Autopilot — Page 373, Lesson 453

I want to walk you through the pitch modes of the autopilot now. We've already covered the outer loop inputs in lateral navigation, so let's look at what happens in pitch. First, let me finish the L NAV disconnection logic, because it sets the pattern for how these modes behave. L NAV will automatically disconnect for several reasons. It disconnects upon reaching the end of the active route, or upon entering a route discontinuity — that's a gap in the flight plan. It also disconnects upon either intercepting or missing the intercept of an approach path inbound track. Finally, either loss of capture criteria or selecting HDG SEL — that's heading select — will disconnect L NAV. Now, the first pitch mode I want to cover is Altitude Hold, abbreviated ALT HLD. This mode gives pitch commands to hold either the MCP selected altitude, or the uncorrected barometric altitude at which the ALT HOLD switch was pressed. Let me unpack that. The MCP is the Mode Control Panel — that's where you dial in your target altitude. But if you press the ALT HOLD switch when you're not at that selected altitude, it will hold whatever uncorrected barometric altitude you're at that moment. Uncorrected means it's referenced to the raw barometric setting, not corrected to standard pressure. ALT HOLD engages in either of two conditions. First, ALT HOLD at the MCP selected altitude — that's indicated by the annunciation of ALT HOLD, and the ALT HOLD switch light is extinguished, meaning it's off. Second, ALT HOLD not at the MCP selected altitude — that's indicated by the annunciation of ALT HOLD, but the ALT HOLD switch light is illuminated, meaning it's on. That second condition — not at the MCP selected altitude — occurs with either of two actions. Pushing the ALT HOLD switch while not at the MCP selected altitude, or selecting a new MCP altitude while in ALT HOLD at the currently selected altitude. Now, there are important inhibitions here. ALT HOLD is inhibited after glide slope capture — that's when you're locked onto the approach path. And when you're in ALT HOLD at the selected altitude, LVL CHG, V/S, and V NAV climb and descend functions are all inhibited until a new altitude is selected. LVL CHG is level change, V/S is vertical speed, and V NAV is vertical navigation. Here's a critical detail about the altitude reference. The altitude selected on the MCP is referenced to the Captain's barometric altimeter setting for the "A" autopilot and FDS — that's the flight director system. And it's referenced to the First Officer's barometric setting for the "B" autopilot and FDS. So the "A" system uses the left seat's altimeter setting, and the "B" system uses the right seat's. After ALT HOLD engages, changes in the altimeter barometric settings do not change the selected altitude reference — so once you're holding, adjusting the baro setting won't move your target. Next, let's look at Altitude Acquire, abbreviated ALT ACQ. This is the transition manoeuvre entered automatically from a V/S, LVL CHG, or V NAV climb or descent to a MCP selected altitude. So when you're climbing or descending and you approach your target altitude, this mode takes over to level you off smoothly. The altitude acquire mode is also armed while climbing or descending in CWS with an autopilot engaged — CWS is control wheel steering, where you fly manually but the autopilot assists. Altitude acquire engagement is annunciated ALT ACQ in pitch when levelling off in either V/S or LVL CHG. However, V NAV remains annunciated throughout the altitude acquire mode when levelling in V NAV — so the display keeps showing V NAV even though ALT ACQ is doing the work. ALT ACQ engagement is inhibited when the ALT HOLD switch is pressed, or while the glide slope is captured. Now let's cover IAS/MACH Hold, abbreviated SPD. This mode holds a selected IAS — that's indicated airspeed — or Mach number. It does this by comparing the selected value with the actual value from the ADC — that's the air data computer — and pitching the aircraft up or down to decrease or increase speed. So if you're too fast, it pitches up to slow down; if you're too slow, it pitches down to speed up. Finally, Vertical Speed, abbreviated V/S. This mode gives pitch commands to hold the selected vertical speed, and it engages the autothrottle in the SPEED mode to hold the selected airspeed. So the autopilot handles the pitch to maintain your climb or descent rate, and the autothrottle manages the speed. The V/S mode has both an armed and engaged state — armed means it's ready to activate, engaged means it's actively controlling. Let me show you how these fit together on the Mode Control Panel. That's the panel where you select your altitude, vertical speed, and modes. And here's how the flight mode annunciations display these modes to you. And this shows the inputs feeding into the autoflight computer. So the key takeaway is that each pitch mode has a specific purpose, a specific way it engages, and specific conditions that inhibit it. ALT HOLD locks your altitude, ALT ACQ transitions you to it, SPD manages your speed, and V/S manages your climb or descent rate. Each one has its own annunciation and its own logic for when it can and cannot operate.