Autopilot — Page 377, Lesson 459

Let's pick up with the autopilot, and I want to walk you through the CWS modes in detail, because this is where the autopilot starts behaving like a smart assistant rather than just a robot holding a heading. First, the big picture. CWS stands for Control Wheel Steering. With CWS engaged, the autopilot manoeuvres the aeroplane in response to control pressures applied by either pilot. So you, the pilot, physically push on the controls, and the autopilot reads that pressure and flies the aeroplane accordingly. The key point here is that the control pressure required is similar to what you'd need for manual flight. It's not a heavy override, it's a natural feel. When you release that control pressure, the autopilot holds the existing attitude. So you push, it moves; you let go, it freezes the attitude. Now, there's a very specific behaviour in roll. If you release aileron pressure with 6 degrees or less angle of bank, the autopilot rolls the wings level and holds the existing heading. So under 6 degrees of bank, letting go means it levels the wings and keeps the heading. That's the heading hold feature. But — and this is critical for your exam — this heading hold feature with bank less than 6 degrees is inhibited under three conditions. Let me list them precisely. First, below 1500 feet RA with the landing gear down. RA is radio altimeter, so that's your height above the ground. Second, after FD VOR capture with TAS 250 knots or less. FD is flight director, VOR is the VHF omnidirectional range navigation aid, and TAS is true airspeed. So after the flight director captures the VOR radial, and you're at 250 knots or less true airspeed, the heading hold is inhibited. Third, after FD LOC capture in the APP mode. LOC is the localizer, and APP is approach mode. So once the flight director captures the localizer in approach mode, again, no heading hold. Why does this matter? Because in those situations, you don't want the autopilot suddenly rolling wings level and holding a heading when you're trying to track a radial or a localizer. So it's inhibited to let the navigation modes take over. Now let's move to pitch CWS with a CMD engage switch selected. CMD is the command mode, the normal autopilot engaged mode. So here's the scenario: the pitch axis engages in CWS while the roll axis is in CMD. That happens under two conditions. First, when a command pitch mode has not been selected, or was de-selected. Second, when the autopilot pitch has been manually overridden with control column force. And note this carefully — the force required for this override is greater than the normal CWS control column force. So it's a deliberate, heavier push. And there's a limitation: this manual pitch override is inhibited in the APP mode with both autopilots engaged. So in approach mode with both A/Ps on, you cannot manually override pitch this way. While this mode is engaged, CWS P is annunciated on the Flight Mode Annunciators, the FMA. That's your display telling you what the autopilot is doing. And once you're in CWS P, command pitch modes can then be selected. So you can re-engage a normal pitch mode. Now here's a beautiful sequence for altitude. When approaching a selected altitude in CWS P with a CMD engage switch selected, CWS P changes to ALT ACQ. ALT ACQ is altitude acquire. And when at the selected altitude, ALT HOLD engages. So it captures the altitude and holds it. But what if you override it? If pitch is manually overridden while in ALT HOLD, the annunciator changes back to CWS P. Now, if you release the control force within 250 feet of the selected altitude, CWS P changes to ALT ACQ and the autopilot returns to the selected altitude, and ALT HOLD engages again. So within 250 feet, it recaptures. But if you hold the elevator force until you're more than 250 feet from the selected altitude, pitch remains in CWS P. It does not recapture. So that 250-foot window is the decision boundary. Now roll CWS with a CMD engage switch selected. Symmetric logic. The roll axis engages in CWS while the pitch axis is in CMD under two conditions. First, when a command roll mode has not been selected or was de-selected. Second, when the autopilot roll has been manually overridden with control wheel force. And again, the force required for override is greater than the normal CWS control wheel force. CWS R is annunciated on the FMA while this mode is engaged. And here's a very practical use. CWS R with a CMD engage switch illuminated can be used to capture a selected radio course while VOR/LOC or APP mode is armed. So you're armed for a VOR or localizer capture, and you use CWS R to steer toward it. Upon intercepting the radial or localizer, the FD and autopilot annunciation changes from CWS R to VOR/LOC engaged, and the autopilot tracks the selected course. So you manually steer with CWS R until the autopilot takes over and tracks the radial. Finally, Touch Control Steering, TCS. TCS also permits a pilot to manoeuvre the aircraft in pitch or roll. But unlike CWS, the appropriate automatic control channels and servomotors are disengaged while the TCS button is held depressed. So while you hold that button, the autopilot's servos are out of the loop, and you fly the aircraft to the desired attitude using the controls directly. When you release it, the autopilot re-engages and holds that attitude. So the contrast to remember: CWS keeps the autopilot in the loop and reads your pressure; TCS physically disengages the servomotors while you hold the button. Both let you manoeuvre, but the mechanism is different. Let me show you the actuator arrangement in parallel, because that's how these forces are applied. That's the parallel actuator. And here's the trimming arrangement with the variable incidence tailplane. The output is applied to the trim tab actuator or the horizontal stabilizer. That's how the autopilot trims the aircraft in pitch. So to tie it together: CWS gives you a natural, pressure-based way to steer with the autopilot holding the result, with specific inhibitions and altitude-capture logic. TCS gives you a direct, servo-disengaged way to fly to a new attitude. Both are tools you'll use in the cockpit, and the annunciations on the FMA tell you exactly which mode is active.