Autothrottle — Page 393, Lesson 489

Let's pick up with the autothrottle system, and I want to start with the A/T Disengage lights, because they're your first line of feedback when the system drops out. The A/T Disengage lights can be extinguished by any of these three actions: returning the A/T Arm switch to ARM, pressing either A/T Disengage light, or pressing either A/T Disengage switch. So you have two ways to physically cancel the light — pressing the light itself, or pressing the dedicated Disengage switch. And there's one important exception: those lights do not illuminate when the A/T automatically disengages after landing. That's a normal, expected disengagement, so it doesn't trigger the warning. Now let's move into the Take-off Mode. You engage it by pressing either TOGA switch — that's Take-Off/Go-Around — while the aeroplane is on the ground, with the A/T armed, and with the desired take-off N1 thrust limit selected from an FMC CDU. N1 is the fan speed of the engine, expressed as a percentage, and the FMC CDU is the Flight Management Computer's Control Display Unit — that's where you've dialled in your take-off thrust limit. When you press TOGA, the A/T annunciation changes from ARM to N1, and the thrust levers advance toward take-off thrust. The A/T then sets take-off thrust, and here's a key annunciation: THR HLD appears at 84 knots — or 64 knots for aeroplanes with earlier model A/T computers. THR HLD means thrust hold. It tells you the A/T can no longer change the thrust lever position, but you can still reposition the levers manually. So the automation locks the levers where they are, and you have manual authority. After lift-off, the A/T stays in THR HLD until you reach 400 feet RA — that's radio altitude — and approximately 18 seconds have elapsed since liftoff. At that point, the annunciation changes from THR HLD back to ARM. Now you can reduce to climb thrust by pressing the N1 switch. There's a timing restriction here: until 2½ minutes after lift-off, automatic reduction to climb thrust is inhibited when you're in LVL CHG or V/S mode — that's Level Change or Vertical Speed. But if you're in V NAV, ALT ACQ, or ALT HOLD — Vertical Navigation, Altitude Acquire, or Altitude Hold — during that same 2½-minute period, automatic thrust reduction occurs normally. So the inhibition only applies to those two climb-management modes. Next is the N1 Mode. The A/T maintains thrust at the N1 limit you selected from the FMC CDU. N1 is annunciated for the A/T, and the N1 switch illuminates. Pressing the N1 switch changes the A/T mode from N1 back to ARM. And here's the engine-failure behaviour: if an engine fails while the A/T is in N1 mode, the thrust lever of the failed engine will advance forward a few degrees, then return to or below the other thrust lever's position. So it briefly pushes forward, then settles back down. Finally, the Speed Mode. This is available throughout the flight once the take-off phase is completed. You select it by pressing the MCP Speed Select switch — that's the Mode Control Panel — provided it's compatible with the engaged AFDS pitch mode, the Autopilot Flight Director System. When active, MCP SPD is annunciated for the A/T mode, and the Speed Mode switch illuminates. The speed or Mach shown in the MCP IAS/MACH display is your target speed. The A/T will not set power above the displayed N1 limit — but it can exceed an N1 value that you've manually set with the N1 manual set knob. So the automatic limit is respected, but a manual override can be exceeded. If an engine fails while in speed mode, both thrust levers advance together to maintain the target speed — that's different from N1 mode, where only the failed engine's lever moves. On final approach in landing configuration, it's not recommended to set the A/T command speed to allow for wind or gust corrections. The A/T corrects for normal wind gusts through airspeed and acceleration sensing. Higher command speed settings just result in excessive approach speeds. The recommended A/T approach speed setting is VREF +5 — that's the reference landing speed plus five knots. And one last point: below 400 feet RA, the A/T thrust level response rate and engine power levels are sufficient to place the engines in the rapid acceleration range. So in the critical final phase, the system is capable of spooling the engines up quickly if you need a go-around. That's the full autothrottle picture — disengage logic, take-off, N1, and speed modes, with their annunciations and failure behaviours.