Autopilot — Page 377, Lesson 456

I want to walk you through the vertical modes of the autopilot now — the modes that control whether the aircraft climbs, descends, or holds altitude. We've already covered the basics of how the Mode Control Panel, or MCP, works, so now let's look at the specific vertical modes in detail. Let's start with the V/S mode — that's the Vertical Speed mode. When you press the V/S switch on the MCP, it engages the V/S mode, but there's an important condition: it won't engage if ALT HOLD is already engaged, and it also won't engage after glide slope capture. So if you're holding an altitude or you've already captured the glide slope on an approach, pressing V/S won't take effect. When V/S is engaged, you'll see it annunciated on the Flight Mode Annunciator — the FMA. At the same time, the Vertical Speed Display changes from blank to showing your present vertical speed. That's the key point: before V/S engages, that display is blank; once engaged, it shows your current rate of climb or descent. Then you can select a desired vertical speed using the vertical speed thumb-wheel on the MCP. Now here's where it gets interesting — V/S can also become armed, not just engaged. That happens if you're in ALT HOLD at your selected altitude, and you select a new MCP altitude that is more than 100 feet different from the previously selected altitude. In that case, V/S becomes armed, and you'll see "V/S armed" annunciated. To actually engage it from that armed state, you move the vertical speed thumb-wheel. There's also a third path: V/S automatically engages when the altitude acquire mode is engaged and you select a new altitude that's more than 100 feet different from the previously selected one. When that happens, V/S annunciates as engaged, your existing vertical speed appears in the vertical speed display, and you can change the commanded V/S with the thumb-wheel. And note this — you can select vertical speeds that command flight toward or away from the selected altitude. So you can climb away from it or descend toward it, depending on what you command. Now let's move to the Level Change Mode, abbreviated LVL CHG. This mode coordinates pitch and thrust commands together to make automatic climbs and descents to a preselected altitude at a selected airspeed. You initiate a LVL CHG climb or descent by selecting a new altitude and then engaging the LVL CHG mode. During a LVL CHG climb, the annunciations on the FMA are MCP SPD for pitch and N1 for the autothrottle — that's the A/T. During a LVL CHG descent, the annunciations are MCP SPD for pitch and RETARD for the A/T, while the thrust is being reduced toward idle. Once the thrust reaches idle, the A/T annunciation changes to ARM. Now, about the target speed: 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. But if you engage LVL CHG with no active speed mode, then the IAS/Mach display and the airspeed cursors synchronize to your existing speed, and your present speed becomes the LVL CHG target speed. After engagement, you can change the target speed with the MCP speed selector. Finally, let's look at the Vertical Navigation Mode, or V NAV. With V NAV engaged, the FMC — the Flight Management Computer — commands the AFDS pitch and A/T modes to fly the vertical profile that you've selected on the FMC CDUs — the Control Display Units. That profile includes preselected climbs, cruise altitudes, speeds, descents, and it can also include altitude constraints at specified waypoints. The profile may even end with an ILS approach to the destination airfield. To engage V NAV, you press the V NAV switch, but only if FMC performance initialization is complete. When it engages, the mode selector switch illuminates, the MCP IAS/Mach display becomes blank, and the airspeed cursors position themselves at the FMC commanded airspeed. On the FMA, you'll see either V NAV SPD or V NAV PTH for the AFDS pitch mode, and for the A/T mode you'll see FMC SPD, N1, RETARD, or ARM. V NAV climbs and descents are constrained by the selected MCP altitude — so the FMC won't take you beyond what you've set on the MCP. And you can change V NAV commanded speeds using the FMC CDUs. One more important detail: during V NAV path cruise flight, if you select a lower MCP altitude, that arms the FMC to automatically begin the descent when the aircraft arrives at the FMC calculated top of descent point. So the FMC is watching ahead and will start the descent for you at the right moment. Let me show you the Mode Control Panel and the annunciations so you can see how these modes appear in the cockpit. So to tie it together: V/S gives you direct control of your rate of climb or descent, LVL CHG coordinates pitch and thrust to get you to a new altitude at a set speed, and V NAV lets the FMC fly the entire vertical profile for you. Each has its own engagement conditions, annunciations, and constraints — and those are the details you'll need to recognize on the FMA in the cockpit.