Autopilot — Page 373, Lesson 453

We're starting a fresh topic here: the autopilot, and specifically the outer loop inputs in pitch. Before we dive into the modes, I want to set the stage with something important: the LNAV, or Lateral Navigation, mode. I want to walk you through when it disconnects, because it's a perfect example of how these systems think. LNAV will automatically disconnect for several reasons. It will disconnect upon reaching the end of the active route, or upon entering a route discontinuity. A route discontinuity is a gap in the flight plan—a break between two waypoints that aren't connected. Additionally, it will disconnect upon either intercepting, or missing the intercept of, an approach path inbound track. Finally, either a loss of capture criteria, or selecting HDG SEL—that's Heading Select—will disconnect LNAV. So, if you manually select a heading mode, you're telling the autopilot to stop following the lateral navigation. Now, let's move into the pitch axis, starting with 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. "Uncorrected barometric altitude" means the raw altitude reading without any correction for local pressure settings. ALT HOLD engages in one of two conditions. First, ALT HOLD at the MCP selected altitude. This is indicated by the annunciation of ALT HOLD, and the ALT HOLD switch light is extinguished—so the light is off. Second, ALT HOLD not at the MCP selected altitude. This is indicated by the annunciation of ALT HOLD, but now the ALT HOLD switch light is illuminated—the light is on. That second condition, not at the MCP selected altitude, occurs with either of the following: 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. So, if you're holding at 10,000 feet and you dial in 12,000 on the MCP, you're now in ALT HOLD but not at the selected altitude—the light comes on to tell you that. There are some important inhibitions here. ALT HOLD is inhibited after glide slope capture. That means once you're locked onto the glide slope for an approach, you can't engage altitude hold. Also, when in ALT HOLD at the selected altitude, LVL CHG, V/S, and V NAV climb and descend functions are inhibited until a new altitude is selected. LVL CHG is Level Change, V/S is Vertical Speed, and V NAV is Vertical Navigation. So, if you're holding altitude and you want to climb or descend, you must first dial in a new target altitude. 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, and to the First Officer's barometric setting for the "B" autopilot and FDS. FDS stands for Flight Director System. So, the "A" system uses the Captain's altimeter setting, and the "B" system uses the First Officer's. After ALT HOLD engages, changes in the altimeter barometric settings do not change the selected altitude reference. Once you're locked in, adjusting the baro setting won't move your target. Next, we have 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're about to level off at your target, ALT ACQ takes over to smooth that transition. The altitude acquire mode is also armed while climbing or descending in CWS with an autopilot engaged. CWS is Control Wheel Steering—that's where you manually fly the aircraft 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, if you're in V NAV, the display keeps showing V NAV even while the acquire mode is working. ALT ACQ engagement is inhibited when the ALT HOLD switch is pressed, or while the glide slope is captured. Now, let's look at IAS/MACH Hold, abbreviated SPD. This mode will hold a selected IAS or Mach number. IAS is Indicated Airspeed. It does this by comparing the selected value with the actual value from the ADC, 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, we have Vertical Speed, abbreviated V/S. The V/S 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, you set a vertical speed, like 1,500 feet per minute, and the autopilot holds that rate of climb or descent. Meanwhile, the autothrottle manages your speed. The V/S mode has both an armed and engaged state—so it can be standing by, ready to activate, or actively controlling. Let me show you the Mode Control Panel so you can see where these switches live. That's the panel you'll be interacting with. And here's a look at the inputs feeding into it. So, to tie it together: ALT HOLD locks you at an altitude, ALT ACQ transitions you to a new altitude, SPD manages your speed, and V/S manages your vertical speed. Each has its own annunciation and its own set of conditions and inhibitions. That's the foundation of pitch control in the autopilot.