Autothrottle — Page 401, Lesson 492

We're starting the Autothrottle chapter now. This is the automatic throttle system that works hand-in-hand with the autopilot and flight management computer. Let's walk through the modes one by one, because each one has a specific job and a specific condition that triggers it. First, the FMC Speed Mode. FMC stands for Flight Management Computer. This is an autothrottle mode commanded by the FMC during V NAV operation — V NAV being vertical navigation, where the FMC is steering the aircraft's vertical path. When this mode is engaged, the MCP IAS/Mach display goes blank. MCP is the Mode Control Panel, and IAS is Indicated Airspeed. So the speed readout on the panel blanks out, and instead, the airspeed cursors — those little pointers on the speed tape — position themselves at the FMC-commanded airspeed. The autothrottle then maintains that commanded speed. But here's the limit: the autothrottle is limited to the N1 value shown on the thrust mode annunciators. N1 is the fan speed of the engine, expressed as a percentage. So the system can command speed, but it can't exceed that N1 limit that's displayed. Next, N1 Equalization. The autothrottle tries to equalize N1 — that is, balance the fan speeds between the two engines — through the dual servo individual thrust lever control. Each thrust lever has its own servo, so they can be moved independently. But the equalization control is limited to 8 degrees of thrust lever separation. So if the levers drift more than 8 degrees apart, the system won't try to force them back together beyond that point. Now the ARM Mode. The autothrottle annunciates ARM — that's the mode annunciation on the display — when the A/T Arm switch is at ARM and no A/T mode is engaged. In this state, the thrust levers can be manually positioned without any interference from the autothrottle system. So you have full manual control. The system automatically transfers to ARM from the SPEED or N1 mode when that mode is deselected by pressing the respective mode selector switch while the switch light is illuminated. So if you're in SPEED mode and you press the SPEED selector while its light is on, the system drops back to ARM. Then we have the Descent Retard Mode. The autothrottle engages and annunciates RETARD during LVL CHG and V NAV descents. LVL CHG is Level Change, a pitch mode on the MCP. RETARD is the mode that's pulling the thrust levers back. RETARD changes to ARM when the thrust levers reach the aft stop — that's the full rearward position — or when they are manually prevented from reaching the aft stop. So if you hold the levers back yourself, the system recognizes that and goes to ARM. Now the Landing Flare Retard Mode. This is the one that brings the thrust to idle as you touch down. During landing, the RETARD mode engages, reduces thrust, and annunciates RETARD 2½ seconds after FLARE mode engagement, or at 27 feet radio altitude — whichever occurs first. Radio altitude is the height above the ground measured by the radio altimeter. For a non-precision or visual approach with flaps extended to 15 or greater, and the AFDS — that's the Autopilot Flight Director System — not in ALT ACQ or ALT HOLD, the RETARD mode engages at 27 feet radio altitude. ALT ACQ is altitude acquire, ALT HOLD is altitude hold. The autothrottle then automatically disengages approximately 2 seconds after landing touchdown. Finally, the Go-around Mode. With the A/T Arm switch at ARM, the go-around mode is armed when descending below 2000 feet radio altitude, with or without the AFDS engaged. Once armed, it can be engaged until 2 seconds have elapsed after landing touchdown. Pressing either TO/GA switch — that's Takeoff/Go-Around — engages the mode. GA is annunciated for the autothrottle, and the thrust levers advance to the reduced go-around thrust setting. That reduced setting produces a 1000 to 2000 feet per minute rate of climb. After reaching reduced go-around thrust, pressing either TO/GA switch a second time signals the autothrottle to advance thrust to the full go-around N1 limit. After reaching reduced or full go-around thrust, the GA mode can be terminated by selecting another AFDS pitch mode, or when ALT ACQ annunciates engaged. And during a single-engine FD go-around — FD being Flight Director — the autothrottle will increase thrust to the full N1 limit. Now the Autothrottle Disengage Switches. Pressing one of these disengages the autothrottle. The A/T Disengage light flashes, and the A/T ARM switch on the MCP trips off. Pressing the switch a second time extinguishes the A/T warning. And after an automatic A/T disengagement — meaning the system dropped out on its own — pressing the Disengage switch will extinguish the A/T warning. That's the full autothrottle mode structure. The key thread through all of it: every mode has a trigger, an annunciation, and a way out. ARM is your manual fallback, RETARD handles the descent and flare, and GA gets you climbing again.