2000’ or 150 seconds after lift-off and the command bars will — Page 348, Lesson 403

I want to walk you through what happens with the Flight Director, or FD, right after you lift off — this is the take-off mode and the climb-out logic, and it's a sequence you'll need to know cold. First, the trigger. The FD command bars — those are the two little bars on the PFD that tell you where to pitch and roll — they appear automatically at 2000 feet or 150 seconds after lift-off, whichever comes first. Before that, during the actual take-off, the FD is already giving you pitch commands, but the bars themselves show up at that 2000-foot or 150-second point. Now, the pitch logic. After lift-off, the FD commands 15 degrees nose-up pitch. It holds that until you achieve a sufficient climb rate. Once you've got that climb rate, it switches to commanding pitch to maintain the MCP speed plus 20 knots indicated airspeed. MCP is the Mode Control Panel — that's where you set your target speeds and modes. That MCP speed plus 20 knots is calculated and set during the pre-flight. And here's a neat detail: when either autopilot is engaged, or when you rotate the MCP speed selector, that 20 knots is added automatically to the MCP IAS display — so the display itself shows the plus-20 value. For roll, the FD commands wings level from take-off mode engagement all the way through the take-off climb-out. So it's keeping you straight and level in roll the whole time. Now, how do you get out of take-off mode? It depends on your height. Below 400 feet RA — that's radio altitude, the height above the ground measured by the radio altimeter — you terminate take-off mode only by turning both FD switches off. Both of them, not one. Above 400 feet RA, you have more options: you can select other FD pitch modes, or you can engage an autopilot, and that will terminate the take-off mode. Let me walk you through what happens when you engage an autopilot after an FD take-off. The autopilot and FD automatically engage in LVL CHG for pitch — that's Level Change — and HDG SEL for roll — that's Heading Select. But if you had already changed the FD mode from TO/GA — that's Take-Off/Go-Around — to LNAV, HDG SEL, or VOR/LOC, then the autopilot initially engages in the same roll mode as the FDs. So it matches whatever roll mode you'd already selected. And when LVL CHG engages, the MCP IAS/MACH display and the airspeed cursors change to V2 plus 20 knots. V2 is your take-off safety speed — the speed at which you can safely climb out with an engine failed. Now, the engine failure case during take-off — this is critical. If an engine fails before you reach V2, the FD pitch commands are referenced to V2. If the engine fails after reaching V2 but below V2 plus 20 knots, the reference speed is the speed at which the engine failure occurred — so you hold that exact speed. If the failure happens at or above V2 plus 20 knots, then V2 plus 20 knots is the commanded speed. And the key rule: the reference speed is never less than V2 for the current flap setting. So V2 is your floor. Roll control in an engine failure remains the same as for all engines operating — the FD keeps commanding wings level. Now let's move to the cruise phase — this is the Altitude Acquire and Altitude Hold logic. In cruise, we want to fly at a constant level, and the FDS — the Flight Director System — helps achieve that. Signals from the ADC — that's the Air Data Computer — go through the FDS. When you're at the selected level, the FD command bars indicate a neutral position — no pitch command. If the aircraft deviates above the desired altitude, the command bars indicate a pitch-down command. And vice versa — if you deviate below, you get a pitch-up command. So the bars are always telling you to return to that selected altitude. Let me show you the annunciators and the altitude acquire/hold behaviour on the PFD so you can see exactly what these modes look like in the cockpit. So to tie it together: take-off mode gives you 15 degrees nose-up until you have climb rate, then MCP speed plus 20. Roll stays wings level. You exit below 400 feet RA only by switching both FDs off; above that, by selecting other modes or engaging an autopilot. The autopilot picks up LVL CHG and HDG SEL, or matches your existing roll mode, and the speed target becomes V2 plus 20. Engine failure references V2 as the floor. And in cruise, the FD bars hold you level by commanding pitch back to the selected altitude.