'gain' of the system to a given level of input signal. Gain is essentially the amplification factor — how much output you get for a given input. You can liken this to changing gear ratios in a mechanical system — just as you change gears to match speed and load, the autopilot changes its gain to match flight conditions. Gain adaptation is particularly important for maintaining handling characteristics with changes in IAS — that's Indicated Airspeed — during the different phases of flight. And it's similar to the gain scheduling in the flight director system. So the autopilot and the flight director both use this same principle of adjusting gain to keep the aircraft's response consistent. Finally, let's touch on the approach and land mode. During an autoland sequence, the autopilot has to execute many important manoeuvres. These are described in the Autoland notes — that's a separate section that details the full autoland procedure. Let me show you a figure that illustrates how the autopilot actuator works in parallel with the flight controls — this will help you visualise how the autopilot actually moves the control surfaces. So to summarise what we've covered: the disengage warnings with their red and amber indications and the four steady-red conditions, the ±10° pitch and ±30° roll limits that aren't legally mandated, gain adaptation as the system's way of adjusting its response to changing flight conditions, and the approach/land mode that handles the autoland sequence. That's the full picture of autopilot limitations and operational restrictions.