Autothrottle — Page 393, Lesson 489

I want to walk you through the autothrottle system now, and we're going to start with the disengage lights and the take-off mode, then move into the N1 and speed modes. Let's begin with those A/T Disengage lights. The A/T Disengage lights can be extinguished by any of three actions. First, returning the A/T Arm switch to ARM. Second, pressing either A/T Disengage light. Third, pressing either A/T Disengage switch. Now, an important detail here — these lights do not illuminate when the A/T automatically disengages after landing. So if the system drops out on its own after touchdown, you won't see those lights come on. Now let's look at the take-off mode. The take-off mode is engaged by pressing either TOGA switch while the aeroplane is on the ground, with the A/T armed and the desired take-off N1 thrust limit selected from an FMC CDU. Let me unpack that. TOGA stands for Take-Off/Go-Around. N1 is the fan speed of the engine, expressed as a percentage of maximum. FMC CDU is the Flight Management Computer and its Control Display Unit — that's where you select the 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. THR HLD annunciates at 84 knots — or 64 knots for aeroplanes with earlier model A/T computers. THR HLD means thrust hold. It indicates that the A/T cannot change the thrust lever position, but you can reposition the thrust levers manually. So the autothrottle locks the levers in place, but you still have manual authority over them. After lift-off, the A/T remains in THR HLD until 400 feet RA is reached and approximately 18 seconds have elapsed since liftoff. RA is radio altitude — the height above the ground measured by the radio altimeter. At that point, the A/T annunciation changes from THR HLD to ARM. Reduction to climb thrust can now be made by pressing the N1 switch. Now here's a timing restriction. Until 2½ minutes after lift-off, automatic reduction to climb thrust is inhibited when you're in LVL CHG or V/S mode. LVL CHG is level change, and V/S is vertical speed. But if V NAV, ALT ACQ, or ALT HOLD is engaged during that 2½ minute period, automatic thrust reduction occurs normally. V NAV is vertical navigation, ALT ACQ is altitude acquire, and ALT HOLD is altitude hold. So the inhibition only applies in those two specific pitch modes. Let's move to the N1 mode. The A/T maintains thrust at the N1 limit selected from an 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 to ARM. So the N1 switch is your way out of N1 mode back to armed. Now, engine failure behaviour in N1 mode. If an engine fails while the A/T is in the N1 mode, the thrust lever of the failed engine will advance forward a few degrees and then return to or below the other thrust lever position. So the lever briefly moves forward, then settles back to or below the position of the good engine's lever. Finally, the speed mode. The speed mode is available throughout the flight once the take-off phase is completed. Pressing the MCP Speed Select switch selects the speed mode if compatible with the engaged AFDS pitch mode. MCP is the Mode Control Panel, and AFDS is the Autopilot Flight Director System. 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 the target speed. IAS is indicated airspeed. Here's a key limitation. The A/T will not set power above the displayed N1 limit. However, the A/T can exceed an N1 value that has been manually set by the N1 manual set knob. So the displayed limit is a hard ceiling, but a manually set value can be exceeded. If an engine fails while the A/T is in a speed mode, both thrust levers advance together to maintain the target speed — that's different from the N1 mode, where only the failed engine's lever moves. Now, approach guidance. When on final approach in landing configuration, it is not recommended to set the A/T command speed to allow for wind or gust corrections. Through airspeed and acceleration sensing, the A/T corrects for normal wind gusts. Higher command speed settings result in excessive approach speeds. The recommended A/T approach speed setting is VREF +5. VREF is the reference landing speed, so you add 5 knots. And one more detail. 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 close to the ground, the system is capable of spooling the engines up quickly if you need to go around. That's the full picture of the autothrottle modes — take-off, N1, and speed — along with the disengage light logic and the engine failure behaviour in each mode.