Autopilot — Page 377, Lesson 458

We're now in the heart of the autopilot chapter, and I want to walk you through what happens when the autopilot is actually in operation, phase by phase. This is where the theory becomes a working system. First, let's look at a specific descent scenario. During a V NAV path descent — that's a vertical navigation descent following a computed path — V NAV stays engaged until one of four things happens. It disengages when you capture the glide slope, when you select another pitch mode, when you extend the flaps beyond 15 degrees, or when L NAV is disengaged without a glide slope capture. L NAV is lateral navigation, the roll channel's way of following a route. So the rule is: V NAV keeps controlling pitch until something takes over, and the flap limit of 15 degrees is a hard boundary you must respect. Now, proper MCP altitude selections ensure correct altitude alerting. MCP is the Mode Control Panel — that's where you dial in your target altitude and select your modes. If you set it correctly, the system gives you the right altitude alerts at the right times. Let me walk you through the modes of operation during each flight phase. I'll read this table as a working pilot would. In take-off, the roll channel is in TOGA — take-off/go-around — and the pitch channel is also TOGA, with the autothrottle in THR REF, which is thrust reference. In climb, roll can be LNAV, HDG, or VOR — lateral navigation, heading, or VOR tracking. Pitch can be FLCH SPD, VNAV, or V/S — that's flight level change speed, vertical navigation, or vertical speed. The autothrottle runs THR REF, SPD, or MACH — thrust reference, speed, or Mach. In cruise, roll is again LNAV, HDG, or VOR, pitch is ALT HOLD or VNAV — altitude hold or vertical navigation — and the throttle is SPD or MACH. In descent, roll is LNAV, HDG, or VOR, pitch is FLCH SPD, VNAV, or V/S, and throttle is THR REF, SPD, or MACH. In approach, roll is LNAV, HDG, or LOC — localizer — pitch is ALT, G/S, or FLARE — altitude, glide slope, or flare — and throttle is SPD. Then we have land, which is ROLLOUT, and go-around, which returns to TOGA on both roll and pitch with THR REF on the throttle. Now let's look at the other autopilot features. Approach mode, abbreviated APP, couples the localizer and glide slope elements of the ILS — the Instrument Landing System — to the roll and pitch channels of the autopilot. This allows automatic control down to decision height, or even a fully automatic landing. The localizer gives you lateral guidance, the glide slope gives you vertical guidance, and the autopilot follows both. Then there's Control Wheel Steering, or CWS, provided in some automatic flight control systems like the Boeing 737-400. Its purpose is to let the pilot manoeuvre the aircraft in pitch and/or roll through the automatic control system without disconnecting the autopilot. The signals for the autopilot are produced by transducers in the control column — those are devices that convert your physical control inputs into electrical signals. When you press a CWS engage switch, it engages the autopilot's pitch and roll axes in CWS mode, and the flight mode annunciators — the FMAs — display CWS P and CWS R, for pitch and roll respectively. So you're flying through the system, not against it. That's the operational picture: the phase table tells you what's available, and the features tell you how you can intervene.