Autopilot — Page 377, Lesson 456

Let’s pick up the autopilot modes where we left off — the vertical speed mode, the level change mode, and the vertical navigation mode. These are the three big climb-and-descent tools on the Mode Control Panel, and each one has a specific logic for when it engages, arms, or annunciates. Start with the V/S mode — that’s the vertical speed mode. Pressing the V/S switch engages it, but there are two conditions that block it: it will not engage if ALT HOLD is already engaged, and it will not engage after glide slope capture. So if you’re holding an altitude or you’ve already locked onto the glideslope on an approach, the V/S switch won’t take you into vertical speed mode. When V/S does engage, you get an annunciation — the flight mode annunciator shows V/S engaged. At the same time, the vertical speed display changes from blank to showing the present vertical speed. That’s important: the moment you engage, the display stops being blank and starts showing the actual vertical speed you’re currently flying. From there, you can select a desired vertical speed using the vertical speed thumb-wheel on the Mode Control Panel. Now, V/S can also be armed rather than engaged. That happens if you’re in ALT HOLD at the selected altitude, and you dial in a new MCP altitude that is more than 100 feet different from the previously selected altitude. In that situation, V/S becomes armed — it’s annunciated as armed — and you can then engage it by moving the vertical speed thumb-wheel. So the arming condition is: ALT HOLD active, plus a new altitude selected that differs by more than 100 feet. There’s also an automatic engagement path. V/S automatically engages when the altitude acquire mode is engaged and a new altitude is selected that is more than 100 feet different from the previously selected altitude. When that happens, V/S annunciates engaged, and the existing vertical speed appears in the vertical speed display. You can still change the commanded vertical speed with the thumb-wheel. And note this: you can select vertical speeds that command flight either toward the selected altitude or away from it — so you can climb away from your selected altitude if you want, not just toward it. Now the Level Change mode — LVL CHG. This mode coordinates pitch and thrust commands to make automatic climbs and descents to a preselected altitude at a selected airspeed. So it’s a coupled pitch-and-thrust mode. You initiate a LVL CHG climb or descent by selecting a new altitude and engaging the LVL CHG mode. The annunciations tell you what’s being commanded. During a LVL CHG climb, the annunciations are MCP SPD for pitch and N1 for the autothrottle. MCP SPD means the pitch is holding the speed selected on the Mode Control Panel, and N1 means the autothrottle is commanding a target engine N1 — that’s the fan speed percentage. During a LVL CHG descent, the annunciations are MCP SPD for pitch and RETARD for the autothrottle while the thrust is being reduced toward idle. RETARD is the autothrottle command that’s pulling the thrust levers back. Once you’re at idle thrust, the autothrottle annunciation changes to ARM. There’s a speed-retention logic here. If a speed mode was active before you engaged LVL CHG, the previous speed is retained as the target speed for the LVL CHG mode. If LVL CHG is engaged with no active speed mode, then the IAS/Mach display and the airspeed cursors synchronize to the existing speed, and the present speed becomes the LVL CHG target speed. After engagement, you can change the target speed with the MCP speed selector. Finally, the Vertical Navigation mode — V NAV. With V NAV engaged, the Flight Management Computer — the FMC — commands the AFDS pitch and the autothrottle modes to fly the vertical profile that you’ve selected on the FMC CDUs. The CDUs are the control display units, the FMC’s interface screens. The profile includes preselected climbs, cruise altitudes, speeds, and descents, and it can also include altitude constraints at specified waypoints. The profile may even end with an ILS approach to the destination airfield. Pressing the V NAV switch selects V NAV mode, provided FMC performance initialization is complete. So you need the FMC performance data initialized first. When it engages, the mode selector switch illuminates, the MCP IAS/Mach display becomes blank, and the airspeed cursors are positioned at the FMC-commanded airspeed. The Flight Mode Annunciator — the FMA — displays either V NAV SPD or V NAV PTH for the AFDS pitch mode, and for the autothrottle it displays FMC SPD, N1, RETARD, or ARM. Two constraints to remember. V NAV climbs and descents are constrained by the selected MCP altitude — so the FMC profile can’t take you beyond what you’ve dialed on the Mode Control Panel. And V NAV commanded speeds can be changed with the FMC CDUs, not the MCP speed selector. One more behavior: during V NAV path cruise flight, if you select a lower MCP altitude, that arms the FMC to automatically begin the descent upon arrival at the FMC-calculated top of descent point. So you set your lower altitude, and the FMC will start the descent by itself at the computed top of descent. So the key contrasts: V/S is a manual vertical speed command with thumb-wheel control, LVL CHG is a coupled pitch-and-thrust climb or descent to a selected speed, and V NAV hands the vertical profile over to the FMC, constrained by your MCP altitude.