Autopilot — Page 377, Lesson 459

Let's pick this up with the Control Wheel Steering system, because it's the heart of what we're about to look at. I want to walk you through how the autopilot behaves when you, the pilot, want to take over and fly the aeroplane by hand without actually disengaging the system. First, the big picture. With CWS engaged, the autopilot manoeuvres the aeroplane in response to control pressures applied by either pilot. So you're not moving the control surfaces directly with brute force — you're applying a pressure, a force, to the control column or control wheel, and the autopilot senses that pressure and moves the aeroplane accordingly. The key point is that the control pressure required is similar to that required for manual flight. It feels like normal flying. And here's the elegant part: when you release that control pressure, the autopilot holds the existing attitude. Whatever attitude you've flown the aeroplane into, that's what it locks onto and maintains. Now let's look at a 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 you've got a heading hold feature that only works when the bank angle is small — 6 degrees or less. But this heading hold feature is inhibited, meaning it's disabled, under three specific conditions. I want you to remember these exactly. First, it's inhibited below 1500 feet RA with the landing gear down. RA is radio altimeter height, so that's your height above the ground. Second, after FD VOR capture with TAS 250 knots or less. FD is the 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 your true airspeed is 250 knots or less, the heading hold is inhibited. Third, after FD LOC capture in the APP mode. LOC is the localizer, the lateral guidance for an instrument landing system, and APP is approach mode. So once the flight director captures the localizer in approach mode, again the heading hold is inhibited. Now, let's move to pitch. We have a situation called "Pitch CWS with a CMD Engage switch selected." CMD is command mode — that's the normal autopilot mode where it's actively flying. 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 autopilot pitch has been manually overridden with control column force. And note this important detail: the force required for this override is greater than the normal CWS control column force. So it takes more effort to override the autopilot in pitch than it does to fly in normal CWS. And there's a limitation: this manual pitch override is inhibited in the APP mode with both autopilots engaged. While this mode is engaged, "CWS P" is annunciated on the Flight Mode Annunciators — the FMA. That's the display that tells you what the autopilot and flight director are doing. And from this state, command pitch modes can then be selected. Now here's a really important 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 — the autopilot is now capturing the altitude. And when you reach the selected altitude, ALT HOLD engages — altitude hold. So you get this automatic transition: CWS P, then ALT ACQ as you approach, then ALT HOLD at the altitude. But what if you manually override pitch while in ALT HOLD? The annunciator changes back to CWS P. And here's the subtle part with the 250-foot rule. 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. So you're within 250 feet, you let go, 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. You've flown it far enough away that it won't try to recapture — it just stays in CWS. Now the roll side, symmetric to pitch. "Roll CWS with a CMD engage switch selected." 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 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. While this mode is engaged, "CWS R" is annunciated on the FMA. 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 flying in CWS, you've armed the VOR or localizer approach, and you use CWS to steer toward the course. 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 hand it over — you fly it in manually via CWS until the autopilot captures the navigation signal, then it takes over and tracks. Finally, let's look at Touch Control Steering — TCS. This also permits a pilot to manoeuvre the aircraft in pitch or roll. But here's the critical difference from CWS: unlike CWS, the appropriate automatic control channels and servomotors are disengaged while the TCS button is held depressed. So while you hold that button down, the autopilot's servos are actually disconnected — you're flying the aircraft directly, by hand, to the desired attitude. The excerpt cuts off there, but that's the fundamental contrast: CWS keeps the autopilot in the loop and it responds to your pressures, whereas TCS physically disengages the servos while you hold the button, and you fly directly. Let me make sure you've got the key contrasts. CWS — you apply pressure, autopilot moves the aeroplane, release and it holds. TCS — you hold the button, servos disengage, you fly directly. And remember those three inhibitions on the heading hold with bank less than 6 degrees: below 1500 feet RA with gear down, after FD VOR capture at 250 knots or less, and after FD LOC capture in APP mode. Those are the ones examiners love.